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

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: Steve Waldstein <swldstn@xxxxxxxxx>
  • Date: Mon, 23 Aug 2010 07:41:45 -0700

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 application notes on:  electrometers, 
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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