Hello Kai,
As a step of learning good design practices, i just tried to analyse the LM324 circuit that delivers the Usig o/p. But the experience raises lots of doubts when tried to find the way it was designed.
Here, is what i managed to get from the circuit but, couldnt match it resonably with our intended application.
kindly have a look at it and guide me for a suitable way of understanding the circuit.



Voltage at non inverting i/p = 1.75V

Inverting i/p:
freq of mod voltage = 1111HZ
100uF with 2.2K series resistor maintains the freq below 3dB. It blocks the DC component too.
not quite sure about the 10nF cap - seem to be a low pass filter. the 10K with Rf wuld be used for gain adjustment.
the cap accross the Rf makes the circuit works as a integrator at high frequencies.
So the difference between two i/p's is amplified with the above setup. so the difference voltage amplitude is equal to 1.75 - .85 = .9 and 1.75 - .51 = 1.24 (without gain)
but is it not that, we only need to get .85 and .51 at the o/p?

I would require your help with this.
also,
could you give your analysis for the same so that i could compare it with mine for any mistakes and hence could have better understanding?

suresh.