Increasing noise performance by replacing mechanical pots

Adrian Nievergelt

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Apr 4, 2012, 10:58:10 PM4/4/12

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Hi

While designing a analog end user circuit which is noise critical, i
came across the problem that i need variable resistors in the signal
path of my op amps. Now wiring signal currents out onto a user interface
pot is somehow just not feeling right. I was thinking of ofsetting the
pot, by designing in a jfet based variable resistor (
http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=00335682 for those
with access to IEEE ). Which is basically linearizing a jfet with an
opamp and use that as resistor. This part of the circuit could then be
addressed with a quasi DC voltage from aforementioned pot. I have two
worries:
1. That the jfet in combination with the op amp will in the end
introduce more noise into the system than wiring out the pot would bring.
2. That the jfet would have the signal thermally drifting, even with the
linearizing op-amp in place.

Does anyone have experiences or thoughts to this, or maybe just simply a
much better solution (no digital electronics please)?

Sincerely
Adrian

Phil Allison

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Apr 4, 2012, 11:06:49 PM4/4/12

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"Adrian Nievergelt"

>
> While designing a analog end user circuit which is noise critical, i came
> across the problem that i need variable resistors in the signal path of my
> op amps. Now wiring signal currents out onto a user interface pot is
> somehow just not feeling right.

** Really ?

That is more like voodoo science not engineering.

You MUST explain what the app is and not just assume that others accept
your un-stated and probably false assumptions.

Kindly get real.

.... Phil

Adrian Nievergelt

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Apr 4, 2012, 11:34:48 PM4/4/12

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Sorry, didn't want to confuse people with my brabbling. So the
application is essentially a q-control circuit for dynamic atomic force
microscopy. It takes an oscillation signal and feeds back a delayed
version into the oscillator, thus adjusting the damping by actively
driving or actively damping. In electronics that means taking the input
signal, using a manually adjustable phase shifter (first pot) to shift
the phase, then running it through a variable gain stage (second pot),
and finally adding the result to the original input and outputting that.
The typical signals are in the ~100mV range at frequencies of 100kHz to
1MHz. I hope this helps as to what I am trying to do, please say if it
does not.

Sincerely
Adrian

Phil Allison

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Apr 4, 2012, 11:51:12 PM4/4/12

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"Adrian Nievergelt"

Phil Allison wrote:
>> "Adrian Nievergelt"
>>
>>>
>>> While designing a analog end user circuit which is noise critical, i
>>> came
>>> across the problem that i need variable resistors in the signal path of
>>> my
>>> op amps. Now wiring signal currents out onto a user interface pot is
>>> somehow just not feeling right.
>>
>>
>> ** Really ?
>>
>> That is more like voodoo science not engineering.
>>
>> You MUST explain what the app is and not just assume that others accept
>> your un-stated and probably false assumptions.
>>
>> Kindly get real.
>>
>

> Sorry, didn't want to confuse people with my brabbling.

** So stop babbling and be precise.

> In electronics that means taking the input signal, using a manually
> adjustable phase shifter (first pot) to shift the phase, then running it
> through a variable gain stage (second pot), and finally adding the result
> to the original input and outputting that. The typical signals are in the
> ~100mV range at frequencies of 100kHz to 1MHz.

** So why do you think using good quality pots would cause excessive " noise
" ?

Any analogue alternative will have serious temperature drift problems and
/or non linearity.

I suspect that fine adjustment, resettability and good temp stability are
important in your device - so consider the use of multi-turn wire wound
types.

.... Phil

miso

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Apr 5, 2012, 1:28:07 AM4/5/12

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> Sorry, didn't want to confuse people with my brabbling. So the
> application is essentially a q-control circuit for dynamic atomic force
> microscopy. It takes an oscillation signal and feeds back a delayed
> version into the oscillator, thus adjusting the damping by actively
> driving or actively damping. In electronics that means taking the input
> signal, using a manually adjustable phase shifter (first pot) to shift
> the phase, then running it through a variable gain stage (second pot),
> and finally adding the result to the original input and outputting that.
> The typical signals are in the ~100mV range at frequencies of 100kHz to
> 1MHz. I hope this helps as to what I am trying to do, please say if it
> does not.
>
> Sincerely
> Adrian

Does this need to be infinite resolution? If not, perhaps you can use a
MDAC. Sadly, the MDAC is probably cheaper than a good 10 turn pot. [I've
got a nice selection of 10 turn pots with dials, pulled from old test
instruments and scrapped biomedical gear. I started that treasure hunt
when it occurred to me that multiturn pots were on the endangered
species list, at least if you want them on the cheap.]

