[sdiy] Guard Rings

[David Ingebretsen]

First, I really appreciate all the feedback.

I got hold of Nyle today and asked him about R13. He said when they started
building their circuits, they noticed some high frequency oscillations in
the voltage followers which was removed when they added a 1k or 1.2k
resistor in series with the transistor base. He didn't remember if they had
the same problem with FET followers, but thought he added R13 as a
preventative measure. 

He also thought the best idea for this PCB would be to just huddle the nodes
of Q11-S, C5, R13 and Q7-G close together and not put any other pads close
by and forego the guard ring. He said if he had used an FET input OpAmp, he
would recommend a guard ring.

But, what do I know. :) 

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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