[sdiy] Newbie questions about summing buffers

[Ian Fritz]

>
>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.

Ian