I don't have IEEE online access, but jfets as resistors are kind of
temperature sensitive. I don't think I've seen them used in circuits
without feedback control.

Tim Wescott

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Apr 5, 2012, 1:40:22 AM4/5/12

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Noise how? Are you worried about picking up EMI? Are you worried that
the pots themselves will generate thermal noise? What?

If it's thermal or excess noise in the pots, choose a pot material that's
not subject to that problem. If it's EMI, consider shielding the leads
going to and from the pot -- if you're really concerned, build a little
housing for the back of each pot, that surrounds it and makes a good
solid connection to the shielding on the wire. Better yet, just use a
good enclosure.

Or, try reposting without any restrictions against digital electronics,
and see what answers you get.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

Dennis

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Apr 5, 2012, 2:13:17 AM4/5/12

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"Tim Wescott" <t...@seemywebsite.com> wrote in message
news:v_idnWOIXvfbreDS...@web-ster.com...

......or mount the pots on the board and mount the board to the rear of the
user panel, eliminating wiring to/from the pots.

Adrian Nievergelt

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Apr 5, 2012, 3:27:11 AM4/5/12

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On 04.04.2012 22:40, Tim Wescott wrote:
> On Wed, 04 Apr 2012 19:58:10 -0700, Adrian Nievergelt wrote:
>
>> Hi
>>
>> While designing a analog end user circuit which is noise critical, i
>> came across the problem that i need variable resistors in the signal
>> path of my op amps. Now wiring signal currents out onto a user interface
>> pot is somehow just not feeling right. I was thinking of ofsetting the
>> pot, by designing in a jfet based variable resistor (
>> http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=00335682 for those
>> with access to IEEE ). Which is basically linearizing a jfet with an
>> opamp and use that as resistor. This part of the circuit could then be
>> addressed with a quasi DC voltage from aforementioned pot. I have two
>> worries:
>> 1. That the jfet in combination with the op amp will in the end
>> introduce more noise into the system than wiring out the pot would
>> bring. 2. That the jfet would have the signal thermally drifting, even
>> with the linearizing op-amp in place.
>>
>> Does anyone have experiences or thoughts to this, or maybe just simply a
>> much better solution (no digital electronics please)?
>
> Noise how? Are you worried about picking up EMI? Are you worried that
> the pots themselves will generate thermal noise? What?

I don't think the pots will generate much thermal noise, but they might
pick up EMI. At the back of my head is the design paradigma that you
should make signal paths between op-amps as short as possible. cabling
out to a pot seems somehow wrong. I'm not sure if I don't worry about
something that is not a problem. Apart from that the wiring and the
larger pot are quite a parasitic inductance/capacitance and might cause
problems and oscillations with a 1MHz signal. again I might completely
misjudge the problem.

>
> If it's thermal or excess noise in the pots, choose a pot material that's
> not subject to that problem. If it's EMI, consider shielding the leads
> going to and from the pot -- if you're really concerned, build a little
> housing for the back of each pot, that surrounds it and makes a good
> solid connection to the shielding on the wire. Better yet, just use a
> good enclosure.

With the answers I have been getting it will probably turn out like this
yes, shielded cables and housing.

>
> Or, try reposting without any restrictions against digital electronics,
> and see what answers you get.

I might use a combination of a digital pot driven by an analog signal. I
was trying to avoid this route as I'm a bit limited on board space and
this adds quite a bit of complexity.

I was mostly curious if that other thing might work, and as I said if
some people have maybe tried it. I saw floating resistors with OTA's,
but from what i could judge those were also massively thermally not stable.

Thanks
Adrian

John Devereux

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Apr 5, 2012, 3:44:00 AM4/5/12

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Or use extension rods like the Tek scopes?

[...]

--

John Devereux

Nico Coesel

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Apr 5, 2012, 5:36:19 AM4/5/12

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Pots go bad at one point. Back when I designed audio and telecom gear
I always used a VGA (voltage controlled gain amplifier) to control
signal levels. The pot supplied the controlling voltage (with some
filtering). There are many VGA chips available with analog and digital
control interfaces.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

Dennis

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Apr 5, 2012, 7:37:02 AM4/5/12

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"John Devereux" <jo...@devereux.me.uk> wrote in message
news:87bon66...@devereux.me.uk...

