[sdiy] Another SSM2164 question

[ASSI]

On Dienstag 12 August 2008, Tom Wiltshire wrote:
> Now, what I'd like to know is what do the 500R/560pF do and how
> are their values calculated? All the datasheet says about it is
> that "a 500R resistor in series with a 560pF capacitor must be
> added from each input to ground to ensure stability". Great.

This is adding a dominant pole so that there will be enough phase 
margin over the bandwidth of interest.  In other words, when the 
phase shift becomes large enough to allow oscillation, the gain 
is already sufficiently far below 1 and it's just not happening.

> Practically, it seems you can vary the values a bit without
> incident (I've used 560R/470pF and nothing bad has happened)
> but it's a bit hard to know when you're not sure what they're
> supposed to prevent.

This moves the pole frequency from 568kHz to 605kHz, so you surely 
shouldn't expect too much of a change.  Such datasheet values 
normally have more than 10% margin.


Achim.
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