I need to sense 200 V (to GND) 0.5-20 mA DC with minimum component count. After a lot of searching I think an isolation amplifier followed by a post-amp would give the lowest component count.

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An isolation amp usually has low gain (around 8 in this case), so I chose an instrumentation amplifier as post-amp to reduce component count (no impedance matching, no feedback).

I'm rather inexperienced with op-amps. Please point out any problems in the circuit. I'm unsure if it's OK to tie the Ref pin of the in-amp to ground, but there is no negative signal. Using another Ref value would require another op-amp as a buffer and increase component count a lot.


There are high-gain isolation amps, so a post-amp is not required.

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  • \$\begingroup\$ Follow some guidelines to make a "readable" circuit schematics. The caps and resistors are wrapped all around,...etc. Using an isolated converter and then binding terminals on the same GND is totally useless. You do need two separate PSUs with separate grounds!! \$\endgroup\$ Commented Oct 31, 2021 at 14:44
  • \$\begingroup\$ 7E10FC9A, Is this in any way related to another question you asked about providing a current source? Or independent of that? \$\endgroup\$
    – jonk
    Commented Oct 31, 2021 at 18:42

1 Answer 1


I'm unsure if it's OK to tie Ref pin of in-amp to ground

Read the data sheet on page 3 and note this: -

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Valid reference voltages must be 1.6 volts higher than the negative supply \$-V_S\$ and, given that the \$-V_S\$ supply rail in your circuit is 0 volts, the lowest valid voltage on \$V_{REF}\$ is +1.6 volts.

  • \$\begingroup\$ Can this be avoided if I use rail-to-rail in-amp ? \$\endgroup\$
    – 7E10FC9A
    Commented Oct 31, 2021 at 13:18
  • \$\begingroup\$ The relevant data sheet will tell you. \$\endgroup\$
    – Andy aka
    Commented Oct 31, 2021 at 13:19
  • \$\begingroup\$ @7E10FC9A are we done here now? Do you have any further questions on this subject? \$\endgroup\$
    – Andy aka
    Commented Nov 9, 2022 at 9:34

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