Hardening a simple op-amp circuit against EMI
Russell Warren
environment. It looks like below (hopefully the ascii drawings work
out)...
___10k_______
| |
GND--------|\ 20k
| \____|___
--100----------|+/
|/
The numbers are resistor values and the + input comes from a unity
gain buffer.
On first inspection my first obvious thought was to limit the
bandwidth to avoid EMI issues (circuit only actually needs kHz'ish
bandwidth) so I put a simple RC filter on the input as follows:
___10k_______
| |
GND--------|\ 20k
| \____|___
--100----------|+/
| |/
1u =
|
GND
But this still had EMI issues and was probably bad because hanging a 1
uF cap off the input is a bad idea? Not that I have any fundamental
basis for making that statement.
Consulting an EMC design book which has been *extremely* helpful in
the past (EMC for Product Designers by Tim Williams), one of the
things it recommends is putting low value resistors at each of the
inputs, right at the pins. It also indicates that the values should
be no larger than a few hundred Ohms. ie: adding 200 Ohms to each pin
of the circuit above:
___10k_______
| |
GND----200-|\ 20k
| \____|___
--100------200-|+/
| |/
1u =
|
GND
I'd like to understand what is going on here. My questions are:
1. Is hanging a large capacitance on the input of an op-amp a bad
idea? If so, why? I saw some rumblings somewhere about CMRR issues,
but don't get it.
2. With the op-amp having a high input impedance, what is the reason
for limiting the resistors at the input pins to "up to a few hundred
Ohms"?
I'm trying to simulate the impact of it all in Switchercad, but I'm a
hack and my attempts have not been that illuminating. I've spent some
time trying to work out how to simulate and understand when an
amplifier circuit is stable or not, but haven't been able to fully
grasp phase margin and certainly not how to simulate it properly.
Any help is much appreciated! I have an extreme dislike for doing
anything because "that's just how you do it" and want to understand
what is going on underneath.
Russ
Jeroen Belleman
> I've got a simple op-amp circuit that is misbehaving in an EMI
> environment. It looks like below (hopefully the ascii drawings work
> out)...
>
> ___10k_______
> | |
> GND--------|\ 20k
> | \____|___
> --100----------|+/
> |/
>
really mean this?
> ___20k_______
> | |
> GND--10k--+----|\ |
> | \____|___
> --100----------|+/
> |/
>
That would be a gain of three amplifier. Certainly much better
behaved.
Jeroen Belleman
bill....@ieee.org
> I've got a simple op-amp circuit that is misbehaving in an EMI
> environment. It looks like below (hopefully the ascii drawings work
> out)...
>
> ___10k_______
> | |
> GND--------|\ 20k
> | \____|___
> --100----------|+/
> |/
The ASCII art doesn't seem to have worked as it should have. I can
only magine that you meant
GND -10k---+---20k-----+
| |
+----|\ |
| \____|___
--100----------|+/
|/
Which is a straight-forward follower with a gain of three.
> The numbers are resistor values and the + input comes from a unity
> gain buffer.
>
> On first inspection my first obvious thought was to limit the
> bandwidth to avoid EMI issues (circuit only actually needs kHz'ish
> bandwidth) so I put a simple RC filter on the input as follows:
>
> ___10k_______
> | |
> GND--------|\ 20k
> | \____|___
> --100----------|+/
> | |/
> 1u =
> |
> GND
>
> But this still had EMI issues and was probably bad because hanging a 1
> uF cap off the input is a bad idea? Not that I have any fundamental
> basis for making that statement.
It should have worked if the high-frequency interference was coming
from the output of your unity gain buffer, always assuming that the
unity gain buffer was up to driving a 1uF load;
most of them won't like it.
> Consulting an EMC design book which has been *extremely* helpful in
> the past (EMC for Product Designers by Tim Williams), one of the
> things it recommends is putting low value resistors at each of the
> inputs, right at the pins. It also indicates that the values should
> be no larger than a few hundred Ohms. ie: adding 200 Ohms to each pin
> of the circuit above:
>
> ___10k_______
> | |
> GND----200-|\ 20k
> | \____|___
> --100------200-|+/
> | |/
> 1u =
> |
> GND
>
> I'd like to understand what is going on here. My questions are:
>
> 1. Is hanging a large capacitance on the input of an op-amp a bad
> idea? If so, why? I saw some rumblings somewhere about CMRR issues,
> but don't get it.
It shouldn't be problem for the op amp y0u are looking at, but may
present a problem for the device driving that input.
> 2. With the op-amp having a high input impedance, what is the reason
> for limiting the resistors at the input pins to "up to a few hundred
> Ohms"?
The input impedance of an op amp can be seem as a high value resistor
in parallel with a couple of pF of capacitance to ground. Your extra
resistors adds a delay around the feedback loop. If you limt the
resistance to a few hundred ohms, the extra delay is less than a
nanosecond, which is negligible. Higher resistance mean more delay,
which can wreck the stability of the negative feedback loop.
> I'm trying to simulate the impact of it all in Switchercad, but I'm a
> hack and my attempts have not been that illuminating. I've spent some
> time trying to work out how to simulate and understand when an
> amplifier circuit is stable or not, but haven't been able to fully
> grasp phase margin and certainly not how to simulate it properly.
>
> Any help is much appreciated! I have an extreme dislike for doing
> anything because "that's just how you do it" and want to understand
> what is going on underneath.
Your problem is that you need to understand how your electromagnetic
interference is getting into your op amp and producing the
"misbehaviour" you are complaining aobut on the out of the amplifier.
It would help if you told us what the misbehaviour looks like, and
what the electromagnetic interference you are complaining about
consists of.
I've seen the audio signal from local radio stations on op amp
outputs ...
The usual rules of thumb are to keep the connections to the inputs of
an op amp as short as possible, and to route them over a solid ground
plane if this is at all possible.
You have to keep in mind that noise on the power supplies to an op amp
tends to appear on the output - more at high frequencies than at low
frequencies. The op amp data sheet should include a plot of the Power
Supply Rejection Ratio against frequency for both the positive and the
negative power pins. I've put low pass filters on both power pins of
fast op amps to deal with this problem; in one case I tried to get
away with just a ferrite bead and a capacitor, and managed to set up a
resonant circuit, which I ended up critically damping with a small
resistor in series with the bead - ferrite beads are supposed to be
lossy inductors, but mine wasn't anywhere near lossy enough at the
frequency in question.
--
Bill Sloman, Nijmegen
D from BC
Is is conducted noise, capacitively coupled noise, RF pickup noise or
magnetic induction noise?
D from BC
myrealaddress(at)comic(dot)com
BC, Canada
Posted to usenet sci.electronics.design
Eeyore
Russell Warren wrote:
> I've got a simple op-amp circuit that is misbehaving in an EMI
> environment. It looks like below (hopefully the ascii drawings work
> out)...
Yes, well first you'll have to describe what part of the circuitry it is
in and what upsets it and the effect. An input buffer ? Easy to fix.
What kind of misbehaviour ? Maybe you've not fully considered stability
issues.
Graham
Jim Thompson
Did you know you can place a (small) bypass capacitor between the +
and - inputs of an OpAmp... without much, if any, effect on the GBW?
Now you do ;-)
Discovered that probably 30+ years ago while debugging a noise pick-up
problem PCB at Interface, Inc.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
If I'm talking, you should be taking notes.
D from BC
Let's say it's magnetic.
I think using the lowest component impedances possible helps if it's
magnetic induction noise (I think it's called that.)
Example: IF the op amp is next to a leaky transformer.
An induced noise current will yield a noise voltage of I(Z).
Jim Thompson
<myreal...@comic.com> wrote:
>On Tue, 17 Mar 2009 10:53:12 -0700, Jim Thompson
><To-Email-Use-Th...@My-Web-Site.com> wrote:
>
[snip]
>>
>>Did you know you can place a (small) bypass capacitor between the +
>>and - inputs of an OpAmp... without much, if any, effect on the GBW?
>>Now you do ;-)
>>
>>Discovered that probably 30+ years ago while debugging a noise pick-up
>>problem PCB at Interface, Inc.
>>
>> ...Jim Thompson
>
>Let's say it's magnetic.
Why say that?
>I think using the lowest component impedances possible helps if it's
>magnetic induction noise (I think it's called that.)
>Example: IF the op amp is next to a leaky transformer.
>
>An induced noise current will yield a noise voltage of I(Z).
>
>
>D from BC
>myrealaddress(at)comic(dot)com
>BC, Canada
>Posted to usenet sci.electronics.design
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine Sometimes I even put it in the food
Joerg
Ok, Russell, I won't be able to see your posts and replies because you
use gmail (maybe use a better address?). But I want to alert you to
something that is rarely known in the trade:
Opamps with bipolar transistors in the input can cause RF to be
rectified. A cell phone transmitting in the GHz range can cause that
trouble even with an opamp that has only 10MHz or so GBW. This is
because the BE junctions at IN+ and IN- rectify RF and that rectified
signal shows up in your signal. With GSM you'd have pretty hard
"rat-tat-tat" style bursts when the phone connects to a new tower or
checks in once in a while. CMOS opamps suffer much less from this
behavior because there is no BE junction that is slightly conducting,
there are usually only substrate diodes that are sufficiently reverse
biased (as long as your input swings stay withing the rails).
