I want to take two independent differential signals (audio line inputs with unknown properties which may be present or not), remove any common-mode noise from each of them (they could each have different common-mode noise), then sum the desired signals together. I believe I can do this all with 1 op-amp. (Actually I want to output the desired signal differentially, so I will use two of these with inverted output polarity.)
You can assume Rfeedback = Rground and all the input resistors are equal. So then I believe the output is:
$$V_\mathrm{out} = {R_\mathrm{f} \over R_\mathrm{in}} ( A + B )$$
with all the common-mode noise cancelled. Correct?
Are there any problems with this op-amp configuration?
This page calls it a "generic linear operator":
Will the CMRR be reduced by summing on the non-inverting node or anything? I did the math, and CM sources are completely cancelled out at Vout, but I feel vaguely uneasy about it. 🙂
The input impedance of the inverting inputs varies with the signal, as explained here, but shouldn't matter as long as it's much larger than the source impedance. The common-mode impedance is the important part, and should be the same for all input pairs.
If I weren't trying to do it all in one, I'd use two diff amps followed by a summing amp (followed by an inverter for the differential output). Does that method have any benefits over the single op-amp method?
Best Answer
One potential issue I see with this approach is that since the inputs of the op amp will not remain at a fixed voltage, some of the voltage seen on the "sum" inputs of your circuit will be "visible" on the all of the inputs (sum and difference alike). If those inputs are not being driven by low-impedance sources, that could pose a problem.
A better design in some cases, if you have the power-supply margin to accommodate it, may be to have the summing inputs feed into one amplifier wired as an inverter (with a non-inverting input that sits at a fixed voltage), and have the output of that inverter fed as one of the inputs to the amplifier that accepts all the difference inputs. If your sum and difference inputs are intended to be grouped as differential pairs this approach won't be great (the non-inverting inputs will flow through two op amps while the inverting inputs only flow through one) but your original approach won't be either. Your best approach in that case would be to either pass all the sum inputs into one inverting amp, all the difference inputs into another amp, and then take the difference of the two outputs, or else to pass each differential pair through its own instrumentation amp and sum the results. Using a separate instrumentation for each pair would give the by far the best CMRR, but would of course require more amplifiers.