Re: how to raise input impedance of small signal

eehinjor wrote: 
Thank you.John.

No matter 3.3mV or 2.5mV,The two results are difference is the fact.
I want to know the reason and how to solve this problem.

My point is that if the input is actually at 3.3 mV (but is not being measured) when you are measuring the output at 3.3 mV but is loaded down to 2.5 mV by the meter only when you are measuring the input (and at that moment, the output also follows the loaded input down to 2.5 mV, but is not being measured at that instant), then there may not actually be any difference between input and output at any given instant.

Have you ruled out this possibility? Are you measuring both voltages simultaneously (or applying a dummy load to the input that matches the meter impedance, while you measure the output)?

If you have, then your opamp is not a very good example of the type. If your result is taking place in a simulation, then the simulated opamp is a worst case example, not a typical one.

If you really need a small fraction of a millivolt input accuracy under wide temperature range, you may have to change the opamp to a self zeroing (chopper) type. These can have DC errors down to less than .1 mV.
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