[sdiy] Unused SSM2164 cell

[Neil Johnson]

David wrote:
> The RC circuit at the input of 2164 is a "snubber" circuit.  AFAIK, it is
> not for eliminating HF oscillation.  It is for protecting the transistors
> inside the 2164 from current switching transients.  If nothing is connected
> to the input, then the RC does nothing.  Leave it off.

Umm....no.

To quote the SSM2164 datasheet:

"Additionally, a 500 Ohm resistor in series with a 560 pF capacitor
must be added from each input to ground to ensure stable operation."

Or, put another way, it's a lead-lag compensator:
http://www.ti.com/lit/an/sloa020a/sloa020a.pdf

It's obvious why it is needed once you understand the internal
structure and history of the SSM2164 design.

So even with no signal passing through the 2164 there is still the
potential for undesirable instability.  Sometimes you get lucky,
sometimes not.

Cheers,
Neil
--
http://www.njohnson.co.uk