Subject: Re: Current Source Multiplexer
Date: Sat, 23 Nov 2002 13:09:42 GMT
NNTP-Posting-Date: Sat, 23 Nov 2002 08:09:42 EST
Winfield Hill wrote in news:firstname.lastname@example.org:
> Ken wrote...
>> 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.
> - Win
The application is intended to select one of 8 (or more) PIN diodes, which
are current sources. These PIN diodes detect light in a fibre system. The
photocurrents can range between 1 nA and 10 mA, a huge range. Output should
settle within 500 nS for the high currents, but 4-5 uS is acceptable for the
low current sources.