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non-linear?

Hello

I'm wondering if amplifier stages based on operational amplifiers can be non-linear for small signals but OK for bigger signals.

I'm amplifying two signals from a single PSD (position sensitive device) and at low light conditions I don't get the same rations between the two signals anymore.

I have heard that PSD's should be linear down to very small light levels. So that leads me to think if it is some non-linearity in by amplifier circuits.

What can be done to prevent this non-linear behavioral?

Reply to
Elektro
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Nonlinear how? Is it just an offset? An x^2 nonlinearity? Something weird?

And how much nonlinearity? If it's more than a few tens of PPM then even with the worlds cheapest opamp it's probably not the electronics.

I'd suspect the sensor, first.

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Tim Wescott
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Reply to
Tim Wescott

What about the background light? Usually PSDs use a modulated light source, but even then reflections, glare, cheap lenses etc. can cause this phenomenon. I would say the reason you suspect is pretty unprobable. Still maybe you're using wrong opamps or circuits. Give some more information.

--
ciao Ban
Apricale, Italy
Reply to
Ban

This could be an apparent non linearity that in reality is offset voltage drift or noise related problems, or it could be the way you are using to measure the signal. Some questions:

1) How small is the signal? 2) What is the op-amp.

3) How are you making your measurements.

Op amps do not tend to have any inherent non linearity unless you are running them close to the rails (I assume your small signals are centred in the middle of the op-amps suplly, i.e. they are relative to

0V and you have +/- supplies ;-) But there are an awful lot of things can can throw you at low levels.
Reply to
Roger

I can't give you any schematics because I am doing this work at a company. So I can't publish the schematic.

But I found a data sheet for a distance sensor from SHARP that has the same "problem". You can look at the data sheet at

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If you look at FIG .1 in that document there is also a difference in the measured position between white and grey surfaces.

What can cause this effect?

Reply to
Elektro

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