I'm having trouble with getting an accurate measurement with this circuit and I was hoping for some more help.
See the circuit, which I have breadboarded and am testing. I step down a voltage through a 1 to 100 voltage divider. The intent is that I can connect to different parts of the divider, and get a different step down ratio.
If I leave the divider and the unity gain buffer out of the circuit, I get excellent performance from the AD620 in-amp. I can input a voltage between -1 and 1V, and get effectively exactly the same voltage out, which is what I want. My input point would be right before the two 1k resistors.
When I introduce the divider, the voltage I get at the output is no longer accurate. I thought it was because the impedance of the divider was too high for the 1nA input bias current of the AD620, so I added the buffer that you see on the positive input. But this didn't help things! I still don't have an accurate voltage at the output. The buffer is an OPA376AID.
Is there something wrong with my approach to stepping down the input voltage with the divider? I know this worked on another design with a single ended input. Is it because the positive and negative side of the instrumentation amp are being loaded differently?