Multiplexing 8 RTD

[Vindhyachal.takniki]

1. I am using Max31865 , resistance to digital converter for interfacing PT100. (for 2 wire & 4 wire only)
2. I want to multiplex 8 rtd to it so that I don't have to use separate IC for each.
3. I have searched for analog mux like ADG708(internal resistance is 3 ohm) & DG408(RDS = 100ohm).

4.They have large internal resistance. I think these will effect the acuracy of PT100 since it have 0.4ohm/C.
5. Or they won't effect the accuracy since I am using PT100 is 4 wire configuration so drop across wires won't effect the accuracy?
6. Also for 2 wire configuration, I will use it as 4 wire only by connecting two RTD wires across RTD+ & RTD- & shorting FORCE+ with RTD+ & FORCE- with RTD-.
7. Schematic is attached. What should be best way to multiplex.
8. I think I will need 3 (8:1) multiplexer as shown in schematic. Because 4 wire of all RTD can be shorted together. Or I should use 4 (8:1) mux.

[tszaboo]

If you use 4 wires than the mux is part of your wiring assembly so it doesnt effect the accuracy. You only need 4 wire if you are aiming at extreme precision, usually self heating and other sources of noise, EMF etc. will be bigger source of error.

[SeanB]

For each RTD use the switches from a single package, so that the on resistance is equal for each switch. The resistance differs from package to package slightly, and you really want then to be both close and to track with temperature of the switch. You want a mux with lower resistance, or to use a Pt1000 sensor, which will have reduced error from the switch resistance.