[sdiy] 2164 -> TL074, 100 pF in feedback needed?

Mattias Rickardsson mr at analogue.org
Wed Nov 25 17:43:44 CET 2015


Hi fellow 2164 addicts,  ;-)

the typical application circuit in the SSM2164 datasheet shows a 100
pF capacitor in parallel with the 30k feedback resistor on the op-amp,
saying

"The 100 pF capacitors ensure stability and reduce high frequency noise.
They can be increased to reduce the low pass cutoff frequency for
further noise reduction."

[...]

"If additional SSM2164s are added, the 100 pF capacitor may
need to be increased to ensure stability of the output amplifier.
Most op amps are sensitive to capacitance on their inverting
inputs. The capacitance forms a pole with the feedback resistor,
which reduces the high frequency phase margin. As more
SSM2164’s are added to the mixer circuit, their output capacitance
and the parasitic trace capacitance add, increasing the
overall input capacitance. Increasing the feedback capacitor will
maintain the stability of the output amplifier."

The problem is that they already lowpass filter the sound a little bit
- more than ½ dB rolloff at 20 kHz - and sometimes you might need a
bigger resistor than 30k in order to optimize other aspects of the
circuit, resulting in even more HF rolloff. Not awfully much, but you
might have several of these stages in an audio path adding up to some
decibels.

But is 100 pF really needed?
Is TL07x sensitive to capacitance on their inverting inputs?
How about other common op-amps?

Using the bread-and-butter op-amp TL074, has anybody tried out any
variations on the 100 pF theme and run into problems?

I've used as low value as 33 pF & 47k after *two* summed 2164s,
withouth known problems yet. The datasheet pole recommendation would
be 127 pF for this scenario. Should I worry? :-)

(I'm sorry if this has been discussed already)

/mr


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