With strong EMI none of the external methods such as ferrite beads and
capacitors help much. I had one such case where the EMI went straight
through the SO-8 plastic smack dab into the chip. So I recommended a
CMOS opamp to that client or, as an alternative, shielding. We then
tried both and both fixed it. Since they really liked the BJT-based amp
for audio noise level reasons they opted for shielding.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
RST Engineering (jw)
Jim
DJ Delorie
Joerg <notthis...@removethispacbell.net> writes:
> With GSM you'd have pretty hard "rat-tat-tat" style bursts when the
> phone connects to a new tower or checks in once in a while.
I had that on my truck's radio with my old sprint cell phone.
Eeyore
Jim Thompson wrote:
> D from BC <myreal...@comic.com> wrote:
> > Jim Thompson <To-Email-Use-Th...@My-Web-Site.com> wrote:
> >>
> >>Did you know you can place a (small) bypass capacitor between the +
> >>and - inputs of an OpAmp... without much, if any, effect on the GBW?
> >>Now you do ;-)
> >>
> >>Discovered that probably 30+ years ago while debugging a noise pick-up
> >>problem PCB at Interface, Inc.
> >
> >Let's say it's magnetic.
>
> Why say that?
In this instance maybe of no relevance but it's astonishing how much
magnetic / inductive pickup can destroy your signal. Even down to the level
of PCB layout.
Graham
Eeyore
Joerg wrote:
> Ok, Russell, I won't be able to see your posts and replies because you
> use gmail (maybe use a better address?). But I want to alert you to
> something that is rarely known in the trade:
>
> Opamps with bipolar transistors in the input can cause RF to be
> rectified.
Tell me about it ! I've changed designs to JFET input to avoid exactly that
problem.
> A cell phone transmitting in the GHz range can cause that
> trouble even with an opamp that has only 10MHz or so GBW. This is
> because the BE junctions at IN+ and IN- rectify RF and that rectified
> signal shows up in your signal. With GSM you'd have pretty hard
> "rat-tat-tat" style bursts when the phone connects to a new tower or
> checks in once in a while. CMOS opamps suffer much less from this
> behavior because there is no BE junction that is slightly conducting,
> there are usually only substrate diodes that are sufficiently reverse
> biased (as long as your input swings stay withing the rails).
>
> With strong EMI none of the external methods such as ferrite beads and
> capacitors help much. I had one such case where the EMI went straight
> through the SO-8 plastic smack dab into the chip. So I recommended a
> CMOS opamp to that client or, as an alternative, shielding. We then
> tried both and both fixed it. Since they really liked the BJT-based amp
> for audio noise level reasons they opted for shielding.
Horses for courses.
Graham
D from BC
<To-Email-Use-Th...@My-Web-Site.com> wrote:
>On Tue, 17 Mar 2009 11:09:25 -0700, D from BC
><myreal...@comic.com> wrote:
>
>>On Tue, 17 Mar 2009 10:53:12 -0700, Jim Thompson
>><To-Email-Use-Th...@My-Web-Site.com> wrote:
>>
>[snip]
>>>
>>>Did you know you can place a (small) bypass capacitor between the +
>>>and - inputs of an OpAmp... without much, if any, effect on the GBW?
>>>Now you do ;-)
>>>
>>>Discovered that probably 30+ years ago while debugging a noise pick-up
>>>problem PCB at Interface, Inc.
>>>
>>> ...Jim Thompson
>>
>>Let's say it's magnetic.
>
>Why say that?
>
>>I think using the lowest component impedances possible helps if it's
>>magnetic induction noise (I think it's called that.)
>>Example: IF the op amp is next to a leaky transformer.
>>
>>An induced noise current will yield a noise voltage of I(Z).
>>
>>
>>D from BC
>>myrealaddress(at)comic(dot)com
>>BC, Canada
>>Posted to usenet sci.electronics.design
>
> ...Jim Thompson
Magnetic induction is just a possibility. The OP didn't mention what
this circuit is next to.
For all the circuits I've dreamed up, power supply noise or crappy
mixed mode PCB layout was the problem.
Fixable with filters and/or better routing.
Jim Thompson
<j...@rstengineering.com> wrote:
>"Small" relative to ... ???
>
>Jim
>
>
>> Did you know you can place a (small) bypass capacitor between the +
>> and - inputs of an OpAmp... without much, if any, effect on the GBW?
>> Now you do ;-)
>>
>> Discovered that probably 30+ years ago while debugging a noise pick-up
>> problem PCB at Interface, Inc.
>>
>> ...Jim Thompson
[snip]
Try sizes until you get what you want ;-) Say 100pF.
It CAN be calculated. This is left as an exercise for the student ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
John Larkin
<russand...@gmail.com> wrote:
Try a different opamp maybe. Jfet and CMOS opamps are usually much
less sensitive to rfi pickup than bipolars. The bipolar diff input
pair tends to rectify.
I recently saw an ad for an opamp that's specifically EMI hard, but I
can't recall whose. LTC maybe?
John
Rich Grise
On Tue, 17 Mar 2009 11:39:46 -0700, RST Engineering (jw) wrote:
Lack of Attribution Repaired:
>> ...Jim Thompson wrote
>
>> Did you know you can place a (small) bypass capacitor between the +
>> and - inputs of an OpAmp... without much, if any, effect on the GBW?
>> Now you do ;-)
>>
>> Discovered that probably 30+ years ago while debugging a noise pick-up
>> problem PCB at Interface, Inc.
> "Small" relative to ... ???
Large?
And don't top post, and leave your victim's "Xxxx wrote:" line in.
Rich
Joerg
The only BJT-input opamps I remember being quite EMI-proof were those in
metal TO-cans. But that has kind of gone out of fashion ;-)
Back then I needed that, since about 1-2 feet from the respective gear
was my kilowatt-size RF amp, often without the lid and then a Pringles
can size inductor would be looking at you. So I used uA709 in TO cans
since in our neck of the woods the uA741 only came in plastic dip. This
was a few decades ago when copper foil was unobtanium.
John Larkin
The DIY stained-glass-supplies shops have all sorts of cool stuff,
like solderable sticky-back copper foil of various widths, and some
amazingly klutzy soldering irons.
John
Jamie
You could have issues with the type of OP-AMP you're using..
bipolar type op-amps in some environments will actually act as
R.F. detectors. If the EMI is surrounding the chip boundary, chances are
you won't get rid of it.
If you can test this circuit with the input disconnected from the
remote source and shunted to common with an R to verify if this is
really happening or not, it would be a great help.
Using a MosFet type of Op-Amp on the front end many times alleviates
the problem.
Have you tried shielding around the circuit and employed some bypass
caps?
bill....@ieee.org
wrote:
You can express this more formally, by pointing that the long-tailed
pair at the input of a bipolar op amp goes non-linear if the voltage
between the inputs goes above about 26mV.
At low frequencies - within the op amps bandwidth - negative feedback
stop this from happening, but at higher frequencies, there isn't any
feedback.
The long-tailed pair on the input of a FET- or MOSFET-input op amp
goes non-linear in the same way, but only at signal levels around a
volt.
It explains the rectifier action ...
--
Bill Sloman, Nijmegen
Joerg
In Europe they were called Tiffany lamp shops. But the pricing in those
places was far beyond reach for someone in high school. But grandpa gave
me a big stash of copper and I thought I'd be in heaven. From then on I
could make shielded enclosures without having to convince mom that we
all "needed" a can of butter cookies. Because then she make fresh ones,
no can.
Joerg
Happens much earlier. Some hundred microvolts from a cell phone and your
low noise pick-up circuit is toast. Rectification happens the instant RF
hits a forward biased diode path such as a BE junction of a properly
biased BJT amp stage, whether diff or not. Ye olde LM324 and his
brethren can't regulate that away via feedback.
John Larkin
<notthis...@removethispacbell.net> wrote:
James A is a big fan of Altoids cans.
John
Bill Sloman
"Joerg" <notthis...@removethispacbell.net> schreef in bericht
news:rkZvl.26516$ZP4....@nlpi067.nbdc.sbc.com...
It shouldn't be. Much below 26mV and the input stage is linear, so what the
collector current gains on the highs should be exactly cancelled by what it
loses on the lows. Once you get above the 26mV region, whichever of the
two transistors is turned on is pretty much fully turned on, and the other
one
is close to off, and - for NPN transistors - the input starts following the
upper
envelope of the incoming RF, rather than its average. If you've managed to
set
up a resonant circuit for the cell-phone frequency around the op amp inputs,
you may see higher voltages at the inputs than you'd expect.
>Rectification happens the instant RF hits a forward biased diode path such
>as a BE junction of a properly biased BJT amp stage, whether diff or not.
Seems unlikely.
>Ye olde LM324 and his brethren can't regulate that away via feedback.
Absolutely not. Cell phones used to use an 800MHz carrier - I think that
this has
now gone up to a few GHz.
Very few op amps have any gain left at these frequencies.
