Re: HELP, I have non-linear gain in two channels


"Elektro" <skolpojken72@xxxxxxxx> wrote in message
news:1191445520.451785.162570@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I have two identical 3 stage active band pass filters consisting of 3
op amps each. The problem is that I get varying gain in the channels
depending on the input signal level.

For a test I ran the same signal to both channels at the same time and
sample the output from each channel. Those sampled signal are then
adjusted so the static gain is the same for both channels. And then
the bias is adjusted.

If I then plot the signals they seem to be exactly the same. But if I
plot the deferens between the channels I can se the sometimes ch1 is a
bit more than ch2 and vise versa. It's dependent of the input signal
amplitude.

What can cause this, any tips?

In my application it is important that both channels change the same
percentage. If the input signal to ch1 and ch2 attenuates 1% the
outputs from ch1 and ch2 must attenuate 1% also.

ADC's can have small non-linearity errors. So maybe I'm going to
connect one band pass signal to both ADC's to check if I get the same
result.

I wouldn't have thought it possible that 2 channels of 9 opamps and shed
loads of passive components are ever going to match within 1%.
If you're plotting the deferens on a cycle by cycle basis then tiny phase
shifts will give big errors. Even with an RMS calculation for each channel
there will be differences, due to the numerous capacitors changing their
values slightly with differing signal levels.
God forbid you using anything remotely like ceramic caps. These can offer a
(useful?) 2X value change for a 2X signal change.


.

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