# Bandpass filter for summing amplifier input

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?

• Filters are usually defined by -3dB band pass and -xx dB band stop rejection levels. What would you like? Commented Mar 27, 2017 at 20:44
• "The best way"..... Hmm... Commented Mar 27, 2017 at 21:00
• How about "any way" lol. I just want to have a low and high frequency cut off, so each cutoff frequency is at -3 dB... Commented Mar 27, 2017 at 21:12
• I'd use a few amps to do this. One to sum and one to do the filtering after summing. If you need different high pass poles, I would use three different amps and then one summing amp Commented Mar 27, 2017 at 21:53
• @donut a more specific question would get you a better answer instead of leaving it open-ended. Commented Jul 11, 2017 at 15:11

## 1 Answer

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.