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From: "John Jardine"
Subject: Re: Current Source Multiplexer
Date: Thu, 21 Nov 2002 18:06:04 -0000
NNTP-Posting-Date: 21 Nov 2002 17:54:33 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
Ken wrote in message
> 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:)
> Typical multiplexers I have seen are intended to operate on voltage
> and would have too high of a crosstalk (~25 dB) to be of much use for this
Main problem with switched current sources is that when unused they will
saturate at a supply rail in a vain attempt to maintain their now unwanted
current. When switched into circuit they then need a long time to come out
of saturation and into control.
If all unused sources are switched to a voltage near to their normal
operating point (eg 0V). Then little voltage slewing is required once they
are switched into circuit. This current changeover can be very fast.
Downside is that 2 switches plus an inverter (eg 4066 type) is required for
each current source.
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