[SI-LIST] Re: Electrostatic HiZ PCBs

Not my area of experience, but I believe that electrometers are generally
very low frequency devices (near DC).  If that is the case, any FR-4 like
product would not seem inappropriate.  Is the product simple enough that the
high Z section could be built discreetly on PTFE stand-offs and the low Z
stuff done on FR-4?

Just a thought.....

With respect,

Brent DeWitt
redmond, WA

> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of bbolton
> Sent: Monday, July 18, 2005 11:03 AM
> To: si-list
> Subject: [SI-LIST] Electrostatic HiZ PCBs
>
>
> Hello SI-LIST,
>
> I am looking for advice on minimizing surface and resistive
> parasitic leakages
> for a fempto-amp electrometer
> design.
>
> At the moment, teflon PCBs look attractive due to their
> hydrophobic (water
> wicking) property and consistently
> high volume resistivity (10^17 to 10^18) whereas FR4 is (10^10 to
> 10^17 Ohm).
> It seems that if one could
> constrain the fabrication of an FR4 PCB to achieve 10^17, that would be
> preferred over teflon due to
> manufacturing difficulty.
>
> How would I specify a very low loss (high volume resistivity) FR4
> PCB?  Which
> layer materials or vendors
> (rogers or others for the dielectric layers) would be used to
> specify such a
> PCB?
>
> Does anyone know of a vendor who would produce a teflon PCB in
> the case that I
> chose to go that route?
>
> Any advice on hydrophobic coatings would be greatly appreciated.
> I know that
> Humiseal makes this sort of
> product, but have not investigated the dry resistivity of thier products.
>
> This is an electrostatic analog signal integrity versus a low-Z
> digital SI
> type of application.  I'd be glad to know
> about other list-serves that might be better for this sort of application.
>
> If you can advise specific shielding or lessons-learned or good books /
> articles on this topic, I'd be glad to
> know.  It seems that these issues might be addressed in
> capacitive sensing
> applications.
>
> Best Regards,
> Bryce Bolton
> bbolton at vt dot edu
>
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