• Guard Traces

    From Cursitor Doom@21:1/5 to All on Mon Aug 26 12:44:31 2024
    Anyone still believe in 'em?

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  • From Jan Panteltje@21:1/5 to cd@notformail.com on Mon Aug 26 13:05:28 2024
    On a sunny day (Mon, 26 Aug 2024 13:38:46 +0100) it happened Cursitor Doom <cd@notformail.com> wrote in <0qtocjp22309o1ck937mle5qa610sip3eu@4ax.com>:

    On Mon, 26 Aug 2024 13:28:59 +0100, liz@poppyrecords.invalid.invalid
    (Liz Tuddenham) wrote:

    Cursitor Doom <cd@notformail.com> wrote:

    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.

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  • From Cursitor Doom@21:1/5 to Liz Tuddenham on Mon Aug 26 13:38:46 2024
    On Mon, 26 Aug 2024 13:28:59 +0100, liz@poppyrecords.invalid.invalid
    (Liz Tuddenham) wrote:

    Cursitor Doom <cd@notformail.com> wrote:

    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.

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  • From Liz Tuddenham@21:1/5 to Cursitor Doom on Mon Aug 26 13:28:59 2024
    Cursitor Doom <cd@notformail.com> wrote:

    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.


    --
    ~ Liz Tuddenham ~
    (Remove the ".invalid"s and add ".co.uk" to reply)
    www.poppyrecords.co.uk

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  • From Phil Hobbs@21:1/5 to Cursitor Doom on Mon Aug 26 15:22:01 2024
    Cursitor Doom <cd@notformail.com> wrote:
    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
    --
    Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

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  • From john larkin @21:1/5 to All on Mon Aug 26 08:56:05 2024
    On Mon, 26 Aug 2024 12:44:31 +0100, Cursitor Doom <cd@notformail.com>
    wrote:

    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.

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  • From Liz Tuddenham@21:1/5 to Cursitor Doom on Mon Aug 26 18:39:05 2024
    Cursitor Doom <cd@notformail.com> wrote:

    On Mon, 26 Aug 2024 13:28:59 +0100, liz@poppyrecords.invalid.invalid
    (Liz Tuddenham) wrote:

    Cursitor Doom <cd@notformail.com> wrote:

    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.)


    --
    ~ Liz Tuddenham ~
    (Remove the ".invalid"s and add ".co.uk" to reply)
    www.poppyrecords.co.uk

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  • From john larkin@21:1/5 to Liz Tuddenham on Mon Aug 26 16:14:49 2024
    On Mon, 26 Aug 2024 18:39:05 +0100, liz@poppyrecords.invalid.invalid
    (Liz Tuddenham) wrote:

    Cursitor Doom <cd@notformail.com> wrote:

    On Mon, 26 Aug 2024 13:28:59 +0100, liz@poppyrecords.invalid.invalid
    (Liz Tuddenham) wrote:

    Cursitor Doom <cd@notformail.com> wrote:

    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.

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  • From Bill Sloman@21:1/5 to Cursitor Doom on Tue Aug 27 16:07:20 2024
    On 26/08/2024 9:44 pm, Cursitor Doom wrote:
    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.

    --
    Bill Sloman, Sydney

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