--
Bill Sloman, Nijmegen
James Arthur
> Joerg wrote:
>
>> John Larkin wrote:
>>> Joerg wrote:
>>>
>>>> The only BJT-input opamps I remember being quite EMI-proof were those in
>>>> metal TO-cans. But that has kind of gone out of fashion ;-)
>>>>
>>>> Back then I needed that, since about 1-2 feet from the respective gear
>>>> was my kilowatt-size RF amp, often without the lid and then a Pringles
>>>> can size inductor would be looking at you. So I used uA709 in TO cans
>>>> since in our neck of the woods the uA741 only came in plastic dip. This
>>>> was a few decades ago when copper foil was unobtanium.
>>>
>>> The DIY stained-glass-supplies shops have all sorts of cool stuff,
>>> like solderable sticky-back copper foil of various widths, and some
>>> amazingly klutzy soldering irons.
>>>
>> In Europe they were called Tiffany lamp shops. But the pricing in those
>> places was far beyond reach for someone in high school. But grandpa gave
>> me a big stash of copper and I thought I'd be in heaven. From then on I
>> could make shielded enclosures without having to convince mom that we
>> all "needed" a can of butter cookies. Because then she make fresh ones,
>> no can.
>
>
> James A is a big fan of Altoids cans.
>
> John
>
Sure. They tin easily and they're Curiously Strong(tm). Clabber
Girl baking powder cans too.
Cheers,
James Arthur
Joerg
It has nothing at all to do with gain. What happens is this:
RF gets in, GHz range. The amount of RF current present in the IN+
region is not 100% the same phase/amplitude as in the IN- region. Now
some of that gets rectified in the BE junctions. This rectified amount
is what the opamp sees as a real input signal because, in the case of
GSM and other cell phone system, the bursts are spectrally very low in
frequency. RF on, RF off, RF on, and so forth. Once this stuff is in
there, there ain't nothing you can do to get rid of it if it's in your
band of interest.
Joerg
Watch your stomach when eating all those Altoids. They are delicious but
some of them seem quite strong. I liked the Fishermen's Friends cans but
that stuff is really potent. I remember someone who took one and then
fell asleep on the couch. He was so tired that he woke up the next
morning. One cheek was completely numb, just like the Novocaine from the
dentist would do.
Jim Thompson
Do you use stake pins as feed-throughs ?:-)
John Larkin
Horrors! Don't you know that stuff is mostly cornstarch?
(Dan sez hi; he's in the conference room digging through old
submetering drawings.)
John
Russell Warren
this group is very active and helpful! Tricky to respond individually
to all the helpful insights and questions, but I'll try to get the
main ones...
Bill Sloman said:
> The ASCII art doesn't seem to have worked as it should have. I can
> only magine that you meant
>
> GND -10k---+---20k-----+
> | |
> +----|\ |
> | \____|___
> --100----------|+/
> |/
>
> Which is a straight-forward follower with a gain of three.
Argh. You are absolutely correct... that is a consequence of a last
minute edit to the drawings that I obviously screwed up. Sorry about
that and thanks for clarifying.
Since I completely messed up the drawing (in more ways than that), let
me do it more completely and accurately...
GND -10k---+------20k------+
| |
------ +--------|\ |
| | |\ | \____|___
| DAC|-----|+\__100_______100_________|+/
| | +--| / | |/
| | | |/ | TS921ID
| | | OP727 |
|____| |____________|
There were several informative comments about BJT op-amps being more
sensitive to RFI. The OP727 is listed as having a bipolar pnp input
stage and CMOS output stage, and the TS921ID is listed as BiCMOS.
Where is the "bi" split in BiCMOS? Is that the same as the OP7272?
ie: BJT input stage and CMOS output stage?
The info on reasons for RFI affecting the op-amp were particularly
useful. I still have some confusion as to the impact of RFI pickup at
the op-amp inputs, but I'll save that for another post.
Regarding the capacitive load on the OP727 follower which I previously
said was a unity gain buffer... the data sheet says it can drive up to
500 pF loads. I always hear that the 100 Ohm load in the feedback
path helps drive capacitive loads (but again don't fully understand
why). With the addition of the RC filter in the circuit it actually
looks like below.
GND -10k---+------20k------+
| |
------ +--------|\ |
| | |\ | \____|___
| DAC|-----|+\__100_______100_________|+/
| | +--| / | | |/
| | | |/ | 1uF = TS921ID
| | | OP727 | |
|____| |____________| GND
Bill Sloman said:
> It should have worked if the high-frequency interference was coming
> from the output of your unity gain buffer, always assuming that the
> unity gain buffer was up to driving a 1uF load;
> most of them won't like it.
Is the 1 uF load still a problem in the corrected circuit drawing
above, or is the 100 Ohm in the OP727 feedback sufficient?
Bill Sloman said:
>> 2. With the op-amp having a high input impedance, what is the reason
>> for limiting the resistors at the input pins to "up to a few hundred
>> Ohms"?
>
> The input impedance of an op amp can be seem as a high value resistor
> in parallel with a couple of pF of capacitance to ground. Your extra
> resistors adds a delay around the feedback loop. If you limt the
> resistance to a few hundred ohms, the extra delay is less than a
> nanosecond, which is negligible. Higher resistance mean more delay,
> which can wreck the stability of the negative feedback loop.
Interesting... thanks. When you say the resistor is causing a delay,
do you mean the phase change resulting from the filter created by the
serial resistor and the input capacitance?
Also - if it is delay in the feedback loop which is of concern, what
would the reason for limiting the resistor value directly on the +
input pin?
I've had trouble finding specs on the input capacitance of an op-amp
(I've seen other threads on this, too). Seems like something that
would be important to have if/when putting serial resistors in place
like recommended for RFI hardening. How high can it realistically get
when unspecified? Does it vary radically unit to unit? At something
like 5 pF, 200 Ohms limits to 150 MHz which is "ok" since I would
assume if I need that much bandwidth I better know what I'm doing
regarding the op-amps input capacitance anyway. Clearly another case
of it being better to know what you are doing rather than blindly
following a rule of thumb. Aren't they all, though.
Bill Sloman wrote:
> > I'm trying to simulate the impact of it all in Switchercad, but I'm a
> > hack and my attempts have not been that illuminating. I've spent some
> > time trying to work out how to simulate and understand when an
> > amplifier circuit is stable or not, but haven't been able to fully
> > grasp phase margin and certainly not how to simulate it properly.
>
> > Any help is much appreciated! I have an extreme dislike for doing
> > anything because "that's just how you do it" and want to understand
> > what is going on underneath.
>
> Your problem is that you need to understand how your electromagnetic
> interference is getting into your op amp and producing the
> "misbehaviour" you are complaining aobut on the out of the amplifier.
>
> It would help if you told us what the misbehaviour looks like, and
> what the electromagnetic interference you are complaining about
> consists of.
The device is in a temperature chamber and I see problems at certain
points in temperature cycling. I expect it is EMI kicking out of the
compressor when it is on.
The nature of the bad behaviour is hard to describe because we are
picking it up very indirectly and subtly in another measurement. It
is not a huge pickup, it just affects our signal of interest (a low
bandwidth LC reading elsewhere) by < 1%, but this is is quite
significant in the application. I do not see it directly in that op-
amp because it is hard to get to and measure in the setup. However -
we do know the root of the problem is in the circuit above (by
elimination). We have not yet reproduced the problem in a setup where
it is easier to both induce and measure the problem.
> The usual rules of thumb are to keep the connections to the inputs of
> an op amp as short as possible, and to route them over a solid ground
> plane if this is at all possible.
>
> You have to keep in mind that noise on the power supplies to an op amp
> tends to appear on the output - more at high frequencies than at low
> frequencies. The op amp data sheet should include a plot of the Power
> Supply Rejection Ratio against frequency for both the positive and the
> negative power pins. I've put low pass filters on both power pins of
> fast op amps to deal with this problem; in one case I tried to get
> away with just a ferrite bead and a capacitor, and managed to set up a
> resonant circuit, which I ended up critically damping with a small
> resistor in series with the bead - ferrite beads are supposed to be
> lossy inductors, but mine wasn't anywhere near lossy enough at the
> frequency in question.
Thanks... I'm certainly looking at better decoupling on the supply
rails and what you say is good advice to keep in mind for the future.
Now - from lab testing yesterday (where I was rather than responding
to the thread) the following circuit change eliminates the observed
problem on a few units.
GND -10k---+------20k------+
| |
------ +--------|\ |
| | |\ | \____|___
| DAC|-----|+\__100_____12k7_____100__|+/
| | +--| / | | |/
| | | |/ | 10nF = TS921ID
| | | OP727 | |
|____| |____________| GND
The 12.7k + 10 nF RC filter was added to add some bandwidth limiting
to more what the circuit actually cares about, and the 100 Ohm was
added to the TS921ID input pin (pin lifted and stuffed under) based on
the general RFI recommendations, filtering direct-to-pin pickup with
the input capacitance of the op-amp.
I know that although it fixed the observed problem, it was hardly a
full EMC test, so it isn't where I'll leave the circuit, but that was
all I did due to time constraints and the joy of hand soldering 0402
SMT components. The circuit still needs more investigation and
testing.
eg: I still have to determine if the 10 nF will be a problem for the
OP727. It seems like not based on performance, but it is obviously
greater than the 500 pF OP727 spec and I don't know if the 100 Ohm
feedback resistor actually fixes it or why, as stated above. Although
it performs ok, is it surfing the edge of stability?
