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Analog Multiplexer for current

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analog_fever

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analog multiplexer

I need to design an analog multiplexer that can switch between different current sources to the output.

I am looking at transmission gate switches (Each current source connected to a transmission gate input. Only the Tgate of the required output is turned on), and do not see why they will not work? Is there a reason why we should not use Tgates switches for current muxes?
 

FvM

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Disconnecting an ideal current source (e.g. by a multiplexer) raises it's output voltage to infinity.

Fortunately, ideal current sources don't exist. But you have to consider the current source's output charateristics as well as the multiplexer's voltage rating or respective means of voltage limiting in a real design.
 

dick_freebird

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If your current source of interest is very low, then you
may find that the switch side-leakage injects an error
term you will have to deal with. Voltage sources of low
impedance would not be bothered much. You can look
at the specs and assign a maximum error, to decide.

If your currents are provided by circuitry that takes a
long time to settle, then you may want a more complex
multiplexer that routes all unconnected currents to
an image of the mux output / ADC input voltage
so that no large DC offset (that opened sources might
rail off to) has to be driven back out with a settling
time / slew rate penalty. If the sense voltage point
is always ground (or virtual ground) then diode clamps
might do. But current should only be steered to one
loop or another at the same (roughly) voltage, not
interrupted, if you want to minimize time to acquire.
 

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