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TMUX1309: Is it advisable to use this multiplexor to connect RTD's?

Michael Ratterman
Prodigy 10 points
Part Number: TMUX1309
Other Parts Discussed in Thread: SN74CB3Q3253, TMUX1109

Hi,

I am designing a 4 channel RTD sensor and I would like to multiplex the inputs of the 4 RTDs.  They are all 3 wire PT100's.  

Can I use the TMUX1309 to do this?  Do I need to add Ron in software to compensate?

Thanks!

Mike R

  • 0
    Guru 276620 points

    Which of the 3 wires are switched? The 3-wire connection can compensate for the lead resistance, but assumes that the leads have the same resistance. In your circuit, the Ron is part of the lead resistance.

    You could use a switch with lower Ron like the TMUX1109 or SN74CB3Q3253, but the Ron is still noticeable. If you cannot use a 4-wire connection, you probably have to calibrate your measurements.

  • +1
    TI__Genius 10552 points

    Hi Mike,

    Thanks for reaching out and welcome to the forum! So in this case the TMUX1309 is not ideal due to the relatively high on resistance and leakage on this part. The resistance component can cause unwanted attenuation and high leakage can cause offsets larger than the voltage you want to measure. As Clemens suggested, I would go with the TMUX1109 here. You'll have a low and flat on resistance curve and the added bonus of this being a precision part which means very low leakage. 

    Let me know if you have any other questions!

    Thanks!

    Alex 

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Thanks Clemens!  I will look into this.