Thanks to all for your help. The goal of my post wasn't to have my
problem solved for me, but more to get a better understanding of what
the heck I'm doing and why. All the posts are certainly helping and
are much appreciated.
Russ
Rich Grise
> On Tue, 17 Mar 2009 15:57:16 -0700, Joerg
>>Back then I needed that, since about 1-2 feet from the respective gear
>>was my kilowatt-size RF amp, often without the lid and then a Pringles
>>can size inductor would be looking at you. So I used uA709 in TO cans
>>since in our neck of the woods the uA741 only came in plastic dip. This
>>was a few decades ago when copper foil was unobtanium.
>
> The DIY stained-glass-supplies shops have all sorts of cool stuff,
> like solderable sticky-back copper foil of various widths, and some
> amazingly klutzy soldering irons.
On late-night PBS here, there's a show about hobbyist glassworking,
and one of the show's sponsors is Weller. ;-)
Cheers!
Rich
James Arthur
Oh not to worry--I don't EAT them. My Mom does & gives me the cans.
Win-win.
> I liked the Fishermen's Friends cans but
> that stuff is really potent. I remember someone who took one and then
> fell asleep on the couch. He was so tired that he woke up the next
> morning. One cheek was completely numb, just like the Novocaine from the
> dentist would do.
>
That's a timely tip. I broke a molar Thursday, crowned yesterday.
Grins,
James Arthur
James Arthur
I just sort of make up whatever's handy and appropriate--
- 1/4w resistor leads thru teflon tubing for routine stuff,
usually soldered/anchored to a PCB within,
- add ferrite bead and 2 bypass caps to ground for others,
- a 1/4" chunk of hardline coax makes a nice teflon-insulated
feedthru,
- actual, official feed-thru caps when you really need them,
- good old turret ground terminals for ground connections,
- or official SMA rf connectors if need be.
The casing's soft, so you can just punch through it with an
awl.
There's something neat about an enclosure that's all solderable
ground plane, 100% shielded, pops open easily for access, and
smells good when you solder it.
Cheers,
James Arthur
Joerg
The final crown? That's pretty fast. My last one was around $800 without
root canal :-(
James Arthur
> James Arthur wrote:
>
>> John Larkin wrote:
>>>
>>> James A is a big fan of Altoids cans.
>>>
>>> John
>>>
>> Sure. They tin easily and they're Curiously Strong(tm). Clabber
>> Girl baking powder cans too.
>
> Horrors! Don't you know that stuff is mostly cornstarch?
Gaaack! _Now_ he tells me!
> (Dan sez hi; he's in the conference room digging through old
> submetering drawings.)
>
> John
Hi Dan!
James
James Arthur
No, a temp, and $1,400. $1k for the crown, and $400 for the prep
complicated by the lost cusps. She screwed a couple Ti pins into
the tooth--cool. I didn't know they did that.
James Arthur
Joerg
$1400? Yikes! I assume that includes the final crown but it's still very
expensive. Probably there comes a time when it would make sense to
travel to some foreign country, have major dental work done there, and
use the savings to tack on a nice vacation there.
The screws may have been a dowel, that would be pretty much standard
procedure if the crown needs it for support.
James Arthur
It's the first time I needed Ti. As far as the $1k for the crown,
sounds high to me too, but that's the standard quote.
That's another side-effect of government payment schedules--it
coordinates the price of all the providers.
James Arthur
Joerg
Hmm, since your address is Verizon I assume you are in the US. AFAIK
dentist charges are not regulated by the government. All I can tell you
is that it's a whole lot less expensive out here (east of Sacramento),
or at least was when I got my last one about a couple years ago. Still
painful though because as a self-employed I must pay 100% out of pocket.
Luckily we have HSA now so it's pre-tax. But post SE-tax :-(
Jim Thompson
You are correct. Even Medicare does not cover dental work.
James Arthur
The charges aren't regulated, just the payments.
http://www.denti-cal.ca.gov/WSI/Default.jsp?fname=Default
If companies do this, it's called 'collusion' or 'price fixing,'
and it's illegal.
Last time I shopped I got three quotes, all identical.
C'est la vie
Cheers,
James Arthur
Joerg
Ok, I don't know about Medi-Cal but once when a new dentist moved into
town we went there and requests pricing for certain common treatments.
He was more than 15% higher than our current dentist so we did not switch.
Now, with propane companies, that's a whole 'nother matter. Last time I
compared they were "miraculously" all pretty much the same.
James Arthur
Like you, I pay cash, but, I assure you, whenever a large payer
prints a list of what they'll pay, they set a minimum price.
(Those prices vary by area, too.)
A related thing Dad used to say: when the government sets a
minimum standard, they've also set the maximum standard.
Cheers,
James Arthur
Bill Sloman
"Russell Warren" <russand...@gmail.com> schreef in bericht
news:676b611a-aec2-4a58...@13g2000yql.googlegroups.com...
Wiith any luck, the relevant datasheet will tell you.
> The info on reasons for RFI affecting the op-amp were particularly
> useful. I still have some confusion as to the impact of RFI pickup at
> the op-amp inputs, but I'll save that for another post.
>
> Regarding the capacitive load on the OP727 follower which I previously
> said was a unity gain buffer... the data sheet says it can drive up to
> 500 pF loads. I always hear that the 100 Ohm load in the feedback
> path helps drive capacitive loads (but again don't fully understand
> why). With the addition of the RC filter in the circuit it actually
> looks like below.
>
> GND -10k---+------20k------+
> | |
> ------ +--------|\ |
> | | |\ | \____|___
> | DAC|-----|+\__100_______100_________|+/
> | | +--| / | | |/
> | | | |/ | 1uF = TS921ID
> | | | OP727 | |
> |____| |____________| GND
>
> Bill Sloman said:
>> It should have worked if the high-frequency interference was coming
>> from the output of your unity gain buffer, always assuming that the
>> unity gain buffer was up to driving a 1uF load;
>> most of them won't like it.
>
> Is the 1 uF load still a problem in the corrected circuit drawing
> above, or is the 100 Ohm in the OP727 feedback sufficient?
http://www.analog.com/static/imported-files/data_sheets/OP777_727_747.pdf
shows the output impedance of the part peaking at about 200R at
around 5MHz. 100R might be enough.
> Bill Sloman said:
>>> 2. With the op-amp having a high input impedance, what is the reason
>>> for limiting the resistors at the input pins to "up to a few hundred
>>> Ohms"?
>>
>> The input impedance of an op amp can be seem as a high value resistor
>> in parallel with a couple of pF of capacitance to ground. Your extra
>> resistors adds a delay around the feedback loop. If you limt the
>> resistance to a few hundred ohms, the extra delay is less than a
>> nanosecond, which is negligible. Higher resistance mean more delay,
>> which can wreck the stability of the negative feedback loop.
>
> Interesting... thanks. When you say the resistor is causing a delay,
> do you mean the phase change resulting from the filter created by the
> serial resistor and the input capacitance?
Exactly.
> Also - if it is delay in the feedback loop which is of concern, what
> would the reason for limiting the resistor value directly on the +
> input pin?
Symmetry. If you are using small resistors right up against the inputs to
linit RF pick-up, it makes sense to use identical resistors.
> I've had trouble finding specs on the input capacitance of an op-amp
> (I've seen other threads on this, too). Seems like something that
> would be important to have if/when putting serial resistors in place
> like recommended for RFI hardening. How high can it realistically get
> when unspecified? Does it vary radically unit to unit? At something
> like 5 pF, 200 Ohms limits to 150 MHz which is "ok" since I would
> assume if I need that much bandwidth I better know what I'm doing
> regarding the op-amps input capacitance anyway. Clearly another case
> of it being better to know what you are doing rather than blindly
> following a rule of thumb. Aren't they all, though.
I got caught by a Texas Instruments CMOS input op amp - I think it
was the TLC2201. No input capacitance on the data sheet, guessed
about 3pF - found about 15pF (based on the amount of feedback
capaitance it took to stop it oscillating). Fourteen years ago now, and
it still rankles.
The OP727 won't have any trouble driving the 12k7, and won't notice
10nF at the other end of the resistor
--
Bill Sloman, Nijmegen
David L. Jones
news:w4Svl.9967$jZ1....@flpi144.ffdc.sbc.com...
>D from BC wrote:
>> On Tue, 17 Mar 2009 08:52:33 -0700 (PDT), Russell Warren
>> <russand...@gmail.com> wrote:
>>
>>> I've got a simple op-amp circuit that is misbehaving in an EMI
>>> environment. It looks like below (hopefully the ascii drawings work
>>> out)...
>>>
>>> ___10k_______
>>> | |
>>> GND--------|\ 20k
>>> --100----------|+/
>>> |/
>>>
>>> gain buffer.
>>>
>>> On first inspection my first obvious thought was to limit the
>>> bandwidth to avoid EMI issues (circuit only actually needs kHz'ish
>>> bandwidth) so I put a simple RC filter on the input as follows:
>>>
>>> ___10k_______
>>> | |
>>> GND--------|\ 20k
>>> --100----------|+/
>>> | |/
>>> |
>>> GND
>>>
>>> But this still had EMI issues and was probably bad because hanging a 1
>>> uF cap off the input is a bad idea? Not that I have any fundamental
>>> basis for making that statement.
