Electronic – Identifying function of passive components in op amp circuit

acaudiooperational-amplifier

Guitar Pedal Input

I was reading this report on a digital guitar effects pedal design and I came across the circuit above. "AUDIO_IN" is where the guitar signal enters the pedal. The signal is then filtered through the op amp circuit and ends up at a high res ADC. According to the report, the circuit is a "4th-order unity gain Sallen-Key band-pass filter with cutoff frequencies of 25 Hz and 16.5 kHz".

I set myself to the task of justifying each component and got lost pretty quickly. Here's what I think I know:

R9: Weak pull-down resistor on input to ground

C10, C12: Coupling capacitors (Why are there two?)

R10: Pulls AC signal up into the 0V – 5V range, centered around 2.5V

R11: ???

C14: Decoupling/bypass cap

R14: ???

R17: ???

C18: ???

C19: ???

R18: Resistor in RC filter before ADC input

R19: ???

C21: Capacitor in RC filter before ADC input

I'm really curious about R19. I have a fair amount of experience with DC op amp circuits but I don't have a ton of experience with op amp circuits with AC signals. I want to use this circuit in a project but I want to know what everything does first.

Can anyone help me identify the function of the passives I'm unsure about? Reading suggestions on good AC op amp circuit design are also very welcome.

Best Answer

I think you need to look at it as circuits in stead off individual component.

The first op-amp is a second order high pass filter.

Second op-amp is a second order low pass filter.

Both filters are Sallen-Key topology as jonk wrote in his comment. There is a pretty good wikipedia page on Sallen-Key.

The circuit at the ADC input is a balanced low pass filter. You can read about the circuit in the datasheet for the ADC. The exact circuit can found in the Application and Implementation section.