Isolated current loop receiver

[Tim Wescott]

Task: Make a 4-20mA current loop receiver that vampires onto an existing
loop, and delivers a clean signal (either voltage or current) referenced
to my local ground, so that I can stuff it into an ADC. Disturbance to
the existing loop should be minimal -- ideally it would present no load
at all to the loop, but a small resistance (less is better, with 100 ohms
being about as high as I'd like to go) is acceptable. DC bias and large-
signal distortion up to 1% of full range is acceptable, gain errors are
acceptable, but AC performance around any given operating point should be
12-bit-ish sort of good to a 100Hz bandwidth.

External power is available.

You'd think this would be a subject for which dozens or even hundreds of
app notes exist all over the web -- but I only found one, for a Vishay
optoisolator device.

Anyone have any suggested circuits?

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

[John Larkin]

There are lots of commercial loop isolator bricks around, if you only need a
few.

Avago and Analog Devices have isolated instrumentation amp chips. Analog has
some isolated ADCs.

If you don't need full galvanic isolation, you could consider a shunt resistor
and an AD8216.

Flying capacitor isolators are always fun.


--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

[Syd Rumpo]

See the AAV003-10E in http://www.nve.com/analogSensors.php which puts
about 0.3 ohms in the loop. It's isolated and easy to interface to if
it's linear enough for you.

Cheers
--
Syd

[Spehro Pefhany]

On Wed, 12 Jun 2013 10:47:33 -0500, Tim Wescott
<t...@seemywebsite.please> wrote:

So you just need an isolation amplifier or does it have to be powered
from the loop?

If you need a loop powered supply it can be simply an isolated DC-DC
with a clamp across the input (pick a voltage high enough that your
entire circuit including losses will never draw more than, say, 3.5mA
+ perhaps a bit proportional to the loop current. If your circuit
draws (say) 1mA at 3.3V, then a very low burden should be possible. Or
just use an isolated DC-DC converter to get power across the barrier.

Then, in either case, you need a small value precision resistor to
sense the current and an isolation amplifier. The span is 16mA, so for
12-bit-ish accuracy you'll want a few uA stability, so something like
25R or 50R should be fine with a decent amplifier.

Since this stuff tends to involve less in the way of expensive chips
and more in the way of custom magnetics you'll not find much public
domain info on it.

Many, many years ago I used a whack of AD202/204 for a quickie job
similar to this application (handy because they don't require a power
supply on the isolated side). They run about $50 each in small
quantities these days.

BTW, typically they use something like 24V supplies, so there is about
8V to play with in a typical system.

Best regards,
--sp

[Joerg]

Tim Wescott wrote:
> Task: Make a 4-20mA current loop receiver that vampires onto an existing
> loop, and delivers a clean signal (either voltage or current) referenced
> to my local ground, so that I can stuff it into an ADC. Disturbance to
> the existing loop should be minimal -- ideally it would present no load
> at all to the loop, but a small resistance (less is better, with 100 ohms
> being about as high as I'd like to go) is acceptable. DC bias and large-
> signal distortion up to 1% of full range is acceptable, gain errors are
> acceptable, but AC performance around any given operating point should be
> 12-bit-ish sort of good to a 100Hz bandwidth.
>
> External power is available.
>

Now that's pure luxury. Why don't I ever get that? :-)

Anyhow, if you have external power couldn't you just use a hall sensor?
That should pose the least distortion to the loop.

> You'd think this would be a subject for which dozens or even hundreds of
> app notes exist all over the web -- but I only found one, for a Vishay
> optoisolator device.
>
> Anyone have any suggested circuits?
>

When it had to be isolated I (so far) always had to roll my own circuits.

--
Regards, Joerg

http://www.analogconsultants.com/

[Fred Bartoli]

Something like that should do:


IL300/PD1
+------+--------------.
| | | |
| - | |
| ^ ,-----||---. |
| | | | | | /
| | | |\| | | /
| +--+-|-\ | IL300/LED /
| | | >-----+ | /
+----------|+/ | | /
| | |/| <- V -> /+\ /
.-. .-. | - ( ) |
| | | | | | \-/ | VCC
50R | | | |10K | .-. | | +
'-' '-' | | | | | |
| | | | | | | |
| | | '-' | | - IL300/PD2
| | | | | | ^
+-----+------+------+-------' | |
| / +----->
| / |
| / .-.
| / | | R1
| / | | 10K trim
| / '-'
|
|
===
GND


Probably adapt the 10K value that I randomly picked up out of my junk box.


