[SI-LIST] Re: PCB materials, solders, and components for VERY HIGH impedance applications

  • From: DAVID CUTHBERT <telegrapher9@xxxxxxxxx>
  • To: steve weir <weirsi@xxxxxxxxxx>
  • Date: Tue, 24 Aug 2010 06:39:23 -0600

The guard traces will work better if they are bare with no solder mask
covering them.
    Dave Cuthbert

On Mon, Aug 23, 2010 at 7:34 AM, steve weir <weirsi@xxxxxxxxxx> wrote:

> Steve, you will find that active guards / shields are common  in high
> input impedance circuits.  In order to reduce leakage current, the
> adjacent conductor, which can be an inner shield on a cable, or guard
> trace that surrounds the signal trace on the PCB is actively driven by
> typically a voltage follower.  If you have a 1mV offset follower driving
> the environment around a 1V DC signal, the leakage current is 1/1000th
> what you would have to common.
>
> For over 100 GOhms, PCB surface contamination is going to be a major
> concern.  I haven't looked recently, but there used to be substantial
> literature available.  Look up charge amplifiers, and PH monitors.
>
> Steve.
> Steve Waldstein wrote:
> > Steve,
> >
> > Thanks for your reply. Could you elaborate on what you mean by an "active
> > guard ring"?
> > Also, just to restate. We are looking for impedances > 100 G Ohms, not 1
> G
> > Ohm.
> >
> > Steve W
> >
> >
> >> -----Original Message-----
> >> From: steve weir [mailto:weirsi@xxxxxxxxxx]
> >> Sent: Sunday, August 22, 2010 10:29 PM
> >> To: Steven Waldstein
> >> Cc: si-list@xxxxxxxxxxxxx
> >> Subject: Re: [SI-LIST] PCB materials, solders, and components for VERY
> >> HIGH impedance applications
> >>
> >> Steve, very high impedance circuits (1 TOhm) have typically been built
> >> on substrates like Teflon with very rigid cleaning requirements.  With
> >> an adequate cleaning process, and good circuit design:  I highly
> >> recommend active guard rings, you should be able to deal with 1 GOhm
> >> without too much trouble.
> >>
> >>
> >> Steve.
> >> Steven Waldstein wrote:
> >>
> >>> To all,
> >>>
> >>> I'm wondering if anyone has experience with PCB boards for very high
> >>> impedance application (i.e > 100 GOhm). I'm learning about Isolation
> >>> Resistance on capacitors and realizing that surface mount capacitors
> >>> might only offer ~ 1e9 ohms of isolation. I'm now beginning to think
> >>> about PCB materials as well. What kinds of leakage is there on FR-4?
> >>> Do I need to worry about this as well. I also have been told that the
> >>> right solders and fluxes need to be used as well along with good
> >>> baking to ensure all trapped moister, which can lower the impedance,
> >>> is used as well. Anyone have any experience with these as well and
> >>>
> >> can
> >>
> >>> recommend the proper types. Any and all help is appreciated.
> >>>
> >>> The application is a sensor that has a very low capacitance for AC
> >>> coupling, a very high impedance input is used to ensure a very low
> >>> highpass corner.
> >>>
> >>> Thanks in advance for any help
> >>>
> >>> Steve
> >>>
> >>>
> >> --
> >> Steve Weir
> >> IPBLOX, LLC
> >> 150 N. Center St. #211
> >> Reno, NV  89501
> >> www.ipblox.com
> >>
> >> (775) 299-4236 Business
> >> (866) 675-4630 Toll-free
> >> (707) 780-1951 Fax
> >>
> >>
> >
> >
> >
> >
>
>
> --
> Steve Weir
> IPBLOX, LLC
> 150 N. Center St. #211
> Reno, NV  89501
> www.ipblox.com
>
> (775) 299-4236 Business
> (866) 675-4630 Toll-free
> (707) 780-1951 Fax
>
>
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