ADG406: Unpowered usage

Q

Two identical circuits, with 24V source and 32 input channels must sample the
same 32 switches to detect which are open and which closed. One of the
identical circuits is a backup & will be unpowered. So how do I mux the 32
signals so that the unpowered side is not damaged by the powered side. e.g.
the unpowered mux will see the 24V at its inputs (can be potted down).

Will the ADG508 do the job and not be damaged by having say say +12V on its
input pins when it is unpowered?

A

In general, CMOS switches are destroyed when a signal is applied in the absence
of a power supply. There are parasitic diodes which are forward biased and
conduct heavily if not limited by series resistance. As well as heating up the
switch, this will also mess-up your measurement of the other channel because of
the lost current.

Some switches are more robust than others but it's not a good solution.

see following article for an explanation
http://www.analog.com/library/analogDialogue/archives/35-05/latchup/index.html

The correct solution is to use fault protected switches.

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