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

From: "Barnes, Heidi" <heidi.barnes@xxxxxxxxxx>
To: "Steve Waldstein" <swldstn@xxxxxxxxx>
Date: Mon, 23 Aug 2010 10:56:30 -0500

Hi Steve,
The final surface resistivity can be specified for the assembly, and set as a 
requirement.  Some assembly/board fabricators will specialize in this if you 
want to pay extra and it is worth surveying the vendors to see what is 
available.  The design should avoid SMT footprints/components that will trap 
solder fluxes and cleaning solutions.  

A classic on low level measurements is:
Keithley Instruments, Low Level Measurement Handbook 6th Edition, 2010
http://www.keithley.com/knowledgecenter/knowledgecenter_pdf/LowLevMsHandbk_1.pdf

Triax cables with ground, guard, and signal are worth looking at...

Regards,
Heidi B.



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of steve weir
Sent: Monday, August 23, 2010 7:42 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 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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References:
- [SI-LIST] PCB materials, solders, and components for VERY HIGH impedance applications
  - From: Steven Waldstein
- [SI-LIST] Re: PCB materials, solders, and components for VERY HIGH impedance applications
  - From: steve weir
- [SI-LIST] Re: PCB materials, solders, and components for VERY HIGH impedance applications
  - From: steve weir

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

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