Question about unusual feedback arrangement

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,

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Nemo
Reply to
Nemo
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Here's the LTspice circuit list for the  circuit you've described and
its near equivalent:
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Reply to
John Fields

Perhaps he didn't have a 1 pF cap?

Reply to
PeterD

Or maybe the 0.1% resistors he used didn't go up to 1M?

Some spiral-cut thin-film resistors have about 0.3pF of parallel capacitance, and the original designer might have been designing around that. Few capacitor ranges go below 1.0pF, and the tolerance on the lowest capacitances tends to be high.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

YES. Mark

Reply to
makolber

It sounds like he/she was trying to achieve a response similar to what is used for phonograph equalization (per RIAA standards).

See the section called Complete Active RIAA Filter on this website:

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Bob

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Reply to
BobW

Thank you, lots of useful tips there.

I noticed that it was assumed he used thin film 499k resistor. However, they're just normal 1% metal film ones. Maybe there was a bag of them to hand when he started his design. I suspect he wasn't aware that thin film is lower noise (after all, no one knows every trick). Although with a target bandwidth of around 60kHz, maybe thin film has little effect even when the gain is so high (does it only give noticeable improvement at high freqs?)

Thanks again for the feedback, I feel I can tweak this circuit with more confidence now if we hit problems when it goes into production. All the side comments are really useful.

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Nemo
Reply to
Nemo

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