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From: John Popelish
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X-Mailer: Mozilla 4.7 [en] (Win98; U)
Subject: Re: what are guardrings for?
Date: Sun, 29 Sep 2002 00:01:37 GMT
NNTP-Posting-Date: Sat, 28 Sep 2002 20:01:37 EDT
> I've been reading about guard rings in a book (AoE I think) but never
> got a good explanation of how exactly they work. One input of the
> op-amp is laid out on the PCB so it surrounds the other input.
> This is supposed to help against leakage currents when pA level
> current levels are involved, but how exactly does a guard ring
> prevent these leakages?
The guard ring has a voltage applied to it that is a very close match
to the signal it surrounds. Thus, and surface resistance from the
sensitive signal line to its surroundings sees almost no voltage drop
across it, so almost no current flows through that leakage
resistance. And since there is almost no voltage change between that
sensitive line and its surroundings, there is also almost no
capacitive current either.
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