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

  • From: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
  • To: "'steve weir'" <weirsi@xxxxxxxxxx>, "'Steve Waldstein'" <swldstn@xxxxxxxxx>
  • Date: Mon, 23 Aug 2010 08:57:31 -0700

All

Keithley has a great little book about making low level measurements that
has a lot of detail about designing for high impedance circuits.  "Low Level
Measurements Handbook".  The 6th Edition is the current one.

Regards,

Tom Dagostino
Teraspeed Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@xxxxxxxxxxxxx 
www.teraspeed.com 

Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827
www.teraspeed.com  


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of steve weir
Sent: Monday, August 23, 2010 6:34 AM
To: Steve Waldstein
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: PCB materials, solders, and components for VERY HIGH
impedance applications

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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