From: tomb@hplsla.hp.com (Tom Bruhns)
Subject: Re: Do Analog Systems Need Ground Planes?
Organization: HP Lake Stevens, WA
Lines: 25

engp2254@nusunix1.nus.sg (SOH KAM YUNG) writes:

>I understand the need for a ground plane in a digital system. However, I
>would like to know if such a ground plane is also necessary in an analog
>system.
>
>(Note: this ground plane will not be connected, except at one point, to
>the signal ground).
>
>I keep getting conflicting answers over this. Does having a ground plane
>in an analog system help reduce noise also? My system only deals with
>low frequency signals (up to 100Hz only!!!).

Well, one reason for getting conflicting answers is that it depends on
what you want the ground plane to do.  A continuous conductor is a good 
electrostatic shield.  Do you have signals on your board that need
shielding rom other things?  This shielding won't do much good for magnetic
fields, unless you make it continuous around the circuit to be shielded,
like a Faraday cage.

The flip side of the coin:  if you are working with (very) high impedances,
you could end up with capacitances to the gound plane that cause problems.
You aren't likely to encounter this at 100Hz, but at 20kHz, it can be a
real problem (one that bit me once).