Haha - I nearly added that! My first job was reparing TEQ for a telecoms
co.

Phil Allison

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Apr 5, 2012, 7:44:54 AM4/5/12

to

"Adrian Nievergelt"

>
>> Noise how? Are you worried about picking up EMI? Are you worried that
>> the pots themselves will generate thermal noise? What?
>
> I don't think the pots will generate much thermal noise, but they might
> pick up EMI.

** My god you are soooooo stupid.

> At the back of my head is the design paradigma that you should make signal
> paths between op-amps as short as possible. cabling out to a pot seems
> somehow wrong.

** First time you mentioned that fake issue.

Like to play your cards real close to the chest - right ?

> I'm not sure if I don't worry about something that is not a problem.

** My god - a triple negative !!!!!!!!!!!!!!!!

Lawyers will be having orgasisms ...........

> I might use a combination of a digital pot driven by an analog signal.

** The reverse idea makes FAR more sense.

Particularly with multi-turn WW pots.

> I was mostly curious if that other thing might work, and as I said if some
> people have maybe tried it. I saw floating resistors with OTA's, but from
> what i could judge those were also massively thermally not stable.

** With floating resistors

- you always gotta watch out for the Plimsoll line ...

.... Phil

George Herold

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Apr 5, 2012, 9:23:01 AM4/5/12

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> Adrian- Hide quoted text -
>
> - Show quoted text -

Crank up the (fixed) gain as much as possible in first few gain
stages. Then send this signal off for signal processing. (digital or
analog) Then if there is, for some reason, too much gain in the front
end you can attenuate it to the desired level. But do all the signal
processing at a high level. Then you don't care (too much) about
noise and interference.

George H.

George Herold

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Apr 5, 2012, 9:36:31 AM4/5/12

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On Apr 5, 3:44 am, John Devereux <j...@devereux.me.uk> wrote:

> "Dennis" <n...@name.com> writes:
> > "Tim Wescott" <t...@seemywebsite.com> wrote in message
> >news:v_idnWOIXvfbreDS...@web-ster.com...
> >> On Wed, 04 Apr 2012 19:58:10 -0700, Adrian Nievergelt wrote:
>
> >>> Hi
>
> >>> While designing a analog end user circuit which is noise critical, i
> >>> came across the problem that i need variable resistors in the signal
> >>> path of my op amps. Now wiring signal currents out onto a user interface
> >>> pot is somehow just not feeling right. I was thinking of ofsetting the
> >>> pot, by designing in a jfet based variable resistor (

> >>>http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=00335682for those

> >>> with access to IEEE ). Which is basically linearizing a jfet with an
> >>> opamp and use that as resistor. This part of the circuit could then be
> >>> addressed with a quasi DC voltage from aforementioned pot. I have two
> >>> worries:
> >>> 1. That the jfet in combination with the op amp will in the end
> >>> introduce more noise into the system than wiring out the pot would
> >>> bring. 2. That the jfet would have the signal thermally drifting, even
> >>> with the linearizing op-amp in place.
>
> >>> Does anyone have experiences or thoughts to this, or maybe just simply a
> >>> much better solution (no digital electronics please)?
>
> >> Noise how? Are you worried about picking up EMI? Are you worried that
> >> the pots themselves will generate thermal noise? What?
>
> >> If it's thermal or excess noise in the pots, choose a pot material that's
> >> not subject to that problem. If it's EMI, consider shielding the leads
> >> going to and from the pot -- if you're really concerned, build a little
> >> housing for the back of each pot, that surrounds it and makes a good
> >> solid connection to the shielding on the wire. Better yet, just use a
> >> good enclosure.
>
> > ......or mount the pots on the board and mount the board to the rear of the
> > user panel, eliminating wiring to/from the pots.
>
> Or use extension rods like the Tek scopes?

The one problem I've had with switches to low level signals is that
the metal shaft can* act like a little antenna and couple crud in the
room down into the signal.

so non-conductive extension rods.

George H.

*most of the time the metal shaft is grounded to the surrounding
bushing and then front panel... but if you jiggled it just right...

>
> [...]
>
> --
>
> John Devereux- Hide quoted text -

Phil Hobbs

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Apr 5, 2012, 11:25:14 AM4/5/12

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So you're mostly concerned about EMI and cable capacitance running a
cable out to a panel pot, is that right?

