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I found the schematic below for a muxed thermocouple circuit from this web page. This circuit uses a mux to switch just one leg of the thermocouples. While I am sure that this works fine I have a feeling that its causing a couple problems with the noise immunity that one would normally get with a differential signal. First, each leg now has a different impedance to ground which could cause different voltages to be induced on each cable when interference is present. Second, all cabling of the disconnected thermocouples will behave as an antennas inducing noise on the negative input for all of the other channels.

Is this a correct assessment? And, besides using a differential mux, is there a reasonable low complexity, low component count way to accomplish this (muxing thermocouple channels)?

muxed thermocouples

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  • \$\begingroup\$ The CD4097B is a differential 8:1 analog mux. Either something like that or a second mux identical to the one you have will be necessary for true differential. \$\endgroup\$
    – Reinderien
    Commented Mar 31, 2020 at 3:39
  • \$\begingroup\$ Thanks. I suppose its not that much realestate difference to change the one chip. I think it goes from 16 pins to 24 pins. \$\endgroup\$
    – Jay
    Commented Mar 31, 2020 at 9:32
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    \$\begingroup\$ Be careful: the CD4097 is an old part and does not have the same analog performance as the mux you have shown. Analog analysis might rule it out. \$\endgroup\$
    – Reinderien
    Commented Mar 31, 2020 at 21:02
  • \$\begingroup\$ I think you are right. The better option would be using ADG609 which differential analog switch. \$\endgroup\$
    – Ali
    Commented Sep 29, 2022 at 23:46
  • \$\begingroup\$ @Ali This is not a full answer. I've converted it to a comment. You can add a fuller answer if you wish. \$\endgroup\$
    – Russell McMahon ♦
    Commented Sep 30, 2022 at 0:35

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