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Im designing a voltage measurement circuit that needs to measure differential signals in the order of mV in the presence of high voltage common mode noise.

Im using a isolated DC/DC converter that is connected to mains earth by a 1nF capacitor. Im planning to measure the differential voltage with an instrumentation amplifier and an ADC.

I only have a unipolar 3.3V supply to power the amplifier and Im aware that I have to bias the inputs to a suitable common mode voltage.

I have tried the following circuit but the voltage in VDIF+ and VIDF- with respect to my floating ground is to high for the differential amplifier. Reducing R1 and R2 is not an option because I need a high impedance input

What is the correct way to bias the instrumentation amplifier? Circuit 1

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lreyes
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  • You should explain what the unwanted common mode signal is and what the "wanted" signal is under all circumstances of amplitude and frequency. – Andy aka Oct 06 '20 at 15:12
  • The wanted signal VDIF is a +-100mV DC signal. The unwanted common mode noise is 0-100 Vac 50Hz noise – lreyes Oct 06 '20 at 15:24
  • A decent low pass filter appears to be the way to go. Don't try and feed unfiltered into an InAmp or you'll just be disappointed. – Andy aka Oct 06 '20 at 15:28
  • I low cut off low pass filter would require a big resistance or big capacitor. The big resistance could generate big offset due to bias current. On the other hand big capacitors to filter the common mode could generate a differential voltage due to missmatch. I agree that i need to filter the signal but I dont think this resolves my problem – lreyes Oct 06 '20 at 15:46
  • Why have you connected the isolated output of your DC-DC converter to mains earth through that 1nF capacitor? Surely your DC-DC is capable of providing isolation over the 100V mains voltage ... ? – brhans Oct 06 '20 at 18:34
  • I have the capacitor across the isolation barrier to reduce EMI emissions as it discussed here: https://electronics.stackexchange.com/questions/216959/what-does-the-y-capacitor-in-a-smps-do. The Ground of the non isolated circuit is connected directly to mains earth. The DC-DC es capable of 2000Vrms of isolation – lreyes Oct 06 '20 at 20:41
  • Ok - so why did you choose to earth your isolated ground? If you allowed it to float you'd have the option to bias it somewhere near your signal-of-interest's common-mode point instead. – brhans Oct 06 '20 at 23:10
  • It is the non isolated ground the one who its connected to earth – lreyes Oct 07 '20 at 14:32

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