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Subject: Variable resistances in the high-megaohm range

Date: 08/11/03 at 2:36 PM
Posted by: telefrog
E-mail: industrycommunity[AT55]chadgibson.com (replace [AT55] with @ to reply)
Message Posted:

Hello,I'm working on developing an analog simulator for a "pH" probe. The probe itself is a relatively simple design; it consists of two conductors which are immersed in a fluid. We bias these conductors at 0.7V, and measure the current flow inbetween. The current rides between 5-125nA (this requires a conductor/conductor resistance between ~6M-140M). +0.7Vo------------- | \ / 6M to 140MOhm variable \ / |o-------------My first thought is to develop some sort of CR source with a FET, but I have not been able to find anything with a negligible leakage current. A digitally controlled pot in the 200M range would be ideal, but this doesn't exist either! I need the ability to change the value electronically, with a minimum resolution of 1MOhm.Any thoughts? Perhaps some thermal, optical, or electromechanical options to consider? :)Thanks in advance!Chad

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