Suggested low pass filter for a DAC

[T3sl4co1l]

A check with transient simulation is always worthwhile.   (Making sure there's a noise source somewhere, so that it starts up if it does oscillate.)

A pole-zero analysis should show poles in the RHP, although for any nontrivial circuit, there always seems to be a mess of poles and zeroes, or I'm just using the analysis wrong, dunno.

Tim

[peter-h]

Despite me being clueless on filters, I am amazed nobody spotted that this circuit is likely to oscillate, by looking at what the phase is doing in the plot I posted above.

There is only one way this circuit can oscillate and that is the feedback cap. There is no other feedback. And if one starts at say 500pF and plots the amplitude and phase every 500pF, one finds the gain peaks sharply around 2000pF



and after that the phase remains oscillation-favourable on the right hand side, eventually. There is a dramatic change either side of 2000pF.

This is with 1000pF

[T3sl4co1l]

Positive phase shift, as in, predicting the future? A hallmark of RHP poles, yes.

Tim