>>>
>>> Consulting an EMC design book which has been *extremely* helpful in
>>> the past (EMC for Product Designers by Tim Williams), one of the
>>> things it recommends is putting low value resistors at each of the
>>> inputs, right at the pins. It also indicates that the values should
>>> be no larger than a few hundred Ohms. ie: adding 200 Ohms to each pin
>>> of the circuit above:
>>>
>>> ___10k_______
>>> | |
>>> GND----200-|\ 20k
>>> | \____|___
>>> --100------200-|+/
>>> | |/
>>> 1u =
>>> |
>>> GND
>>>
>>> I'd like to understand what is going on here. My questions are:
>>>
>>> 1. Is hanging a large capacitance on the input of an op-amp a bad
>>> idea? If so, why? I saw some rumblings somewhere about CMRR issues,
>>> but don't get it.
>>>
>>> for limiting the resistors at the input pins to "up to a few hundred
>>> Ohms"?
>>>
>>> hack and my attempts have not been that illuminating. I've spent some
>>> time trying to work out how to simulate and understand when an
>>> amplifier circuit is stable or not, but haven't been able to fully
>>> grasp phase margin and certainly not how to simulate it properly.
>>>
>>> Any help is much appreciated! I have an extreme dislike for doing
>>> anything because "that's just how you do it" and want to understand
>>> what is going on underneath.
>>>
>>
>> Is is conducted noise, capacitively coupled noise, RF pickup noise or
>> magnetic induction noise?
>>
>
>
> Ok, Russell, I won't be able to see your posts and replies because you use
> gmail (maybe use a better address?). But I want to alert you to something
> that is rarely known in the trade:
>
> Opamps with bipolar transistors in the input can cause RF to be rectified.
> A cell phone transmitting in the GHz range can cause that trouble even
> with an opamp that has only 10MHz or so GBW. This is because the BE
> junctions at IN+ and IN- rectify RF and that rectified signal shows up in
> your signal. With GSM you'd have pretty hard "rat-tat-tat" style bursts
> when the phone connects to a new tower or checks in once in a while. CMOS
> opamps suffer much less from this behavior because there is no BE junction
> that is slightly conducting, there are usually only substrate diodes that
> are sufficiently reverse biased (as long as your input swings stay withing
> the rails).
I've used "the cell phone test" quite a bit as an extra test on input stages
in various designs.
Hold cell phone an inch away and see what happens :->
Often it's the phone speaker magnetics that cause the trouble though - a two
for one test, bargain!
Very common to get the "rat-tat-tat" effect in your car radio when your
mobile is sitting nearby.
Dave.
JosephKK
Thanks for the reminder. I am trying to finish my big ticket work
while at least some small portion of it is covered. (working stiff)
.
JosephKK
You didn't know??
.
bill....@ieee.org
wrote:
> Bill Slomanwrote:
> > "Joerg" <notthisjoerg...@removethispacbell.net> schreef in bericht
> >news:rkZvl.26516$ZP4....@nlpi067.nbdc.sbc.com...
This begs the question of how it gets rectified. If the RF current
being injected is big enough to sweep out the stored charge in the BE
junction, then you can get rectification.
Otherwise the RF goes through to the collector and gets lost in the
parallel capacitance of the node.
My point is that small RF currents won't get rectified; if they are
big enough to get rectified they are going to show up as a more-
than-26mV voltage excursion between the base connection to the input
transistors.
> This rectified amount
> is what the opamp sees as a real input signal because, in the case of
> GSM and other cell phone system, the bursts are spectrally very low in
> frequency. RF on, RF off, RF on, and so forth. Once this stuff is in
> there, there ain't nothing you can do to get rid of it if it's in your
> band of interest.
True, but the whole point about the advantage of FET-input op amps
over bipolar input op amps is that you need a sufficient voltage
excursion for the RF to get rectified (and its a lot higher for FET-
input amps).
There's a rational arguement that says that the threshold lies close
to 26mV for bipokar op amps, and you seem to want to revise this down
to the microvolt level without providing any explicit justification
for this revision.
I didn't want to disagree with you about this - I didn't want to give
John Larkin more justification for describing me as cranky - but I
think that you've got it wrong on this particular point (even though
you very rarely get things wrong).
--
Bill Sloman, Nijmegen
Jim Thompson
We have a new dentist in the neighborhood offering 15% discount for
cash or check.
I'm hearing more and more tales of good deals for direct pay.
Also a doctor here offers "executive medical care"... flat fee of
$79/month, immediate service, covers testing, too !!
Joerg
> On Mar 18, 3:31 pm, Joerg <notthisjoerg...@removethispacbell.net>
> wrote:
>> Bill Slomanwrote:
>>> "Joerg" <notthisjoerg...@removethispacbell.net> schreef in bericht
>>> news:rkZvl.26516$ZP4....@nlpi067.nbdc.sbc.com...
[ RF into BJT-opamp inputs ]
> This begs the question ...
Oh, oh. Watch the goons from the language police swooping in now ...
(and I still believe that wording is correct so we'll probably share a
holding cell soon)
> ... of how it gets rectified. If the RF current
> being injected is big enough to sweep out the stored charge in the BE
> junction, then you can get rectification.
>
It doesn't have to be big if what you are processing is in the microvolt
range. Whenever they call me out to cases like that this is almost
always so.
> Otherwise the RF goes through to the collector and gets lost in the
> parallel capacitance of the node.
>
Yeah, 99% does. But the remaining 1% is what causes all the grief,
frantic searches for the next flight out there, and so on ;-)
> My point is that small RF currents won't get rectified; if they are
> big enough to get rectified they are going to show up as a more-
> than-26mV voltage excursion between the base connection to the input
> transistors.
>
Ok, I can't show you schematics for reasons of confidentiality. All I
can tell you is that it does get rectified and I did measure RF levels.
Well below 26mV, yet rectification occurred. Enough of it to show up in
the data signal. Heck, we could tell whether the person coming down the
stairway next door was wearing an AT&T cell phone (GSM network) or not.
>> This rectified amount
>> is what the opamp sees as a real input signal because, in the case of
>> GSM and other cell phone system, the bursts are spectrally very low in
>> frequency. RF on, RF off, RF on, and so forth. Once this stuff is in
>> there, there ain't nothing you can do to get rid of it if it's in your
>> band of interest.
>
> True, but the whole point about the advantage of FET-input op amps
> over bipolar input op amps is that you need a sufficient voltage
> excursion for the RF to get rectified (and its a lot higher for FET-
> input amps).
>
Sure. But sometimes they don't offer the low frequency noise performance
you need.
> There's a rational arguement that says that the threshold lies close
> to 26mV for bipokar op amps, and you seem to want to revise this down
> to the microvolt level without providing any explicit justification
> for this revision.
>
Sorry, I can't share the data because it was work under contract for
clients, but it's right here. Typically went like this: RF amplitude
measured at the inputs, usually in the hundreds of uV. Bursts coincide
with the dirt in the data. Rip out BJT amp, replace with FET amp,
problem gone, if the baseband noise performance is still good enough
plus margin the job is done, call airline, try to get an earlier flight
back, wife happy.
If the baseband/audio noise performance ain't good enough with FET amps
it's back to square one. Usually meaning a relayout, better grounding,
shielding, and so on. Call hotel instead of the airline, see if stay can
be extended, wife not so happy.
> I didn't want to disagree with you about this - I didn't want to give
> John Larkin more justification for describing me as cranky - but I
> think that you've got it wrong on this particular point (even though
> you very rarely get things wrong).
>
Thanks for the kudos. But yeah, I am certainly wrong on some things,
just not on this one ;-)
GregS
I subscribed to one of the dental plans for a family member at the suggestion
of the dentists secretaries. At about $125 a year, saves considerable costs.
Just ask the dentist what insurance he accepts. I think I bought
Avia. Saved thousands.
]greg
James Arthur
I knew. John's teasing me--I recently told him this.
FWIW, Rumford brand has beaucoup more active stuff
per $, but Clabber Girl cans and 0.25" copper tubing
make 900MHz cavity filters with Q's of roughly 300.
Cheers,
James Arthur
James Arthur
No, Joerg's right Bill. Operating, bipolars' junctions are
forward biased, drawing bias current from their inputs, and
they'll rectify the slightest signal.
Think of them as pre-biased detectors.
Cheers,
James Arthur
Joerg
Exactly. Bill, you might as well try it out: Take the most sluggish BJT
opamp you find, hook up as 1000:1 amp or whatever, take a 100MHz or
higher frequency source, attenuate down to a millivolt or so, feed into
input. Hang a stereo amp on the output and then pulse the oscillator
hard. You'll be surprised ...
Jim Thompson
<bogus...@verizon.net> wrote:
>bill....@ieee.org wrote:
>> On Mar 18, 3:31 pm, Joerg <notthisjoerg...@removethispacbell.net>
>> wrote:
[snip]
>>
>> My point is that small RF currents won't get rectified; if they are
>> big enough to get rectified they are going to show up as a more-
>> than-26mV voltage excursion between the base connection to the input
>> transistors.
>>
[snip]
>>
>> I didn't want to disagree with you about this - I didn't want to give
>> John Larkin more justification for describing me as cranky - but I
>> think that you've got it wrong on this particular point (even though
>> you very rarely get things wrong).
