Charge injection in chopper stabilized opamps

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schmitt trigger

Joined Jul 12, 2010
136
I remembered that I had an OPA735 (bought brand new from Digikey) and decided to play with it a little. You know, I had nothing better to do on a dreary Sunday afternoon.

I built a classic diff amp, with 1Meg and 100k (0.1%) feedback and input resistors, supplied it with 5 volts, and biased it with a 2.5 volt virtual ground from another g.p. opamp. Proper decoupling caps used throughout. My intention was to amplify a 20 mV DC signal 10 times. Perhaps to use in the future as a current shunt preamp.

My initial verification was to check the offset voltage with inputs shorted. To my surprise, the output was reading 15mV (with a DMM), which meant 1.5 mV on a chopper opamp? Impossible!
I know enough to know when seeing such offsets in a DMM, is good practice to actually see the waveform on a scope, to see if noise, hum or an oscillation was causing the spurious reading.

The scope waveforms showed a quite significant spike, meaning that most likely the problem was being caused by input charge injection.

Reading different chopper opamp datasheets, in addition to the OPA735, there were recurring descriptions to lower the overall loop resistance to minimize charge injection.

For that matter, I replaced the feedback and input resistors to 10k and 1k (0.1%) respectively.
The output offset voltage now dropped to 0.1 mV, which was what I would expect on this opamp.

While I was expecting some chopper induced noise, I was surprised that high impedance resistors would generate some much DC offset......

Has anyone had similar experiences?
 

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