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From: Boris Mohar
Subject: Re: Low leakage parts
References: <firstname.lastname@example.org> <3E17D633.378A@sneakemail.com> <3E184B38.588B@sneakemail.com> <email@example.com> <3E186B3C.5BA0@sneakemail.com> <firstname.lastname@example.org> <3E18805E.4AA@sneakemail.com>
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Date: Tue, 07 Jan 2003 08:38:12 -0500
NNTP-Posting-Date: Tue, 07 Jan 2003 08:41:48 EST
Organization: Bell Sympatico
On Sun, 05 Jan 2003 13:58:38 -0500, Mike Monett
>Boris Mohar wrote:
>> On Sun, 05 Jan 2003 12:28:28 -0500, Mike Monett
>> >Boris Mohar wrote:
>> >[... good info]
>> >> Regards,
>> >> Boris Mohar
>> >> Viatrack Printed Circuit Designs
>> >Thanks for the excellent info. If you find a sub for the 3N155A please
>> >let us know.
>> 3N163 from Siliconix. The specify the IGSS leakage of <-1pA at VGS of
>> -40V. They were also used in same instrument that measured 1fA and up.
>> I would still like to find something better.
>> Boris Mohar
>> Viatrack Printed Circuit Designs
>Do you have to select devices to get low enough leakage?
Not only select but select for matched pairs
>Have you thought about the LMC662: 2fA typ, 1.3uV/C tempco? This would
>raise the minimum current that can be measured, but the drift with
>temperature is attractive for long-term meaurements.
>How important is the low-fA region anyway? Are there many instances where
>being able to measure it directly really matters?
This was for measuring Tritium in air. It was important that once the
instrument was zeroed that it stayed zeroed. Speaking of zeros, to my
dismay I discovered that some high value resistors have a non linear
voltage coefficient. At very low voltage drops the value of one Tohm
resistor start to go through the roof. So just as you zero your
electrometer your feedback resistor value increases making it even harder
Viatrack Printed Circuit Designs
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