[sdiy] Substitute for IMF-3958 ARP 4015 S/H SubModule

[Ian Fritz]

At 10:01 AM 5/28/2008, Dave Manley wrote:

>The guard isn't so much a plane, as a copper trace that surrounds the 
>indicated pins.  If you look at page 3 of the Arp-Tech PDF you can just 
>barely see it (the quality of the image is poor).  In the circuit, the 
>guard ring surrounds the high-impedance node where the charge is held.

Right. But more specifically, the guard ring is to eliminate current 
leakage into the high impedance node.  The guard ring is at the same 
potential as the node being guarded, so there can be no surface leakage 
current flowing between them.  Since the guard ring is at low impedance, it 
can suck up any surface current without affecting the circuit's 
performance.  (And, of course, this is a totally different thing from the 
use of a ground plane.)

Ian