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We're in the midst of designing a device for measuring analog signals and have been testing our initial prototype. We had a single-ended gain stage before the ADC, but due to the connections in the rest of the system we are experiencing ground loops through the signal cable. These are causing offsets and throwing a heap of noise onto the signal.

Our initial solution has been to use an instrumentation amp as a pseudo-diff amp with a pulldown on the inverting input, as shown below.

enter image description here

I understand R3 prevents the inputs from floating when no device is connected while allowing the GND of the two devices to be at different potentials. I'm having a hard time figuring out why the value is so low though (the design is recycled from an older device). The signal we're interested goes down to a few mV and offsets of 0.1mV are acceptable. With input bias currents in the nA range for IN-amps I would think R3 could be much larger.

This comes about because there is a use-case for having numerous (10-20) of these amplifiers in parallel. With 10-20 R3's in parallel the impedance between the two system grounds will drop to 5-10 Ohm and the 'break' in the ground loop will be much less effective.

What might be the reason for keeping R3 low? I'm hoping to optimize it for some larger value but first need to understand the trade-offs.

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  • \$\begingroup\$ If you added the ground loop noise voltage that you are expecting, you might be able to do a proper simulation. \$\endgroup\$ – Andy aka Mar 10 at 9:38
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Probably there are issues with the input common mode range of the inamp. They are very peculiar in that, depending on the internal architecture (often there are 2-3 pages in the datasheet devoted to the CM range in various condition of use).

The 100 ohm is often also a safety drain for remote ground lift: same issue but different purpose.

Maybe you need an insulation amplifier if you have these problems (they are not cheap)

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  • \$\begingroup\$ Thanks for your answer, could you elaborate on the "safety drain for remote ground lift" point? Safety sounds like it could explain why the value is kept low. We're not expecting large common mode voltages, we should see a worst case of 1V difference between the local grounds in each device \$\endgroup\$ – Jordan Cartwright Mar 11 at 20:44
  • \$\begingroup\$ if the common mode stay lows no issues but think about 60-90 volt of ground lift (I've seen worse on long distances) and the current involved on the ground common \$\endgroup\$ – Lorenzo Marcantonio Mar 12 at 8:58
  • \$\begingroup\$ The cables connecting our devices range from 0.3 - 2m and we're dealing with current of up to 5A so I don't expect ground lifts to get anywhere near that high thankfully. I had a look at the CM range for our inamp and the output is fully specced to common mode inputs of approximately +/-10V which is far more than we're expecting to see. \$\endgroup\$ – Jordan Cartwright Mar 14 at 21:14
  • \$\begingroup\$ in 2m there should no ground lift problems then! just tune that resistor to your needs \$\endgroup\$ – Lorenzo Marcantonio Mar 15 at 10:09

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