Hi,
I am designing thermocouple input module for
PLC.Differential analog multiplexer is used
for interfacing two thermocouples and millivolt signal generated on board for performing system calibration .I am using
20 bit ADC for the same purpose .This ADC has
a differential i/p & instrumentation amplifier with i/p impedence of 1200 megaohm.
It uses Sigma-Delta technique for conversion.
It supports direct interfacing of thermocople withount any external amplifier.
It can be configured for +/- 25 mV range.
When I applies millivolt ,say 10 mV,with source connected in differential mode to ADC without multiplexer it works properly and gives cont corresponding to applied voltage.When source is disconneced ,it shows random count (expected)since ADC i/p is floating & when i/p is connected again , it gives proper count .
But this is not the case with multiplexer.
When power is applied very first time to differntial mux with its enable i/p asserted & analog signal connected to proper channel, ADC gives proper count .When mux is disabled,
ADC i/p is floating & gives random count (again expected),but if mux is enabled now with the same channel & i/p ,it never gives normal count .Even ADC is reset & conversion command is given ,it does not return to normal operation .The only way is to reapply power to ADC.

Thanks