Guard Traces

What a coincidence: I've just had a problem with a circuit on some cheap stripboard and had to re-arrange the connections to a CMOS switch so that one acts as a guard trace to prevent leakage currents when the switch is open.

I'll mark you down as a "believer" then, Liz.

I don’t need to believe in them, I’ve seen them.

(And yes, anyplace a hundred picoamps of highly variable current with really horrible 1/f noise would be inconvenient, it’s worth thinking about guarding.)

Cheers

Phil Hobbs

Sometimes, they make sense.

For picoamp or femtoamp measurements, an actively driven guard can reduce surface leakage errors. Or reduce capacitive coupling from other traces.

Sometimes two fast signals might crosstalk, so a grounded guard trace between them helps, and can make both traces into matched-impedance coplanar waveguide. Lots of vias on the guard, of course.

Either case is rare.

I suppose there is a planar trace geometry with a ground on one side, a lopsided CPW. That would be fairly dispersive, which is I guess why it doesn't have a name. We occasionally use ATLC to analyze such impedances, but it doesn't calculate dispersion.

Last year I designed a low-jitter triggered 50 MHz LC oscillator. FR4 is about the world's worst capacitor (tempco around +900 PPM) so I needed to reduce the capacitance of the critical node. I did a proto PCB, and one version had a layer 2 actively driven guard, as a pour under the region. That didn't work well, the way I did it.

It is evidence-based, so it isn't just a belief. (...and it agrees with the evidence, which a lot of so-called beliefs regard as irrelevant.)

A man has to believe in something. I believe I'll have another beer.

Anybody who has read Ralph Morrison's "Grounding and Shielding Techniques in Instrumentation" - ISBN 0-47124518-6 - knows that they work. and how they work.

The ISBN is for the fourth edition, which is what I've got on my book-shelf. I read the first edition when I was a graduate student, and got my employers to buy later editions for the edification of junior engineers.

You need to know about stray capacitance, and how to calculate rough values for it, to grasp the value of guard traces, but once you have got that far it's all evidence-based understanding, rather than any kind of optional belief, or one of Cursitor Doom's demented conspiracy theories.