Subject: Re: Current Source Multiplexer
Date: Sat, 23 Nov 2002 14:47:41 GMT
NNTP-Posting-Date: Sat, 23 Nov 2002 09:47:41 EST
John Larkin wrote in
> On Thu, 21 Nov 2002 13:37:59 GMT, Ken
>>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
>>sources and would have too high of a crosstalk (~25 dB) to be of much
>>use for this design.
> An HC4051 will probably work OK; they leak in the picoamps. Can't be
> specific about AC crosstalk.
> Caution: if the unused current sources are allowed to pull up to the
> 4051 Vcc rail, current will be squirted into the chip substrate diodes
> and all hell will break loose; don't let this happen.
> Uh, "1 nA" and "bandwidth" really don't belong together.
I agree that unused current source will need to be shunted away from the
switch. It is surprising that a standard analog multiplexor does not exist
to act as a "current switch" when unused inputs are automatically shunted.
BW will degrade at low currents due to parasitic and other capacitances, so
"1 nA" and "bandwidth" can be used in the same sentence :)
BTW I forgot to mention that besides switching, the main requirement is
footprint, this needs to to be small.