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Subject: Re: Guard rings and ground planes

Date: 06/19/02 at 5:16 AM
Posted by: David Ashby
E-mail: dashby@quin.co.uk
Message Posted:

In Reply to: Re: Guard rings and ground planes posted by Gillian on 06/14/02 at 8:49 AM:

Hi Gillian,

In order to know what to do with the ground plane, you have to know what voltage is on the pins you're trying to guard.

See John Dunn's earlier comment dated 06/13/02 @ 9:49pm. If you are trying to guard the inputs to a voltage follower, the guard ring has to be tied to the voltage follower output - in that case, any copper poured groundplane will have to be backed off from the guard ring. But if you have an op-amp circuit used in the inverting amplifier configuration (with the positive input tied to ground) then you will not need a guard ring - the grounded copper pour itself will provide a guard at the right potential.

If you are using through-hole components, then whatever guarding arrangement you decide to use will have to be applied around the pins on both sides of the board.

There is one other thing to watch out for: make sure your guard ring is NOT covered with solder mask! Solder mask would allow the leakage currents to flow over the guard ring. In the inverting amplifier case, if you use the grounded copper pour to provide a guard, you'll have to make a slot in the solder mask around the guarded pins to expose the metal and break the leakage current path.

Hope this helps,

David Ashby

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