The usual way to fix that in your frequency range is an optical encoder,
a small microcontroller, and a low-value digital pot. High value dpots
are slow on account of the switch capacitance--check out the AD8403,
which gives you 256 taps down to 2k resistance, and works out to a few
megahertz. You can combine three dpots in a pseudo-Kelvin-Varley
divider to get smaller steps if you need to--connect two pots in
parallel, and run the third off the two wipers. Run the first stage
pots a couple of codes apart to reduce the effect of differential
nonlinearity, and you can get ~15 bit resolution. You don't get
anything like 15-bit accuracy, but at least it'll be monotonic, and for
a manual adjustment that's often all you need.

The JFET trick is also okay but becomes nonlinear fairly rapidly at
higher amplitude, and has a not-too-nice temperature coefficient. (The
trick of feeding back 50% of V_DS to the gate helps but is far from
perfect.)

Another approach would be to use a pair of varactors as a variable
voltage divider. If you use a pair of duals, you can get rid of the
nonlinearity to first order. For a purely analogue method, that would
probably get my vote.

The old-school approach would be to mount the pot somewhere in the
middle of the board, wherever it needs to be, and run a shaft extension
to a panel bearing. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

Jim Thompson

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Apr 5, 2012, 11:45:18 AM4/5/12

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On Thu, 05 Apr 2012 09:36:19 GMT, ni...@puntnl.niks (Nico Coesel)
wrote:

>Adrian Nievergelt <niev...@gmx.ch> wrote:
>
>>Hi
>>
>>While designing a analog end user circuit which is noise critical, i
>>came across the problem that i need variable resistors in the signal
>>path of my op amps. Now wiring signal currents out onto a user interface
>>pot is somehow just not feeling right. I was thinking of ofsetting the
>>pot, by designing in a jfet based variable resistor (
>>http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=00335682 for those
>>with access to IEEE ). Which is basically linearizing a jfet with an
>>opamp and use that as resistor. This part of the circuit could then be
>>addressed with a quasi DC voltage from aforementioned pot. I have two
>>worries:
>>1. That the jfet in combination with the op amp will in the end
>>introduce more noise into the system than wiring out the pot would bring.
>>2. That the jfet would have the signal thermally drifting, even with the
>>linearizing op-amp in place.
>>
>>Does anyone have experiences or thoughts to this, or maybe just simply a
>>much better solution (no digital electronics please)?
>
>Pots go bad at one point. Back when I designed audio and telecom gear
>I always used a VGA (voltage controlled gain amplifier) to control
>signal levels. The pot supplied the controlling voltage (with some
>filtering). There are many VGA chips available with analog and digital
>control interfaces.

Tim Wescott

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Apr 5, 2012, 12:20:50 PM4/5/12

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People really should understand the "whys" -- then the "wherefores" just
make sense, instead of being dimly remembered rote learning.

As long as your op-amp can drive the cable, and as long as you're not
going to excessively attenuate or phase shift the signal over your
frequency range of interest, putting long lengths of cable with pots at
the ends _between_ op amps isn't going to cause problems.

Putting long cables to pots in the _feedback_ path of an op-amp _could_
cause problems, because of cable capacitance and (if you really screw up)
coupling to other circuits. So your challenge is to not do that -- make
sure that your circuits will work well with low-impedance outputs
launching the signal onto the cable, a pot at the end to attenuate it,
and a return. Then you'll be fine.

<balance snipped>

Jan Panteltje

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Apr 5, 2012, 1:02:09 PM4/5/12

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On a sunny day (Thu, 05 Apr 2012 08:45:18 -0700) it happened Jim Thompson
<To-Email-Use-Th...@On-My-Web-Site.com> wrote in
<lefrn7h5sene9frhv...@4ax.com>:

>On Thu, 05 Apr 2012 09:36:19 GMT, ni...@puntnl.niks (Nico Coesel)
>wrote:
>
>>Adrian Nievergelt <niev...@gmx.ch> wrote:
>>
>>>Hi
>>>
>>>While designing a analog end user circuit which is noise critical, i
>>>came across the problem that i need variable resistors in the signal
>>>path of my op amps. Now wiring signal currents out onto a user interface
>>>pot is somehow just not feeling right. I was thinking of ofsetting the
>>>pot, by designing in a jfet based variable resistor (

>My little JVC FS-6000 "hi-fi" in my office has a motor-driven volume
>control pot (if operated via the remote).

