Guard Traces

[Cursitor Doom]

Anyone still believe in 'em?

[Liz Tuddenham]

Post by Cursitor Doom
Anyone still believe in 'em?

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.

[Cursitor Doom]

Post by Liz Tuddenham

Post by Cursitor Doom
Anyone still believe in 'em?

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.

[Jan Panteltje]

On a sunny day (Mon, 26 Aug 2024 13:38:46 +0100) it happened Cursitor Doom

Post by Cursitor Doom

Post by Liz Tuddenham

Post by Cursitor Doom
Anyone still believe in 'em?

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.

There are many places where guard traces make sense.
Believes have nothing to do with it, for that:
you can buy those at the local churches
Pay them enough and you are considered a good believer.

[Liz Tuddenham]

Post by Cursitor Doom

Post by Liz Tuddenham

Post by Cursitor Doom
Anyone still believe in 'em?

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.

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.)

[john larkin]

Post by Liz Tuddenham

Post by Cursitor Doom

Post by Liz Tuddenham

Post by Cursitor Doom
Anyone still believe in 'em?

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.

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.

[Phil Hobbs]

Post by Cursitor Doom
Anyone still believe in 'em?

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

[john larkin]

Post by Cursitor Doom
Anyone still believe in 'em?

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.

[Bill Sloman]

Post by Cursitor Doom
Anyone still believe in 'em?

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.