Re: Isolate or not?
In fact u will not need real OTA and can make a VCCS with two opamp like LM358!
Check the Pic.
u can make it with single supply also, here iss the concept only!
each of your user have a ckt like thiss, the output of several uer (in current not voltage!) i added in a node wichi is connected to a resistor, use a simple buffer for reading the voltage.
for analog switch u can use 4016 or 4066 and supply voltage of up to 15 V, or use small/ low-cost reed relay; I think it iss better tree like addresing with binary coding iss a good solution for usser routing!
BEST!