--
Thanks,
Fred.

[Klaus Kragelund]

Look up current sourced DC/DC converter

Switch a 1:1 transformer at 50% (or very close to 50%) with a couple of MOSFETs push pull or any way you like in series with the loop. Use the external power to drive the power stage gates.

It really cannot be much simpler than that since you don't have to worry about maximum drop voltage

If you don't like external power you can use a royer converter topology.

Regards

Klaus

[Klaus Kragelund]

The last page of this patent:

http://www.freepatentsonline.com/EP2002413.html

But, you are not allowed to use it I guess ;-)

Cheers

Klaus

[Jamie]

http://www.allegromicro.com/Products/Current-Sensor-ICs/Zero-To-Fifty-Amp-Integrated-Conductor-Sensor-ICs/ACS712.aspx

WIth this one, you wouldn't need to worry about unexpected current surges.

Using isolated hall sensors seem to work nicely.

I've not used this particular one but I have used one that didn't have
a filter pin. I did find it not to be precisely linear but that was
corrected down stream.


We do use a lot of isolators that uses isolated DC-DC converter to
power the loop side sensing circuit. Those normally use a PWM or
dual detector in the optical. The PWM tend to be slow, not something
you want for high speed pulse conveyance.


Jamie

[Joerg]

That depends. It looks like a fairly simple chopping scheme that was
already quite common for ECG signal transfer back in the 80's, when I
started into the industry. When you can prove prior art or obviousness
it is feasible to shoot down patent claims. Either via a gentlemen's
agreement with the patent assignee or, if it has to be done, in court.

[George Herold]

On Jun 12, 5:45 pm, Fred Bartoli <mynamewithAdotinbetw...@free.fr>
wrote:

> Fred.- Hide quoted text -
>
> - Show quoted text -

Neat. I never heard of the IL300.
I was wondering if you could just stick the LED half of an opto-
coupler in the current loop and measure the photo current on the other
side.
LED's are sorta linear above ~0.1 mA.. but by 20 mA, I'm not sure.

George H.

[Jamie]

We use dual detector opto's, they're ok but they still need circuit and
power on both sides. If that don't bother you, then you're all set :)

P.S.
We have some high speed analog isolators that use that method. It's
actually cheaper to build with that with a couple of op-amps than it is
using an isolated op-amp ;)

Jamie

[Martin Riddle]

the IL300 has pretty good linearity, not perfect (0.5%) but might work
in this application. But I think the feedback led need to be utilized
for propper linearity vs temperature.

Cheers


cHEERS

[John Devereux]

Martin Riddle <marti...@verizon.net> writes:

> On Wed, 12 Jun 2013 18:10:36 -0700 (PDT), George Herold
> <ghe...@teachspin.com> wrote:
>
>>On Jun 12, 5:45 pm, Fred Bartoli <mynamewithAdotinbetw...@free.fr>
>>wrote:

[...]

>>Neat. I never heard of the IL300.
>>I was wondering if you could just stick the LED half of an opto-
>>coupler in the current loop and measure the photo current on the other
>>side.
>>LED's are sorta linear above ~0.1 mA.. but by 20 mA, I'm not sure.
>>
>>George H.
>
> the IL300 has pretty good linearity, not perfect (0.5%) but might work
> in this application. But I think the feedback led need to be utilized
> for propper linearity vs temperature.

He is using it; see the opamp - input.

--

John Devereux

[Fred Bartoli]

Sure but you'll suffer led aging and both sides tempcos and non linearity.
The IL300 saves all that using 2 "matched" PDs which takes the LED out of
the equation and leaves you with only the 2 PDs mismatch.

