[sdiy] S&H

[John L Marshall]

Yes, most implementations that I have seen are just a (fet)switch to charge
the capacitor (usually polystyrene, polycarbonate, polywogg) followed by a
very low current buffer. Really simple.

I thought I saw really clever circuit by Jerald Graeme


Take care,
John
www.sound-photo.com
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----- Original Message -----
From: "harrybissell" <harrybissell at prodigy.net>
To: "John L Marshall" <john.l.marshall at gte.net>
Cc: "Tim Ressel" <madhun2001 at yahoo.com>; "Paul Maddox"
<P.Maddox at signal.qinetiq.com>; <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, January 15, 2003 7:01 PM
Subject: Re: [sdiy] S&H


> Hi John...
>
> I don't think so (in most cases).  It would depend on the topology of the
S/H.
> Many "music" S/H are an open loop system... where the capacitor charges
> at the RC constant of the switch "on" resistance and the capacitor value.
> These
> don't have any feedback...
>
> Those that employ feedback usually overcome the switch on-resistance by
making
> the applied voltage much higher than the desired value, then switching the
> charging
> current off when the proper value is achieved. Its faster and can
overshoot and
>
> ring. Feedback capacitor might help here to stabilize the system.
>
> Droop from opamp offset voltages or bias currents probably can't be
> compensated,
> but can be minimized by making the cap big with respect to the currents
> involved.
>
> I tried an LM324 as a buffer amp (in a non critical S/H) and boy did I get
a
> big
> surprise.  The input PNP likes to source current to the hold
cap...resulting in
>
> POSITIVE droop.
>
> Cool... like having a car that makes gas... just stop every couple of
miles and
>
> drain off a gallon or two...   ;^P
>
> H^) harry
>
> John L Marshall wrote:
>
> > Tim,
> >
> > Can't the droop be compensated for by a second capacitor in the feedback
> > path of the buffer opamp?
> >
> > Take care,
> > John
>
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>