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Reply-To: "fred bartoli"
From: "fred bartoli"
Subject: Re: Current Source Multiplexer
Date: Thu, 21 Nov 2002 19:57:58 +0100
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Organization: Guest of ProXad - France
NNTP-Posting-Date: 21 Nov 2002 19:55:52 MET
John Jardine a écrit dans le message :
> 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
> > 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
> > design.
> > Thanks,
> > Ken
> 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
> 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
> each current source.
Simpler : one cascode output stage after the swith + N diodes to the cascode
ref voltage will do.
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