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

Richie Burnett rburnett at richieburnett.co.uk
Wed Nov 25 18:07:18 CET 2015


I would recommend making your PCB in such a way that one of the design goals 
is minimising the stray capacitance from the inverting input of the op-amp 
to ground...

Then...

Populate it with 100pF in the feedback position and test it, then decrease 
this capacitance in steps until you observe HF oscillation at the output of 
the op-amp (due to the reduced phase-margin at the crossover frequency that 
they talked about.)  You will have to look with an oscilloscope as it will 
likely be in the MHz range and might only be a volt or so in amplitude.

When you find the critical value that is on the boundary of oscillation, 
triple it.  That should give you sufficient phase margin to prevent 
reoccurrence of HF oscillation during use with the minimal of HF rolloff at 
the top of the audio spectrum.  If you can't tolerate the resulting treble 
rolloff then reduce the feedback resistor and make up the lost gain with 
another amplifier stage.

The same problem must occur in large mixing desks that use long mix busses 
at "virtual earth" potential.  There are probably some other clever tricks 
possible like a phase-lead network to kick-in some extra phase-lead up in 
the MHz where the op-amp would be prone to oscillate due to compromised 
phase-margin.  Also, I'm sure someone on this list mentioned connecting the 
op-amp inverting-input to the mix buss through a small inductor to 
"de-couple" the stray capacitance of the mix buss from the virtual earth 
node of the op-amp.

-Richie,

-----Original Message----- 
From: Mattias Rickardsson
Sent: Wednesday, November 25, 2015 4:43 PM
To: synthdiy diy
Subject: [sdiy] 2164 -> TL074, 100 pF in feedback needed?

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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