I'm familiar with op amps using a capacitor in parallel with the feedback resistor to control rolloff. However, I've inherited a circuit from an analogue specialist (no longer contactable) who used two separate resistors in series, each with its own parallel capacitor, and I'm wondering why he did that. The feedback circuit is something like this:
(op amp input) ----R//C-----R//C------(op amp output)
At first I assumed it was a T feedback network but there's no third leg in the middle - it's like a 1M feedback resistor split in half. The R's are 499k and one capacitor is 2.2pF, the other 1.8pF.
Obviously if you didn't design the circuit you can't say for sure. But I'd be interested in views on what advantage the circuit gains from this. Is this simply to put two different frequency breakpoints in the rolloff characteristics, or does it make the rolloff steeper, is it some clever noise-reduction technique I've not heard of - or is it simply to allow use of slightly larger and thus more controllable capacitors which has some cunning side benefit?
Thanks,