>>
>> --
>> Bill Sloman, Nijmegen
>
>No, Joerg's right Bill. Operating, bipolars' junctions are
>forward biased, drawing bias current from their inputs, and
>they'll rectify the slightest signal.
>
>Think of them as pre-biased detectors.
>
>Cheers,
>James Arthur
Wonder from which orifice "physicist" Slowman pulled 26mV ?:-)
(I know what he misunderstands ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
How severe can senility be? Just check out Slowman.
John Larkin
>John Larkin more justification for describing me as cranky...
Cranky isn't important if you're right. And you're wrong.
John
bill....@ieee.org
Web-Site.com> wrote:
> On Thu, 19 Mar 2009 15:43:37 GMT, James Arthur
>
>
>
>
>
>
>
> >bill.slo...@ieee.org wrote:
> >> On Mar 18, 3:31 pm, Joerg <notthisjoerg...@removethispacbell.net>
> >> wrote:
> [snip]
>
> >> My point is that small RF currents won't get rectified; if they are
> >> big enough to get rectified they are going to show up as a more-
> >> than-26mV voltage excursion between the base connection to the input
> >> transistors.
>
> [snip]
>
> >> I didn't want to disagree with you about this - I didn't want to give
> >> John Larkin more justification for describing me as cranky - but I
> >> think that you've got it wrong on this particular point (even though
> >> you very rarely get things wrong).
>
> >> --
> >>Bill Sloman, Nijmegen
>
> >No, Joerg's right Bill. Operating, bipolars' junctions are
> >forward biased, drawing bias current from their inputs, and
> >they'll rectify the slightest signal.
>
> >Think of them as pre-biased detectors.
>
> >Cheers,
> >James Arthur
>
> Wonder from which orifice "physicist" Slowman pulled 26mV ?:-)
>
> (I know what he misunderstands ;-)
Jim should check out the Ebers-Moll equations or Gummel-Poon.
http://en.wikipedia.org/wiki/Bipolar_junction_transistor
kT/q comes out at 26mV at 300K which is to say room temperature
i(v) = i(0).e^V/26mV
It is a bit of an over-simplification to say that the current is a
linear function of voltage below 26mV, and exponential above it - what
Jeorg is saying is that you may not get much non-linearity below 26mV,
but you can get enough to rectify enough of an RF signal to get you
into trouble - but as an arbitary place to set the boundary it isn't a
bad choice.
In fact, for a long-tailed pair, the voltage difference gets divided
over both transistors, so 52mv might be a better number.
--
Bill Sloman, Nijmegen
bill....@ieee.org
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
I don't think so. The relationship between voltage and current is
still exponential for voltages below 26mV, so you can still get some
rectification, even if it is then only a small proportion of the RF
current that is getting rectified. In Jeorg's case, that small
proportion was enough to create difficulties.
Your arbitary - and unsupported - claim that I'm wrong, really is
cranky.
--
Bill Sloman, Nijmegen
James Arthur
Bill you're on the wrong part of the curve. dV(RFI) is small,
but you've missed the fact that the input junctions are
pre-biased forward, so Vf is 100's of mV, not 26mV.
Cheers,
James Arthur
Jim Thompson
Slowman's ego, chutzpah and senility level are high enough that he
could easily replace Barney Frank as Chief US Fairy ;-)
James Arthur
> On Thu, 19 Mar 2009 19:11:11 GMT, James Arthur
> <bogus...@verizon.net> wrote:
>
>> bill....@ieee.org wrote:
>>> On Mar 19, 5:42 pm, John Larkin
>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
>>>> Cranky isn't important if you're right. And you're wrong.
>>> I don't think so. The relationship between voltage and current is
>>> still exponential for voltages below 26mV, so you can still get some
>>> rectification, even if it is then only a small proportion of the RF
>>> current that is getting rectified. In Jeorg's case, that small
>>> proportion was enough to create difficulties.
>>>
>>> Your arbitary - and unsupported - claim that I'm wrong, really is
>>> cranky.
>>>
>>> --
>>> Bill Sloman, Nijmegen
>>
>> Bill you're on the wrong part of the curve. dV(RFI) is small,
>> but you've missed the fact that the input junctions are
>> pre-biased forward, so Vf is 100's of mV, not 26mV.
>>
>> Cheers,
>> James Arthur
>
> Slowman's ego, chutzpah and senility level are high enough that he
> could easily replace Barney Frank as Chief US Fairy ;-)
>
> ...Jim Thompson
No one could replace Barney Frank! Well, maybe Ted Kennedy.
Cheers,
James Arthur
bill....@ieee.org
Actually, I'm not. The exponential relationship is exponential
relative to whatever you take to be your reference point, and mine is
where the two transistors in the long-tailed pair are both biased for
equal currents in their collectors - needless to say, the equal
current that biasses up the rest of the circuit so that all the active
devices are in state to do something useful.
You can choose your own - equally arbitrary - reference point, but it
isn't likely to be anything like as useful as a starting point.
--
Bill Sloman, Nijmegen
James Arthur
Kindly compare the rectification efficiency of these two
cases (below).
Cheers,
James Arthur
~~~~~~~~~~~
Version 4
SHEET 1 880 680
WIRE 208 48 112 48
WIRE 448 48 272 48
WIRE 528 48 448 48
WIRE 112 80 112 48
WIRE 448 80 448 48
WIRE 528 80 528 48
WIRE 112 176 112 160
WIRE 448 176 448 160
WIRE 528 176 528 144
WIRE 208 240 112 240
WIRE 448 240 272 240
WIRE 528 240 448 240
WIRE 112 256 112 240
WIRE 448 272 448 240
WIRE 528 272 528 240
WIRE 112 352 112 336
WIRE 528 368 528 336
WIRE 448 384 448 352
FLAG 112 176 0
FLAG 112 352 0
FLAG 448 176 0
FLAG 448 384 0
FLAG 528 368 0
FLAG 528 176 0
SYMBOL voltage 112 64 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(400mV 1mV 1meg)
SYMBOL voltage 112 240 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value SINE(26mV 1mV 1meg)
SYMBOL diode 208 64 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D1
SYMBOL diode 208 256 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D2
SYMBOL res 432 64 R0
SYMATTR InstName R1
SYMATTR Value 10k
SYMBOL res 432 256 R0
SYMATTR InstName R2
SYMATTR Value 10k
SYMBOL cap 512 272 R0
SYMATTR InstName C1
SYMATTR Value 50nF
SYMBOL cap 512 80 R0
SYMATTR InstName C2
SYMATTR Value 50nF
TEXT 78 408 Left 0 !.tran 0 2mS 0 200nS
krw
<bogus...@verizon.net> wrote:
Barney hasn't gotten his boyfriend pregnant and then killed him. OTOH,
Slowman is Barney Fag's Doppelganger, right down to the lithp.
bill....@ieee.org
> On Thu, 19 Mar 2009 20:28:54 GMT, James Arthur
>
>
>
>
>
> <bogusabd...@verizon.net> wrote:
> >Jim Thompson wrote:
> >> On Thu, 19 Mar 2009 19:11:11 GMT, James Arthur
>
> >>> bill.slo...@ieee.org wrote:
> >>>> On Mar 19, 5:42 pm, John Larkin
> >>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> >>>>> Cranky isn't important if you're right. And you're wrong.
> >>>> I don't think so. The relationship between voltage and current is
> >>>> still exponential for voltages below 26mV, so you can still get some
> >>>> rectification, even if it is then only a small proportion of the RF
> >>>> current that is getting rectified. In Jeorg's case, that small
> >>>> proportion was enough to create difficulties.
>
> >>>> Your arbitary - and unsupported - claim that I'm wrong, really is
> >>>> cranky.
>
> >>>> --
> >>>> Bill Sloman, Nijmegen
>
> >>> Bill you're on the wrong part of the curve. dV(RFI) is small,
> >>> but you've missed the fact that the input junctions are
> >>> pre-biased forward, so Vf is 100's of mV, not 26mV.
>
> >>> Cheers,
> >>> James Arthur
>
> >> Slowman's ego, chutzpah and senility level are high enough that he
> >> could easily replace Barney Frank as Chief US Fairy ;-)
>
> >> ...Jim Thompson
>
> >No one could replace Barney Frank! Well, maybe Ted Kennedy.
>
> Barney hasn't gotten his boyfriend pregnant and then killed him. OTOH,
> Slowman is Barney Fag's Doppelganger, right down to the lithp.
Jim's fan club seems determined to out do him in the out-of-touch-with-
reality department.
Barney Frank would have to grow a beard and lose some weight before
there would be much of a resemblance, I don't lisp. and my wife was
still female the last time I checked.
--
Bill Sloman, Nijmegen
bill....@ieee.org
My charity doesn't extend spending time on irrelevant examples. We are
talking about the long-tailed pairs at the inputs to op amps, which is
a rather different situation.
If you can manage to get that into yur head, you might then find
something useful to say, but it seems unlikely.
--
Bill Sloman, Nijmegen
James Arthur
> James Arthur wrote:
>> bill.slo...@ieee.org wrote:
>>> James Arthur wrote:
>>>> bill.slo...@ieee.org wrote:
I'm sorry, it's clear I don't understand the relevance of
your explanation.
