Bandpass filter for summing amplifier input

Question

I need to create a bandpass filter for a single input to a summing amplifier. In this example of a summing amplifier, I understand how the capacitor and resistor in series creates the low cutoff frequency of \$1/(2\pi RC)\$ for the individual signal. But what is the best way to create an upper cutoff frequency for an individual signal here?

I know a capacitor in parallel with the feedback resistor like this would create an upper cutoff frequency, but that would affect all input signals:

What's the best solution to this?

Answer 1

The easiest way is to add a passive low pass filter to each mixer input, eg. like this:-

^{simulate this circuit – Schematic created using CircuitLab}

This is fine if you only want a soft roll-off (-6dB/octave), but higher order passive filters are difficult to design because they interact with each other. To reduce interaction you have to greatly increase the resistance in each subsequent stage, which quickly leads to impractically high output impedance.

What's 'best' depends on what is most important to you. If you want sharper filtering then an active filter (requiring one op amp per input) would be a good option. For 3 inputs you would only need a single quad op amp IC.