That must be very very old.

Philips used to make a whole range of analog audio ICs
where they had a DC control voltage for gain, bass and treble too IIRC.
So if your 'signal' is in the audio range....
Do not remember any part numbers though.
But I likely still have some in the attic.

Mr.CRC

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Apr 5, 2012, 1:48:36 PM4/5/12

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It could cause problems if he's talking about putting pots at the end of
a long cable on the other side of the room vs. inside the Faraday shield
of a chassis. This probably isn't the case, but it's not entirely
certain from the information provided.

I'd just add that "it depends" if the cabling to the pots is going to a
high EMI area, or remaining within an EMI controlled area such as inside
the chassis. Even then is it going past some SMPS, deflection coils, or
other noise radiating parts, etc.?

> Putting long cables to pots in the _feedback_ path of an op-amp _could_
> cause problems, because of cable capacitance and (if you really screw up)
> coupling to other circuits. So your challenge is to not do that -- make
> sure that your circuits will work well with low-impedance outputs
> launching the signal onto the cable, a pot at the end to attenuate it,
> and a return. Then you'll be fine.

That is a very useful addition to the discussion also. For someone
learning the ropes, they should gain understanding of why a feedback
path is a different animal than the path between independent stages.

Also consider that the scope makers frequently used a mechanical
extension from the chassis exterior to a pot mounted on a board inside.
This solves various sorts of problems such as a pot at a high potential
offset from ground, keeping signal paths short for a low level signal
within/between stages, etc.

I don't worry too much about adding noise with a pot from the wiper
itself unless there is DC current superimposed on the low level signal.
If that is the case, then I figure out a different approach.

--
_____________________
Mr.CRC
crobc...@REMOVETHISsbcglobal.net
SuSE 10.3 Linux 2.6.22.17

miso

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Apr 5, 2012, 2:00:41 PM4/5/12

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> http://www.thatcorp.com/THAT_IC_Selection_Guide.shtml#VCAs

Bipolar VCA off the shelf. You can also roll your own with some TI chips
and a bit of glue.

Nico Coesel

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Apr 5, 2012, 2:05:35 PM4/5/12

to

Analog devices has several up to many MHz. For audio range I'd look
into NXP.

Adrian Nievergelt

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Apr 5, 2012, 2:18:21 PM4/5/12

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On 05.04.2012 11:05, Nico Coesel wrote:
> miso<mi...@sushi.com> wrote:
>
>>
>>> http://www.thatcorp.com/THAT_IC_Selection_Guide.shtml#VCAs
>>
>> Bipolar VCA off the shelf. You can also roll your own with some TI chips
>> and a bit of glue.
>
> Analog devices has several up to many MHz. For audio range I'd look
> into NXP.
>

I really like the idea of using VCAs, feel kinda stupid that i didn't
get there myself, thanks. I'll probably use one of those. Thanks a lot
to Nico for that suggestion!

Adrian Nievergelt

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Apr 5, 2012, 2:23:24 PM4/5/12

to

What's your problem? Ever considered that not everyone has english as
first language? I ask here because I don't know better, not to show off.

Adrian

George Herold

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Apr 5, 2012, 4:04:19 PM4/5/12

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Wow, I never heard of them. They sell matched transistors too! Are
they on the same die?

Thanks

George H.

Jim Thompson

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Apr 5, 2012, 4:29:55 PM4/5/12

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Yep, same die. I did some VCA designs for a start-up in San Jose in
the '70's that ultimately joined up with THAT. I need to make a trip
to my off-site archive facility to remember the company name :-)

Phil Hobbs

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Apr 5, 2012, 5:03:24 PM4/5/12

to

Yes. They aren't as good as the MAT14, but they're better than the CA3046.

Jim Thompson

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Apr 5, 2012, 5:14:21 PM4/5/12

to

On Thu, 05 Apr 2012 17:03:24 -0400, Phil Hobbs
<pcdhSpamM...@electrooptical.net> wrote:

>On 04/05/2012 04:04 PM, George Herold wrote:
>> On Apr 5, 2:00 pm, miso<m...@sushi.com> wrote:
>>>> http://www.thatcorp.com/THAT_IC_Selection_Guide.shtml#VCAs
>>>
>>> Bipolar VCA off the shelf. You can also roll your own with some TI chips
>>> and a bit of glue.
>>
>> Wow, I never heard of them. They sell matched transistors too! Are
>> they on the same die?
>>
>> Thanks
>>
>> George H.
>>
>
>Yes. They aren't as good as the MAT14,

Nor as expensive.