>>George H.
>
> the IL300 has pretty good linearity, not perfect (0.5%) but might work
> in this application. But I think the feedback led need to be utilized
> for propper linearity vs temperature.
>

I use it :-)

Anyway there's one thing in the datasheet:

http://www.vishay.com/docs/83622/il300.pdf

Top of first page: 0.01% servo linearity. That's good.
Top of page 4: transfer gain linearity (delatK3)=0.25%/0.5% over temp,
which I understand should be the same as servo linearity.
WTF?



--
Thanks,
Fred.

[Klaus Kragelund]

The Avego HCNR201 seems to be better and cheaper:

http://www.avagotech.com/docs/AV02-0886EN

The LOC117 is also better:

http://www.clare.com/home/pdfs.nsf/www/LOC117.pdf/$file/LOC117.pdf


Regards

Klaus

[Fred Bartoli]

Le Thu, 13 Jun 2013 01:13:24 -0700, Klaus Kragelund a écrit:


>> > the IL300 has pretty good linearity, not perfect (0.5%) but might
>> > work
>>
>> > in this application. But I think the feedback led need to be utilized
>>
>> > for propper linearity vs temperature.
>>
>>
>>
>> I use it
>>
>>
>>

>> Anyway there's one thing in the datasheet:
>>
>>
>>
>> http://www.vishay.com/docs/83622/il300.pdf
>>
>>
>>
>> Top of first page: 0.01% servo linearity. That's good.
>>
>> Top of page 4: transfer gain linearity (delatK3)=0.25%/0.5% over temp,
>>
>> which I understand should be the same as servo linearity.
>>
>> WTF?
>>
>>
> The Avego HCNR201 seems to be better and cheaper:
>
> http://www.avagotech.com/docs/AV02-0886EN
>

Yep, thanks. Forgotten about that one. (much better specified)

> The LOC117 is also better:
>
> http://www.clare.com/home/pdfs.nsf/www/LOC117.pdf/$file/LOC117.pdf
>

Not really better. In fact seems to be the same as the IL300 except that
deltaK3 is given as 1% max vs 0.5% typ for the IL300.

Still one strange point: the table lists
deltaK3, Transfer Gain Linearity (___non-servoed___)

Since by definition deltaK3 is IC2/IC1 what does the ***non-servoed***
part mean?
Scratching head...

And the first page still says 0.01% servo linearity.


--
Thanks,
Fred.

[George Herold]

On Jun 13, 3:32 am, Fred Bartoli <mynamewithAdotinbetw...@free.fr>
wrote:

Hmm.. Well knowing absolutely nothing about the part. I'd guess the
servo linearity is how linear the loop (LED ->PD->opamp->and back to
LED) is over the LED current range (and temperature). And the K3
linearity is how closely the two PD's track each other... one could
imagine temperature changing the two PD's differently.. and also
changing the relative alignment.

George H.

[whit3rd]

On Wednesday, June 12, 2013 8:47:33 AM UTC-7, Tim Wescott wrote:
> Task: Make a 4-20mA current loop receiver that vampires onto an existing
>
> loop, and delivers a clean signal (either voltage or current) referenced
>
> to my local ground, so that I can stuff it into an ADC.

A Hall sensor might be useful, if you insist on full isolation (kinda like
those no-contact scope probes).

If the current loop is well conditioned, a sense resistor and an
instrument amp is all you really need. Most 4-20 mA loops can spare a volt,
and stay within 50V of local GND.

Lastly, you can float a little VCO (run a '4046 off a nine volt battery) and
optocouple or transformer-isolate its output into a F/V converter using
another '4046.

[Robert Macy]

On Jun 12, 8:47 am, Tim Wescott <t...@seemywebsite.please> wrote:
> Task:  Make a 4-20mA current loop receiver that vampires onto an existing
> loop, and delivers a clean signal (either voltage or current) referenced

> to my local ground, so that I can stuff it into an ADC.  Disturbance to
> the existing loop should be minimal -- ideally it would present no load
> at all to the loop, but a small resistance (less is better, with 100 ohms
> being about as high as I'd like to go) is acceptable.  DC bias and large-
> signal distortion up to 1% of full range is acceptable, gain errors are
> acceptable, but AC performance around any given operating point should be
> 12-bit-ish sort of good to a 100Hz bandwidth.
>
> External power is available.
>
> You'd think this would be a subject for which dozens or even hundreds of
> app notes exist all over the web -- but I only found one, for a Vishay
> optoisolator device.
>
> Anyone have any suggested circuits?
>

> --
> Tim Wescott
> Control system and signal processing consultingwww.wescottdesign.com

Sounds like monitoring an old RS-232 port, but I've seen those loops
sometimes run at over 235kHz.

