[sdiy] Newbie questions about summing buffers

[David G Dixon]

> >Capacitor across Rf forms a low pass filter, helps dampen any high 
> >frequency oscillations at the output.
> 
> People often write this but it isn't quite true.  The 
> feedback cap stabilizes the circuit by compensating for opamp 
> nonidealities, not by acting as a LP filter.  IOW you can't 
> clculate the cap size just from the frequency of the spurious 
> oscillations.  There is a good TI writeup about this online somewhere.

I've been reading up on this stuff myself -- break points and loop gain and
all the rest of it -- but I must confess that when I've got spurious
oscillations in a circuit, I use good old trial and error to find the cap
value which eliminates them.