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Yesterday I asked how to properly reference a differential signal (in my case from a current transformer) to ground, and got told that the impedance to ground should be matched at the location where my differential signal amplifier is.

What if in addition I have multiple differential pairs which I would like to multiplex into the signal amp using an analog multiplexer IC? The problem is just that certainly all of the pairs still need to be referenced to ground somehow before going into the multiplexer IC, but I don't want to duplicate the amp for each input pair.

Would, for example, connecting one side of each of the differential signals to ground through a ~1 megaohm resistor be suitable?

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The problem is just that certainly all of the pairs still need to be referenced to ground somehow before going into the multiplexer IC

Use 2 multiplexers; one for one wire in your differential pair and one for the other wire. Step and repeat for all differential pairs and, feed the multiplexed (common) output pair into a differential amplifier. This is common practice for wiring (for instance) a multitude of thermocouples that are fitted to several points on a machine to be monitored.

If your inputs are powered in remote locations or they might be grounded (unintentionally or not) then you have a more complex problem because you then have to cope with ground bounce issues that may either cause poor rejection of common mode noise or, may hurt your multiplexer chips.

Would, for example, connecting one side of each of the differential signals to ground through a ~1 megaohm resistor be suitable?

It needs to be done in a balanced way - both wires of each pair can be grounded via a 1 MΩ resistor prior to feeding into the multiplexers. It might also be a good idea to use 1 nF capacitors in parallel to handle a little surge energy. If your bandwidth is low, then 10 nF is also good (used for multiplexing thermocouples for instance).

It also doesn't hurt to use circa 1 kΩ series resistors into each multiplexer input either (balanced still). Place them between the 1 MΩ/10 nF capacitors and the actual multiplexer pins.

And, if you need a little extra security in measurement, reserve a few multiplexed channels for local 0 volts and (say) a nominal accurate voltage just in case there is any attenuation needed to be accounted for.

You should then consider using an instrumentation amplifier (instead of a differential amplifier) now that a multiplexer is being used.

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  • \$\begingroup\$ In my case I've just got a whole bunch of current transformers, so I imagine essentially the same as with the thermocouples---but, isn't there some worry that the floating pairs could themselves damage the multiplexers by e.g. coupling to a strong electric field from a nearby cable and e.g. climbing in voltage relative to ground? \$\endgroup\$ Dec 22 '20 at 16:20
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    \$\begingroup\$ @KeeleyHoek I've added some words as you were commenting to cover that hopefully. \$\endgroup\$
    – Andy aka
    Dec 22 '20 at 16:22
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    \$\begingroup\$ Sounds good to me! \$\endgroup\$ Dec 22 '20 at 16:22
  • \$\begingroup\$ Sorry, what is the reason for your last sentence?: "You should then consider using an instrumentation amplifier (instead of a differential amplifier) now that a multiplexer is being used." \$\endgroup\$ Dec 22 '20 at 16:36
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    \$\begingroup\$ Instrumentation amplifiers are an order more accurate than differential amplifiers but, they need a certain level of protection on their inputs (such as the resistors and capacitors I mentioned fitted before the multiplexer). The multiplexers need to be run from the same supplies as the InAmps. So, you can now use an InAmp and get benefits from this device's extra accuracy. Does that make sense to you @KeeleyHoek \$\endgroup\$
    – Andy aka
    Dec 22 '20 at 16:40

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