Here's an idea that should be simple and almost everthing free, or low
cost.
Take a broadband air core current transformer [ 1 Hz to 1MHz, yes,
I've made some that go 0.01 Hz to 100kHz with 0.1% 'initial' accracy ]
Use air core so it is predictable. Then, attach into your sound card,
like Creative Labs EMU1212, which can digitize to near Nyquist of
their high speed sampling rate of 192000 S/s. You can make the
receiver 0.1% initially, then use the OUTPUT [16 bit output] of the
soundcard to calibrate your receiverr to get around 0.005% accuracy. I
know it's AC coupled, but with the air core AND the fact that the data
comes with a START bit and a STOP bit you can ALWAYS sort out the
slight AC walk from your received signal.

I may be an Analog Designer, but I must admit how the 'Dark
Side' [digital design] has slowly taken me over. It's so easy to let
the digital world sort out what's going on.

Anyway, the system is cheap and using triple coated wire, kapton, and
standard widing techniques; you can get UL approvable isolation, isn't
that around 3kV?

PS: you have two channel input, so simply 'delay' one signal a half
sample amount of time and you can suddenly extend that 24 bit ADC
89kHz up to 198kHz and with calibration you can reliably get over 20
bit function. Albeit the low end is around 10Hz, but again RS-232
data comes in bursts sending a high speed min/max 'marker' so you can
sort it all out.

[Tauno Voipio]

On 14.6.13 4:17 , Robert Macy wrote:
> On Jun 12, 8:47 am, Tim Wescott <t...@seemywebsite.please> wrote:
>> Task: Make a 4-20mA current loop receiver that vampires onto an existing
>> loop, and delivers a clean signal (either voltage or current) referenced
>> to my local ground, so that I can stuff it into an ADC. Disturbance to
>> the existing loop should be minimal -- ideally it would present no load
>> at all to the loop, but a small resistance (less is better, with 100 ohms
>> being about as high as I'd like to go) is acceptable. DC bias and large-
>> signal distortion up to 1% of full range is acceptable, gain errors are
>> acceptable, but AC performance around any given operating point should be
>> 12-bit-ish sort of good to a 100Hz bandwidth.
>>
>> External power is available.
>>
>> You'd think this would be a subject for which dozens or even hundreds of
>> app notes exist all over the web -- but I only found one, for a Vishay
>> optoisolator device.
>>
>> Anyone have any suggested circuits?
>>
>> --
>> Tim Wescott
>> Control system and signal processing consultingwww.wescottdesign.com



> Sounds like monitoring an old RS-232 port, but I've seen those loops
> sometimes run at over 235kHz.

Nope. Probably an industrial control system signal.
You'd surprised how large proportion of the measurement
and control in process industries still run on 4-20 mA
current loops. Besides, the TTY current loop was not
RS-232. RS-232 is explicitly a voltage-level interface.

I'd think of an Analog Devices ADUM series isolator with
a delta-sigma modulator on the line side. The signal is
recoverable with a simple low-pass filter on the other side.
The isolators ship also isolated power to the line side.

--

-Tauno Voipio

[Jim Thompson]

On Wed, 12 Jun 2013 14:53:39 -0700 (PDT), Klaus Kragelund
<klau...@hotmail.com> wrote:

[snip]

>>
>
>Look up current sourced DC/DC converter
>
>Switch a 1:1 transformer at 50% (or very close to 50%) with a couple of MOSFETs push pull or any way you like in series with the loop. Use the external power to drive the power stage gates.

Perhaps a simple-minded H-bridge driving a transformer, DC-restore on
the other side.

>
>It really cannot be much simpler than that since you don't have to worry about maximum drop voltage
>
>If you don't like external power you can use a royer converter topology.
>
>Regards
>
>Klaus

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 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.