>but they're better than the CA3046.

For matching? I don't know. Those large geometry devices match
pretty good. (Although I'll have to look... the CA3046 may have been
laid out for tightness rather than match.)

>
>Cheers
>
>Phil Hobbs

Phil Allison

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Apr 5, 2012, 7:44:52 PM4/5/12

to

"Phil Hobbs"

> The old-school approach would be to mount the pot somewhere in the
> middle of the board, wherever it needs to be, and run a shaft extension
> to a panel bearing.

** Give that man a Kewpie doll ......

.... Phil

Phil Hobbs

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Apr 5, 2012, 7:45:56 PM4/5/12

to

Jim Thompson wrote:
>
> On Thu, 05 Apr 2012 17:03:24 -0400, Phil Hobbs
> <pcdhSpamM...@electrooptical.net> wrote:
>
> >On 04/05/2012 04:04 PM, George Herold wrote:
> >> On Apr 5, 2:00 pm, miso<m...@sushi.com> wrote:
> >>>> http://www.thatcorp.com/THAT_IC_Selection_Guide.shtml#VCAs
> >>>
> >>> Bipolar VCA off the shelf. You can also roll your own with some TI chips
> >>> and a bit of glue.
> >>
> >> Wow, I never heard of them. They sell matched transistors too! Are
> >> they on the same die?
> >>
> >> Thanks
> >>
> >> George H.
> >>
> >
> >Yes. They aren't as good as the MAT14,
>
> Nor as expensive.
>
> >but they're better than the CA3046.
>
> For matching? I don't know. Those large geometry devices match
> pretty good. (Although I'll have to look... the CA3046 may have been
> laid out for tightness rather than match.)
>
> >
> >Cheers
> >
> >Phil Hobbs
>

MAT14s are only a few bucks--when they reintroduced the MAT04 and
renamed it, they also dumped a whole lot of the production tests that
made it so expensive. They *gutted* the datasheet, anyway.

The THAT devices seem to have a large Rbb' and relatively poor beta
linearity compared with the ADI ones. They don't make nearly as good
laser noise cancellers, anyway, as we both have some reason to remember.

The CA3046 devices have low beta and poor beta linearity, and low beta
at low current usually means a lot of 1/f noise.

Phil Allison

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Apr 5, 2012, 7:46:45 PM4/5/12

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"Adrian Nievergelt"

> What's your problem? Ever considered that not everyone has english as
> first language? I ask here because I don't know better, not to show off.

** Your problem is not merely with English - pal.

It is a LOT worse than that.

.... Phil

Mr.CRC

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Apr 5, 2012, 10:22:19 PM4/5/12

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Jim Thompson wrote:
> On Thu, 5 Apr 2012 13:04:19 -0700 (PDT), George Herold
> <ghe...@teachspin.com> wrote:
>
>> On Apr 5, 2:00 pm, miso <m...@sushi.com> wrote:
>>>> http://www.thatcorp.com/THAT_IC_Selection_Guide.shtml#VCAs
>>> Bipolar VCA off the shelf. You can also roll your own with some TI chips
>>> and a bit of glue.
>> Wow, I never heard of them. They sell matched transistors too! Are
>> they on the same die?
>>
>> Thanks
>>
>> George H.
>
> Yep, same die. I did some VCA designs for a start-up in San Jose in
> the '70's that ultimately joined up with THAT. I need to make a trip
> to my off-site archive facility to remember the company name :-)
>
> ...Jim Thompson

Did you happen to design their differential receivers with the nifty
tricks for getting low CMRR out to high audio frequencies?

Those things are gorgeous.

miso

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Apr 5, 2012, 11:52:47 PM4/5/12

to

> What's your problem? Ever considered that not everyone has english as
> first language? I ask here because I don't know better, not to show off.
>
> Adrian
>

His problem is he is a bodily orifice.

miso

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Apr 6, 2012, 12:01:13 AM4/6/12

to

TI has ap notes on how to roll your own with just a bit of glue. I guess
it depends on your budget. My recollection is the ADI parts are full
blown Gilbert multipliers, while TI app notes do the VCA with OTAs. Best
bet is to check both websites. Obviously you don't need four quadrants.

miso

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Apr 6, 2012, 12:06:14 AM4/6/12

to

Formerly Elantec, but that is Milipitas. Probably not your design.