Above, you wrote:
"The relationship between voltage and current is still
exponential for voltages below 26mV, so you can still
get some rectification, even if it is then only a small
proportion of the RF current that is getting rectified.
In Jeorg's case, that small proportion was enough to
create difficulties."
The e-b junction in ordinary bipolar op amps has a
considerable forward bias voltage across it--much more than
26mV--and rectifies RFI by diode action. That, in turn,
produces a DC offset, which I modeled.
Which input junction of your long-tailed pair is forward
biased with 26mV, and what kind of transistor does it use?
A magic one?
Cheers,
James Arthur
Jim Thompson
A Slowman transistor. Never heard of it? No problem, you're just
ignorant ;-)
bill....@ieee.org
The inputs to an op amp are the bases of the two transistors forming
the long tailed pair.
For small voltage differences between the two inputs - less than 26mV
- the consequent difference between the two collector currents is
pretty close to a linear function of the voltage difference, and any
high frequency content in the input signal averages to zero
Once the voltage difference gets much above 26mV, the transistor that
is biassed on starts getting most of the tail current, and NPN inputs
start following the upper envelope of any high frequency contnet in
the input signal.
As Jeorg poits out, there is still some non-linearity below 26mV, so
you can still see some rectification of high frequency content.
Happy now?
--
Bill Sloman, Nijmegen
bill....@ieee.org
Jim used to know this stuff.
--
Bill Sloman, Nijmegen
Michael A. Terrell
Aren't they hand carved out of solid blocks of silicon by European
welfare queens?
--
http://improve-usenet.org/index.html
Goggle Groups, and Web TV users must request to be white listed, or I
will not see your messages.
If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm
James Arthur
Thanks for the clarification--your reasoning's clear.
But, yours is not the mechanism that detects RFI. The mechanism
is RFI being rectified single-ended at each input, by the b-e
junction. The b-e junction is forward-biased, ready, fast,
and willing.
RFI doesn't flow between the inputs; that path doesn't matter.
Cheers,
James Arthur
Eeyore
John Larkin wrote:
> Try a different opamp maybe. Jfet and CMOS opamps are usually much
> less sensitive to rfi pickup than bipolars. The bipolar diff input
> pair tends to rectify.
Came across a classic example of this when doing some 'EMI hard' tests on
low noise pro mic amps. An op-amp follows and input differential pair. A
TL07x made the circuit far less suscetible than the JRC/NJM BA4560 were
mostly using elsewhere.
I put in a small bifilar choke on the last mic amp I did. And don't forget
to use 1% caps too or you can make matters worse.
Graham
Eeyore
Joerg wrote:
> The only BJT-input opamps I remember being quite EMI-proof were those in
> metal TO-cans. But that has kind of gone out of fashion ;-)
I was astonished to see a relatively new National audio op-amp available in
metal can recently. You pay for it of course.
http://www.national.com/pf/LM/LM4562.html
TI do something else that's gorgeous too ( 1.1 nV / sqrt Hz input noise
voltage ) but it's SMT only, so you can't even retrofit DIL packages for
upgrades. I'm surprised they did that. I suppose their marketing people
didn't think it worth their while.
Graham
Eeyore
Joerg wrote:
> This was a few decades ago when copper foil was unobtanium.
I used some copper wire 'fabric' ( 130 mesh per inch IIRC ) once. Wow ! Did
that shut things up. It felt almost like silk to handle. It had to be
'transparent' you see.
Graham
John Larkin
>
>The inputs to an op amp are the bases of the two transistors forming
>the long tailed pair.
>
>For small voltage differences between the two inputs - less than 26mV
>- the consequent difference between the two collector currents is
>pretty close to a linear function of the voltage difference, and any
>high frequency content in the input signal averages to zero
No. Most opamp input diff pairs run at low current and the emitters
(and current source) have significant capacitance hence long time
constants. The transistor that gets high-freq RF on its base has, in
effect, a capacitor to ground on its emitter. At hundreds of MHz, it's
a nice diode peak rectifier.
>
>Once the voltage difference gets much above 26mV, the transistor that
>is biassed on starts getting most of the tail current, and NPN inputs
>start following the upper envelope of any high frequency contnet in
>the input signal.
Where do you get the 26 mV thing from? There's serious rectification
going on at much lower p-p swings.
Another real-life problem is that real systems, in boxes and on pc
boards, tend to have ghastly high-Q resonances all over the spectrum,
which magnify modest RFI into serious voltages; narrowband peaks like
50:1 are not unusual.
>
>As Jeorg poits out, there is still some non-linearity below 26mV, so
>you can still see some rectification of high frequency content.
>
"Some" is enough to cause serious problems in many, many actual
products. I can't use a lot of cheap electronics in my house because
the ambient RF fields make problems. A naiive thermocouple signal
conditioner design *will* get into trouble when people with cell
phones stroll by.
John
Joerg
Yeah, I often did the copper tape thing. Cut, stick to top of chip,
connect to GND on both sides or, when not reachable, to VCC and VEE via
0402 caps.
Eeyore
Joerg wrote:
> Eeyore wrote:
> > Joerg wrote:
> >
> >> This was a few decades ago when copper foil was unobtanium.
> >
> > I used some copper wire 'fabric' ( 130 mesh per inch IIRC ) once. Wow ! Did
> > that shut things up. It felt almost like silk to handle. It had to be
> > 'transparent' you see.
>
> Yeah, I often did the copper tape thing. Cut, stick to top of chip,
> connect to GND on both sides or, when not reachable, to VCC and VEE via
> 0402 caps.
I had to dig around to find copper tape but I found a specialist eventually. Very
useful in transformer construction too.
Graham
bill....@ieee.org
Why do you think that?
>The b-e junction is forward-biased, ready, fast,
> and willing.
Pity it doesn't do what you think it does.
> RFI doesn't flow between the inputs; that path doesn't matter.
In your dreams.
--
Bill Sloman, Nijmegen
bill....@ieee.org
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Fri, 20 Mar 2009 01:52:27 -0700 (PDT), bill.slo...@ieee.org wrote:
>
> >The inputs to an op amp are the bases of the two transistors forming
> >the long tailed pair.
>
> >For small voltage differences between the two inputs - less than 26mV
> >- the consequent difference between the two collector currents is
> >pretty close to a linear function of the voltage difference, and any
> >high frequency content in the input signal averages to zero
>
> No. Most opamp input diff pairs run at low current and the emitters
> (and current source) have significant capacitance hence long time
> constants. The transistor that gets high-freq RF on its base has, in
> effect, a capacitor to ground on its emitter. At hundreds of MHz, it's
> a nice diode peak rectifier.
Any current going through the base to emitter is base drive, and has
the usual effect on the collector current; the long tail drives the
(NPN) emitter up towards the positive rail, turning off the base
current - or at least bringing it back to what the op amp designer had
in mind.
> >Once the voltage difference gets much above 26mV, the transistor that
> >is biassed on starts getting most of the tail current, and NPN inputs
> >start following the upper envelope of any high frequency content in
> >the input signal.
>
> Where do you get the 26 mV thing from? There's serious rectification
> going on at much lower p-p swings.
Ebers and Moll. The "serious rectification" is your hypothesis, not an
established fact.
> Another real-life problem is that real systems, in boxes and on pc
> boards, tend to have ghastly high-Q resonances all over the spectrum,
> which magnify modest RFI into serious voltages; narrowband peaks like
> 50:1 are not unusual.
Funny how difficult it is to get these high-Q resonances when you
actually want them.
> >As Jeorg poits out, there is still some non-linearity below 26mV, so
> >you can still see some rectification of high frequency content.
>
> "Some" is enough to cause serious problems in many, many actual
> products. I can't use a lot of cheap electronics in my house because
> the ambient RF fields make problems. A naiive thermocouple signal
> conditioner design *will* get into trouble when people with cell
> phones stroll by.
Sure. I've seen it.
--
Bill Sloman, Nijmegen
Joerg
Can be bought at any serious electronics retailer such as Digikey. The
prevailing brand seems to be 3M which is also what I am using. Don't
store it in locations where it gets hot such as a garage in summer or
the adhesive backing will lose its stickiness.
Stained glass hobby stores should also have it but probably they'll
charge more.
James Arthur
Theory, plus of actual measurement of real circuits.
>> The b-e junction is forward-biased, ready, fast,
>> and willing.
>
> Pity it doesn't do what you think it does.
>
>> RFI doesn't flow between the inputs; that path doesn't matter.
>
> In your dreams.
Well, obviously differential RFI /could/ flow if your layout
permitted it. Don't do that.
But that's not the usual case, nor the one we were discussing,
nor does it produce the interference seen in real life,
for the very reasons you argue. So, that's not it.
Cheers,
James Arthur
Joerg
> On Mar 20, 5:08 pm, John Larkin
> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
[...]
>> Where do you get the 26 mV thing from? There's serious rectification
>> going on at much lower p-p swings.
>
> Ebers and Moll. The "serious rectification" is your hypothesis, not an
> established fact.
>
Sorry, but in my life it is an established fact, evidenced on lots of
scope plots. For many of my clients is also was. "Was" because we
established serious rectification as the root cause of numerous EMI
problems, fixed it, and they were able to forget about it and move on
with the projects.