[lang...@fonz.dk]

On Jun 14, 5:04 pm, Tauno Voipio <tauno.voi...@notused.fi.invalid>
wrote:

yeh something like an ad7401, will need a clock and an isolated supply

with calibration something like this:
http://dk.mouser.com/ProductDetail/Honeywell/CSLW6B40M/?qs=SFC1ZZvMyCG7C3xNrx5ktw==
might work


-Lasse

[josephkk]

On 12 Jun 2013 21:45:37 GMT, Fred Bartoli

Totally cool part. Isolation, great linearity, with opamp maybe 3 V
burden on the current loop, and good dc and ac accuracy and bandwidth.

?-)

[josephkk]

On Wed, 12 Jun 2013 10:47:33 -0500, Tim Wescott <t...@seemywebsite.please>
wrote:

Wow, so many ideas so little time. Since you have external power
available you can make a version of Fred Bartoli's suggestion zero burden,
dc and ac accurate to about 0.1%, and a couple kHz bandwidth real easy. A
floating "virtual ground (0 v dc offset and controllable dynamics)"
transporting power across the isolation boundary.
Or perhaps similar Fred's circuit with a TL431 shunt regulator providing
an fixed voltage drop of say 3.3 V (preferred for 4 to 20 mA circuits).

?-)

[Robert Macy]

On Jun 14, 8:04 am, Tauno Voipio <tauno.voi...@notused.fi.invalid>
wrote:

Ooops, you're right and I used to design interfaces for TTY machines
and their current loop interface.

[John Larkin]

On Fri, 14 Jun 2013 22:47:08 -0700, josephkk <joseph_...@sbcglobal.net>
wrote:

K3, the opto CTR ratio, varies by almost 3:1.

[lang...@fonz.dk]

On Jun 15, 5:19 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Fri, 14 Jun 2013 22:47:08 -0700, josephkk <joseph_barr...@sbcglobal.net>

> wrote:
>
>
>
>
>
>
>
>
>
> >On 12 Jun 2013 21:45:37 GMT, Fred Bartoli

but binned for +/-6%

hcnr201 is a bit better at +/-5%

but they both need calibration

-Lasse

[John Larkin]

On Sat, 15 Jun 2013 09:15:51 -0700 (PDT), "lang...@fonz.dk" <lang...@fonz.dk>
wrote:

It's not very good over temperature, either. Some PWM or digital scheme would be
more accurate.

[Fred Bartoli]

???

K3 is given for 0.005%/K typ, 0.05%/K max.
Pretty good actually.


--
Thanks,
Fred.

[rickman]

Do these things age similarly? Can you add a reference opto pair to use
in a feedback path to linearize the circuit? Drive both LEDs with the
same current and you should get the same current on the other side.

--

Rick

[rickman]

Ah, I see that *is* what they are doing... lol

--

Rick

[Bill Sloman]

On Saturday, 15 June 2013 01:49:14 UTC+10, Jim Thompson wrote:
> On Wed, 12 Jun 2013 14:53:39 -0700 (PDT), Klaus Kragelund
> <klau...@hotmail.com> wrote:
> [snip]
> >>
> >Look up current sourced DC/DC converter
> >
> > Switch a 1:1 transformer at 50% (or very close to 50%) with a couple of
> > MOSFETs push pull or any way you like in series with the loop. Use the
> > external power to drive the power stage gates.
>
> Perhaps a simple-minded H-bridge driving a transformer, DC-restore on
> the other side.
>
> >
> >It really cannot be much simpler than that since you don't have to worry
> >about maximum drop voltage
> >
> >If you don't like external power you can use a royer converter topology.

If you have got external power, and a +1V to +5V signal range (4mA to 20mA in volts) and don't need too much bandwidth, you could do what I did at George Kent back in 1975, which was to use a three-winding precisely 1:1:1 transformer.

An op amp drove one winding, and sensed the voltage produced across a second winding, so that it exactly matched the input voltage (give or take the op amps input offset voltage). The third winding was on the other side of the isolation barrier.

The current through the driven winding was monitored and when it got too high, the op amp output was clamped to the negative rail and held there until the current through the transformer had dropped back through zero to almost too high in the opposite sense.

