we have an inverting opamp as AC amplifier with gain 100 or 1000 in a shiel ded aluminum box. BNC as input and output. Using chopped light and measurin g noise on top of a signal with a lockin amplifier.
I am wondering why different ops behave differently.
We had it on the table so that input and output BNC crossed each other at c
Not optimal, I know, so we rotated the aluminum case so that input cable an d output cable don't see each other. Less noise.
Next, I tried a collection of opamps for noise.
Here comes the question: on some opamps, noise increases when input/output BNC cables are touching (crossed each other) and input and output 'see' e ach other.
Some other ops don't bother and are always low in noise..... Why? Which quantity in a datasheet would reflect this? I am talking about 0.2% n oise versus 0.05%, so it can't really be the difference in CMRR (114db vs.140dB), I think.