How to isolate and to pass voltage signal between isolated circuits?

DX400

Joined Oct 26, 2017
12
Hi,

What is the best (and quite simple and not very expensive) way to isolate circuit"1" from circuit"3" by some buffer"2"?

The circuit1 is some voltage source with some load. The voltage on load might vary from 0.1 to 1 V, and the maximal current on load - up to 30 mA.
The circuit3 is some ADC input of the PC controlled board to measure the load voltage.
I have to “connect” the load to the ADC input to measure the load voltage, but I am unable to physically connect it. So I need some buffer circuit “2” based, may be, on optocoupler or galvanic (coil) to pass the voltage signal (current no matter) from the load to the ADC input. So the passed through “2”signal have to be equal or proportional to the voltage on load.

I have looked for the optical isolation by some optocoupler, but I haven’t found the suitable example of such circuit.

Give me, please some example(or reference) of the circuit to resolve my problem.

peterdeco

Joined Oct 8, 2019
376
Why can't you connect the ADC board directly? Most ADC inputs are high impedance and shouldn't load circuit #1 down.

ericgibbs

Joined Jan 29, 2010
13,202
hi DX,
This is a linear analog isolated opto coupler.
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peterdeco

Joined Oct 8, 2019
376
Hi Eric. I don't see how you could use this to measure voltage except detecting 1.6V pulses.

DX400

Joined Oct 26, 2017
12
My ADC board has 16 depended channels with common point equal for all of 16 channels. For example the voltage on channel #1 is measured between X1 and Y1 pins. The pin Y1 is connected to analog ground pin “AGND”. The voltage on channel #n is measured between Xn and Yn pins, with Yn pin connected to the AGND - the same for all channels.

I have another circuit not shown on sketch, lets call it circuit “4”, which outputs already connected to the another ADC channels. The common points of the circuit “1” and circuit “4” are different and if I connect it both to some channels of ADC board, it will cause large voltage distortions.
So I want to isolate common point of circuit “1” from common point of circuit “4”, which already connected to AGND. So to connect it to ADC independently.

ericgibbs

Joined Jan 29, 2010
13,202
hi DX,
This LTSpice sim shows one example on how the HCNR can be used.
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ericgibbs

Joined Jan 29, 2010
13,202
hi DX,
Have you tried that circuit in simulation.? If not I could test its performance in LTS.
E

ronsimpson

Joined Oct 7, 2019
1,311
I used the HCNR200. Works, not to 10bits.
Iso-amp I went with this for my isolated scope probe. Not good to 10 bits either but pretty good. There are more parts like this. Isolation amplifiers.

DX400

Joined Oct 26, 2017
12
Have you tried that circuit in simulation.? If not I could test its performance in LTS.
No, I haven't. Could you please simulate and test its performance in LTS.

ericgibbs

Joined Jan 29, 2010
13,202
hi,
E

BTW:
I am sure you appreciate that the Vcc on the input OPA, will be different isolated supply on the 2nd OPA.??

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