SSM was the other company. God was their stuff shit cubed. However, PMI
bought them. Those were Derek Bowers designs. Eh, I said too much already.

Gib Bogle

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Apr 6, 2012, 4:37:59 AM4/6/12

to

Glue is for sniffing, not for smoking.

George Herold

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Apr 6, 2012, 10:44:36 AM4/6/12

to

On Apr 5, 7:45 pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net>
wrote:

> hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text -

>
> - Show quoted text -

Hi Phil, which are the cheaper ones mat14 or mat04's?

In the hundreds of pieces range I see mat14's for $7-$10 each.

The mat04's seeem even more outrageous.
Except for 20 pieces at Quest which I can get for ~$6 each.

George H.

Phil Hobbs

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Apr 6, 2012, 11:16:48 AM4/6/12

to

> Hi Phil, which are the cheaper ones mat14 or mat04's?
>
> In the hundreds of pieces range I see mat14's for $7-$10 each.
>
> The mat04's seeem even more outrageous.
> Except for 20 pieces at Quest which I can get for ~$6 each.
>
> George H.

They discontinued the MAT04s for a year, then brought them back in a
de-spec'd version as the MAT14. I'm pretty sure they're the same part,
but I haven't done an A-B test to make sure. (Some enthusiast might
have thrown away the masks when the part was discontinued.)

miso

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Apr 6, 2012, 6:32:37 PM4/6/12

to

Tooling gets scrapped periodically. Stuff happens. It isn't unheard of
that (speaking in a Mafioso voice) "something unfortunate" happens to a
mask, allowing a different project to take its slot in the queue,
enabling bonus money or some other goodness at the expense of screwing
the company and another engineering team.

Note the "THAT corporation" parts are on DI, while the ADI parts are
bulk. DI should be cheap these days, but my reality distortion field is
not as good as the late Steve Jobs.

Nico Coesel

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Apr 7, 2012, 7:08:45 AM4/7/12

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miso <mi...@sushi.com> wrote:

>On 4/5/2012 11:18 AM, Adrian Nievergelt wrote:
>> On 05.04.2012 11:05, Nico Coesel wrote:
>>> miso<mi...@sushi.com> wrote:
>>>
>>>>
>>>>> http://www.thatcorp.com/THAT_IC_Selection_Guide.shtml#VCAs
>>>>
>>>> Bipolar VCA off the shelf. You can also roll your own with some TI chips
>>>> and a bit of glue.
>>>
>>> Analog devices has several up to many MHz. For audio range I'd look
>>> into NXP.
>>>
>>
>>
>>
>> I really like the idea of using VCAs, feel kinda stupid that i didn't
>> get there myself, thanks. I'll probably use one of those. Thanks a lot
>> to Nico for that suggestion!
>>
>TI has ap notes on how to roll your own with just a bit of glue. I guess
>it depends on your budget. My recollection is the ADI parts are full

Engineering or production costs? An appnote is exactly what it is: an
appnote, not a production ready design!

Robert Baer

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Apr 8, 2012, 12:45:38 AM4/8/12

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Adrian Nievergelt wrote:
> Hi
>
> While designing a analog end user circuit which is noise critical, i
> came across the problem that i need variable resistors in the signal
> path of my op amps. Now wiring signal currents out onto a user interface
> pot is somehow just not feeling right. I was thinking of ofsetting the
> pot, by designing in a jfet based variable resistor (
> http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=00335682 for those
> with access to IEEE ). Which is basically linearizing a jfet with an
> opamp and use that as resistor. This part of the circuit could then be
> addressed with a quasi DC voltage from aforementioned pot. I have two
> worries:
> 1. That the jfet in combination with the op amp will in the end
> introduce more noise into the system than wiring out the pot would bring.
> 2. That the jfet would have the signal thermally drifting, even with the
> linearizing op-amp in place.
>
> Does anyone have experiences or thoughts to this, or maybe just simply a
> much better solution (no digital electronics please)?
>

> Sincerely
> Adrian
>
Possible solution: a digital pot - which can add "zipper" noise and
spikes when being changed; else that gives reasonable isolation.