The output from the isolated winding was disconnected from its output filter while this was going on. I think I used a FET for that job, with the gate tied to the output from the isolated winding.

It was all rather horrible, but exquisitely accurate.

--
Bill Sloman, Sydney

[Fred Bartoli]

Not exactly.
There's no guaranty that leds would be aging similarly. But there's no
aging for the photodiodes.
So, only one led, shared between the forward and feedback PD paths.

--
Thanks,
Fred.

[rickman]

On 6/14/2013 9:17 AM, Robert Macy wrote:
> On Jun 12, 8:47 am, Tim Wescott<t...@seemywebsite.please> wrote:
>> Task: Make a 4-20mA current loop receiver that vampires onto an existing
>> loop, and delivers a clean signal (either voltage or current) referenced
>> to my local ground, so that I can stuff it into an ADC. Disturbance to
>> the existing loop should be minimal -- ideally it would present no load
>> at all to the loop, but a small resistance (less is better, with 100 ohms
>> being about as high as I'd like to go) is acceptable. DC bias and large-
>> signal distortion up to 1% of full range is acceptable, gain errors are
>> acceptable, but AC performance around any given operating point should be
>> 12-bit-ish sort of good to a 100Hz bandwidth.
>>
>> External power is available.
>>
>> You'd think this would be a subject for which dozens or even hundreds of
>> app notes exist all over the web -- but I only found one, for a Vishay
>> optoisolator device.
>>
>> Anyone have any suggested circuits?
>>
>> --
>> Tim Wescott
>> Control system and signal processing consultingwww.wescottdesign.com
>

...snip...

> I may be an Analog Designer, but I must admit how the 'Dark
> Side' [digital design] has slowly taken me over. It's so easy to let
> the digital world sort out what's going on.

If you want to go digital, this design is screaming for a simple, low
power ADC on the current loop. 100 Hz bandwidth means 500 Hz sample
rate would be gravy. Using Manchester encoding to clock the data gives
a 14 kHz bit rate for the 12 data bits plus a couple of framing bits.
Something digital on the downstream side gets the analog back if that is
what is needed or the digital can be used directly.

This can all be done in a small low cost MCU such as a PSOC with very
minimal external circuitry.

> Anyway, the system is cheap and using triple coated wire, kapton, and
> standard widing techniques; you can get UL approvable isolation, isn't
> that around 3kV?

Would that be simpler and easier than a typical opto-isolator? Once you
go digital the opto doesn't need to deal with any of the analog stuff.

--

Rick

[Robert Macy]

WOW! You're right that said 100Hz, not 100kHz !!!

Someday curiosity will outstrip vanity and I'll wear those glasses.

[josephkk]

And the old hen is squawking and scratching at nothing much.

TAIFP

?-)

[John Larkin]

On Mon, 17 Jun 2013 19:20:13 -0700, josephkk <joseph_...@sbcglobal.net>

So, you think it's accurate? And accurate over temperature? Do you admire
trimpots?

Can't invent your own insults?

[Jim Thompson]

On Wed, 12 Jun 2013 10:47:33 -0500, Tim Wescott
<t...@seemywebsite.please> wrote:

>Task: Make a 4-20mA current loop receiver that vampires onto an existing
>loop, and delivers a clean signal (either voltage or current) referenced
>to my local ground, so that I can stuff it into an ADC. Disturbance to
>the existing loop should be minimal -- ideally it would present no load
>at all to the loop, but a small resistance (less is better, with 100 ohms
>being about as high as I'd like to go) is acceptable. DC bias and large-
>signal distortion up to 1% of full range is acceptable, gain errors are
>acceptable, but AC performance around any given operating point should be
>12-bit-ish sort of good to a 100Hz bandwidth.
>
>External power is available.
>
>You'd think this would be a subject for which dozens or even hundreds of
>app notes exist all over the web -- but I only found one, for a Vishay
>optoisolator device.
>
>Anyone have any suggested circuits?

My first thoughts were something along the lines of a V-to-F
converter, and I found such a scheme in US patent 7,477,080

Might also do a classic dual-slope A-to-D, use the signal current for
one side of the integration, a reference current for the other side.

