[sdiy] fanning grounds vs. dirty/clean ground

Neil Johnson nej22 at hermes.cam.ac.uk
Wed Aug 21 22:14:57 CEST 2002


On Wed, 21 Aug 2002 media.nai at rcn.com wrote:

> >You have a series resistor between the opamp output and your (capacitive)
> >load, and one feedback (resistor) goes from _after_the series resistor
> >to the negative opamp input, and a second feedback (capacitor) goes from
> >_before_ the series resitor to the negative opamp input.
>
> So what is that supposed to do again??

I strongly suggest reading Analog Devices App Note AN-257, available in
PDF from their site.  It goes into the theory behind the why's and how's
of stabilising op-amps feeding caapcitive loads.  The canonical diagram is
Figure.2:

  "Adding a resistor in series with the output and a capacitor between the
oputput and the input of an op-amp can reduce phase shift in the feedback
loop to a value close to zero.  This restores stability to an op-amp which
would oscillate when driving a capacitive load."

There's also quite a good discussion on supply decoupling too.  Time to
dig out those control-theory books!

Neil

--
Neil Johnson :: Computer Laboratory :: University of Cambridge ::
http://www.njohnson.co.uk          http://www.cl.cam.ac.uk/~nej22
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