Calculations on paper and SPICE, performance in the real world, two
different things.
My first encounter with this was at age 16 or so. Just had my ham radio
license and a high power endorsement, built a huge amp, and promptly a
neighbor called. I was surprised they let me (being a kid) open their
freaking expensive electric organ. The complaint was a distinct "pock
... pock ... po-po-pock ... pock" when the light in my window was on.
Morse code transients. Aha! Looked at the circuit board and almost
freaked out. This manufacturer had no clue about grounding strategies.
So I took a sawtooth pod to find the most sensitive areas. No chance to
place ferrites or shield anything, this circuit board was an octopus of
traces and wires sprouting to just about everywhere. So I placed small
capacitors over the BE contacts -> bingo, problem gone. Back then I
didn't know what on earth could cause that, now I know.
Bobo The Chimp
> Jim Thompson wrote:
>>
>> Slowman's ego, chutzpah and senility level are high enough that he
>> could easily replace Barney Frank as Chief US Fairy ;-)
>
Sounds like Jim's got a man-crush. ;-)
--
Cheers!
Rich^H^H^H^HBoBo
James Arthur
> bill....@ieee.org wrote:
He's heard it reported from three guys who actually design
electronics, has seen it modeled (he likes models), and he
still doesn't believe it.
He just wants to argue.
James Arthur
Joerg
Maybe we should re-topic the thread to "Hardening of the minds" now :-)
Tim Shoppa
> I've got a simple op-amp circuit that is misbehaving in an EMI
> environment. It looks like below (hopefully the ascii drawings work
> out)...
>
> ___10k_______
> | |
> GND--------|\ 20k
> | \____|___
> --100----------|+/
> |/
>
> The numbers are resistor values and the + input comes from a unity
> gain buffer.
>
> On first inspection my first obvious thought was to limit the
> bandwidth to avoid EMI issues (circuit only actually needs kHz'ish
> bandwidth) so I put a simple RC filter on the input as follows:
>
> ___10k_______
> | |
> GND--------|\ 20k
> | \____|___
> --100----------|+/
> | |/
> 1u =
> |
> GND
>
> But this still had EMI issues and was probably bad because hanging a 1
> uF cap off the input is a bad idea? Not that I have any fundamental
> basis for making that statement.
>
> Consulting an EMC design book which has been *extremely* helpful in
> the past (EMC for Product Designers by Tim Williams), one of the
> things it recommends is putting low value resistors at each of the
> inputs, right at the pins. It also indicates that the values should
> be no larger than a few hundred Ohms. ie: adding 200 Ohms to each pin
> of the circuit above:
>
> ___10k_______
> | |
> GND----200-|\ 20k
> | \____|___
> --100------200-|+/
> | |/
> 1u =
> |
> GND
>
> I'd like to understand what is going on here. My questions are:
>
> 1. Is hanging a large capacitance on the input of an op-amp a bad
> idea? If so, why? I saw some rumblings somewhere about CMRR issues,
> but don't get it.
>
> 2. With the op-amp having a high input impedance, what is the reason
> for limiting the resistors at the input pins to "up to a few hundred
> Ohms"?
>
> I'm trying to simulate the impact of it all in Switchercad, but I'm a
> hack and my attempts have not been that illuminating. I've spent some
> time trying to work out how to simulate and understand when an
> amplifier circuit is stable or not, but haven't been able to fully
> grasp phase margin and certainly not how to simulate it properly.
>
> Any help is much appreciated! I have an extreme dislike for doing
> anything because "that's just how you do it" and want to understand
> what is going on underneath.
I'm unable to really decode your ASCII schematic to see where the
feedback loop around the amp is, but... in an environment with
magnetic fields the area in the feedback loop (PCB trace wise)
determines how big of a signal is induced by the magnetic fields.
Many years ago when there were 60Hz filament transformers everywhere,
people learned real quick to make any loops be very low in cross
sectional area... twisted pairs and lead dressing. This is often
ignored by newbies today.
Tim.
Richard The Dreaded Libertarian
>
> Maybe we should re-topic the thread to "Hardening of the minds" now :-)
Wouldn't it be way cooler to start one on Liberating the mind? There's
certainly enough hardening of the minds to go around several times around,
I seem to notice. >:->
Cheers!
Rich
Rich Grise
>
> I'm unable to really decode your ASCII schematic to see where the
If it's important enough to you to bother, just set your newsreader for
reading in a fixed width font, like courier.
Hope This Helps!
Rich
krw
Anything like a Slow(man)blow(s) fuse? ;-)
Spehro Pefhany
Too bad they can't find a way to get that current noise down.
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
sp...@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Michael A. Terrell
Sloman couldn't blow a picofuse.
Jim Thompson
<notthis...@removethispacbell.net> wrote:
>bill....@ieee.org wrote:
>> On Mar 20, 5:08 pm, John Larkin
>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
>
>[...]
>
>
>>> Where do you get the 26 mV thing from? There's serious rectification
>>> going on at much lower p-p swings.
>>
>> Ebers and Moll. The "serious rectification" is your hypothesis, not an
>> established fact.
Slowman's ignorance is pheeeeeee-nominal ;-)
[snip]
Clifford Heath
> Another real-life problem is that real systems, in boxes and on pc
> boards, tend to have ghastly high-Q resonances all over the spectrum,
> which magnify modest RFI into serious voltages; narrowband peaks like
> 50:1 are not unusual.
I use a nice little foxhunt sniffer made by VK3YNG which is
remarkable for the incredible dynamic range in its front-end
(quite a sensitive lo-noise receiver, but you can still DF when
you're standing on 100W of RF transmitter, it doesn't swamp).
Anyhow, he found out the hard way that a commonly recommended
decoupling method, namely a small cap closely paralleled with
a larger one, often causes major problems when the small C
resonates with the inductance of the larger one.
... just one case in point.
If anyone has an idea how to build a front-end like these have now
that the dual-gate GaAs FET is unobtainium, I'd like to hear it.
Clifford Heath.
krw
True, he'd blow Barney Fag, though.
Eeyore
Spehro Pefhany wrote:
> On Fri, 20 Mar 2009 15:22:16 +0000, the renowned Eeyore
> <rabbitsfriend...@hotmail.com> wrote:
> >Joerg wrote:
> >
> >> The only BJT-input opamps I remember being quite EMI-proof were those in
> >> metal TO-cans. But that has kind of gone out of fashion ;-)
> >
> >I was astonished to see a relatively new National audio op-amp available in
> >metal can recently. You pay for it of course.
> >http://www.national.com/pf/LM/LM4562.html
> >
> >TI do something else that's gorgeous too ( 1.1 nV / sqrt Hz input noise
> >voltage ) but it's SMT only, so you can't even retrofit DIL packages for
> >upgrades. I'm surprised they did that. I suppose their marketing people
> >didn't think it worth their while.
>
> Too bad they can't find a way to get that current noise down.
That's the trouble with low input noise voltage, it usually comes with that
disadvantage.
Graham
JosephKK
Speaking for yourself i see.
.
bill....@ieee.org
Web-Site.com> wrote:
> On Fri, 20 Mar 2009 11:37:39 -0700, Joerg
>
> >bill.slo...@ieee.org wrote:
> >> On Mar 20, 5:08 pm, John Larkin
> >> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
>
> >[...]
>
> >>> Where do you get the 26 mV thing from? There's serious rectification
> >>> going on at much lower p-p swings.
>
> >> Ebers and Moll. The "serious rectification" is your hypothesis, not an
> >> established fact.
>
> Slowman's ignorance is pheeeeeee-nominal ;-)
>
> [snip]
Sure. I don't know a lot of things that Jim Thompson believes to be
true. The ignorance of the real world invoved is entirely Jim's.
--
Bill Sloman, Nijmegen
Phil Hobbs
> John Larkin wrote:
>> Joerg wrote:
>>
>>> John Larkin wrote:
>>>> Joerg wrote:
>>>>
>
>>>>> The only BJT-input opamps I remember being quite EMI-proof were
>>>>> those in metal TO-cans. But that has kind of gone out of fashion ;-)
>>>>>
>>>>> gear was my kilowatt-size RF amp, often without the lid and then a
>>>>> Pringles can size inductor would be looking at you. So I used uA709
>>>>> in TO cans since in our neck of the woods the uA741 only came in
>>>>> plastic dip. This was a few decades ago when copper foil was
>>>>> unobtanium.
>>>>
>>>> The DIY stained-glass-supplies shops have all sorts of cool stuff,
>>>> like solderable sticky-back copper foil of various widths, and some
>>>> amazingly klutzy soldering irons.
>>>>
>>> In Europe they were called Tiffany lamp shops. But the pricing in
>>> those places was far beyond reach for someone in high school. But
>>> grandpa gave me a big stash of copper and I thought I'd be in heaven.
>>> From then on I could make shielded enclosures without having to
>>> convince mom that we all "needed" a can of butter cookies. Because
>>> then she make fresh ones, no can.
>>
>>
>> James A is a big fan of Altoids cans.
>>
>> John
>>
>
> Sure. They tin easily and they're Curiously Strong(tm). Clabber
> Girl baking powder cans too.
>
> Cheers,
> James Arthur
Colman's mustard tins. I made an arc lamp out of one once.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net