Trivial to do as a custom chip ;-) I'll have to ponder if it could be
done with parts available off-the-shelf.

[Tim Wescott]

On Sun, 23 Jun 2013 12:25:45 -0700, Jim Thompson wrote:

> On Wed, 12 Jun 2013 10:47:33 -0500, Tim Wescott
> <t...@seemywebsite.please> wrote:
>
>>Task: Make a 4-20mA current loop receiver that vampires onto an
>>existing loop, and delivers a clean signal (either voltage or current)
>>referenced to my local ground, so that I can stuff it into an ADC.
>>Disturbance to the existing loop should be minimal -- ideally it would
>>present no load at all to the loop, but a small resistance (less is
>>better, with 100 ohms being about as high as I'd like to go) is
>>acceptable. DC bias and large-
>>signal distortion up to 1% of full range is acceptable, gain errors are
>>acceptable, but AC performance around any given operating point should
>>be 12-bit-ish sort of good to a 100Hz bandwidth.
>>
>>External power is available.
>>
>>You'd think this would be a subject for which dozens or even hundreds of
>>app notes exist all over the web -- but I only found one, for a Vishay
>>optoisolator device.
>>
>>Anyone have any suggested circuits?
>
> My first thoughts were something along the lines of a V-to-F converter,
> and I found such a scheme in US patent 7,477,080
>
> Might also do a classic dual-slope A-to-D, use the signal current for
> one side of the integration, a reference current for the other side.
>
> Trivial to do as a custom chip ;-) I'll have to ponder if it could be
> done with parts available off-the-shelf.

Actually my problem is solved. My customer said "oh, I'll do that part".

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

[Bill Sloman]

On Monday, 24 June 2013 05:25:45 UTC+10, Jim Thompson wrote:
> On Wed, 12 Jun 2013 10:47:33 -0500, Tim Wescott
> <t...@seemywebsite.please> wrote:
>
> >Task: Make a 4-20mA current loop receiver that vampires onto an existing
> >loop, and delivers a clean signal (either voltage or current) referenced
> >to my local ground, so that I can stuff it into an ADC. Disturbance to
> >the existing loop should be minimal -- ideally it would present no load
> >at all to the loop, but a small resistance (less is better, with 100 ohms
> >being about as high as I'd like to go) is acceptable. DC bias and large-
> >signal distortion up to 1% of full range is acceptable, gain errors are
> >acceptable, but AC performance around any given operating point should be
> >12-bit-ish sort of good to a 100Hz bandwidth.
> >
> >External power is available.
> >
> >You'd think this would be a subject for which dozens or even hundreds of
> >app notes exist all over the web -- but I only found one, for a Vishay
> >optoisolator device.
> >
> >Anyone have any suggested circuits?
>
> My first thoughts were something along the lines of a V-to-F
> converter, and I found such a scheme in US patent 7,477,080
>
> Might also do a classic dual-slope A-to-D, use the signal current for
> one side of the integration, a reference current for the other side.
>
> Trivial to do as a custom chip ;-) I'll have to ponder if it could be
> done with parts available off-the-shelf.

Shouldn't be difficult. I did one back in 1979 at Chessell Recorders.

It was good to 14-bits, once we got hold of a polypropylene part for the integrating capacitor. Of course it was quad-slope, rather than dual slope.

The bit I was pleased with was the output isolating transformer, which was just a ferrite toroid, held down on the board by six U-shaped bits of insulated wire, which were the windings (plus some printed circuit track). It had very little inductance, but produced a big enough (and sufficiently long sustained) inductive spike to switch half an LM393 through the couple of mV hysteresis I'd designed in - more than the worst case input offset voltage).

We did have a separate isolated power supply for the floating analog bits.

--
Bill Sloman, Sydney

[Joerg]

That can be code for "Well, we'll give it a shot and when things hit the
fan we'll call". Then, the phone rings ... :-)

--
Regards, Joerg

http://www.analogconsultants.com/

[Jim Thompson]

On Mon, 24 Jun 2013 17:05:24 -0700, Joerg <inv...@invalid.invalid>
wrote:

No, that's code for, "We'll take a shot at that, and blame you when we
don't succeed" >:-}