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input/output crosstalk/noise in AC inverting amp

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.

Thanks, Klaus

Reply to
buecherk
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Are you sure you are seeing noise and not low level HF oscillation?

piglet

Reply to
piglet

Exactly my thought.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

good one, I will doublecheck.

But why would one op oscillate, another not? And it is not going from 0.06 to 4%, but only to 0.22%. But a bigger change might be masked by the bandwidth of the lockin amplifier (7265 digital).

thanks, Klaus

Reply to
buecherk

It sounds like a current amplifier with a cable connected to the summing junction. That's a guaranteed way to make the poor amp misbehave.

Op amps are so compact nowadays that there's really no reason not to put the TIA at the photodiode, even in a vacuum chamber.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com
Reply to
Phil Hobbs

How long are the cables? Could be capacitive loading of input and/or output. (The crossed cable thing sounds weird... are you sure that is right?)

George H. (always look at signals on a 'scope.)

Reply to
George Herold

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