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Low Current Measurement with Low Impedance

Question asked by tmandreas on Dec 26, 2014
Latest reply on Dec 26, 2014 by Achim

I have been working on a sensitive test and measurement system and have come across a problem with measuring very low currents with low shunt resistances. I would like to be able to measure bidirectional DC currents as low as 50 nanoamps with only a 10 ohm resistor, while also being able to decipher when there is zero current on the wire. I have tried multiple op-amps and instrumentation amps, all with low noise/low offset characteristics, but always seem to come up with one problem. The output from my amplifier stage, when my input is shunted, or has no current on it is the same as when there is some appreciable amount of current. To clarify, when Iin1 and Iin2 are shunted together, I get the same output as when I pass 1.91 microamps thru from Iin2 to Iin1. Does anyone have any ideas on why this happens, or how to fix it? My first assumption was Offset voltage (that may still be the answer), but I would assume that Vos would be constant with DC. So, if I get 58mV with Iin1 and Iin2 connected to each other (shunted) or when not connected to anything (floating), then that should be my Zero point and current going in either direction should cause a change from that zero point. I am going into an A/D so I can do math based around that being my zero point. However, I find that applying any current into/out of Iin1/Iin2 changes that point dramatically and then causes a new 'Zero Point' which then means that some value of current going through the 10 ohm resistor will also look the exact same as when the Inputs were floating/shunted. I am looking for a way to tell the difference between floating/shunted and that value of current that causes the same reading. Thank you.