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From: Winfield Hill
Subject: Re: Current Source Multiplexer
Date: 21 Nov 2002 06:14:52 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.11
> I have a project about to start that requires the use of a current
> multiplexer. Specifically there are N current sources ranging between
> 1 nA and 10 mA, only one of which should appear at the output.
> Has anyone ever had to deal with a circuit like this? I am hoping for
> a circuit that will not degrade bandwidth or introduce error, even at
> 1 nA (impossible I know, but I can always hope:)
A current source should appear to be a current with an infinite shunt
impedance, even at high frequencies. This means it should have minimal
capacitance. But that's not what you can expect from an N-channel
multiplexer, which likely has a capacitance of 5pF/channel at the wiper
A better solution is to use a small low-capacitance transistor and make
a single current source with switchable current programming. Most small
transistors with under say 0.1nA room temp leakage can also handle 10mA.
Tell us more about your goals for the current source and what you wish
to do with it on the lowest ranges.
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