LTC2984 circuit design for 100x PT100 RTDs sensors

Dear Technical Experts,

I am having a project that required to monitor 100x PT100 RTD sensors within the same circuit board.
In order to save the number of circuit traces and simplify the design work.

For 2 wire RTD configuration, will there be any possibility to group 100x RTDs return common together ? 99 Source +1 Common Return.
Very much appreciate of any advise.

    •  Analog Employees 
    on Apr 27, 2020 10:58 PM 6 months ago

    Hi

        Yes, you could wire the RTDs as single-ended (one leg connected to GND / common, see the thermistor section of the datasheet) and multiplex the pin that connects Rsense to RTD element and to the input channel. Configure the LTC2984 with Rsense on CH2 and the 2W RTD, no sharing, no rotation, on CH4. 

        

       However! Unlike thermistors where the parasitic resistance of PCB traces, mux switch resistance, ground return resistance, are much less than the resistance of the thermistor, accuracy for the 2W RTD will suffer. The mux should have as low an Ron as possible to minimize errors, or you can add an additional mux and mux the excitation current from Rsense separately to the potential across the RTD.  You will need to ensure the ground return is low impedance and that there aren't other circuits with large currents flowing through that section of ground plane as these would cause a shift in the GND potential relative to the LTC2984 - this will cause an error in the RTD reading. 

    Hope it helps,

    -Logan