[sdiy] Guard Rings

[David Ingebretsen]

OK, that makes sense. The potential at the emitter of Q9 won't leak away,
and since it is the same potential as the nodes within the ring, the charge
being held has nowhere to go.

Honestly, I don't understand the circuit well enough to answer your
questions. I'll ask Nyle next time we talk.

David

~~ -----Original Message-----
~~ From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
~~ bounces at dropmix.xs4all.nl] On Behalf Of Dave Manley
~~ Sent: Thursday, November 04, 2010 11:29 PM
~~ To: synth-diy at dropmix.xs4all.nl
~~ Subject: Re: [sdiy] Guard Rings
~~ 
~~ On 11/4/2010 9:24 PM, Oren Leavitt wrote:
~~ 
~~ >
~~ > Looking at the pdf, you'll want to place the guard ring around the
~~ traces connecting (Q11-S, C5, R13) and (R13, Q7-G). That's your "island"
~~ of high impedance while in "hold" mode. The ring is
~~ > connected the output (emitter) of the emitter-follower Q9 which will be
~~ at the same potential but buffered.
~~ > Ditto for Q12, C14, R6, Q8.
~~ 
~~ I agree where the ring should go, and I'd be tempted to optimize the
~~ component
~~ placement.
~~ 
~~ A couple of questions:
~~ 
~~ Is Q9 emitter really at the same potential as the voltage on the cap?
~~ 
~~ What's the point of R13?
~~ 
~~ David - the idea of the guard ring is to surround a high impedance node
~~ holding
~~ a charge with a buffered potential of the same voltage.  This will
isolate
~~ the
~~ node from different nearby voltages so that it doesn't discharge due to
~~ leakage
~~ currents on the board.  If the PCB is clean these leakage currents should
~~ be minimal.
~~ Make sure the board is free of contamination on both sides.
~~ 
~~ -Dave
~~ 
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