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I'm designing a set of PCBS for a midi controller and am wondering what is the best placement for the multiplexers in my design. Here are the details:

Board 1: a microcontroller (Teensy 4.0), power in, usb out
Board 2: A control panel with around 50 pots and switches, all of which will be fed into 74HCT4051 multiplexer chips and then into the Teensy's analog inputs

The boards will be connected with an IDC cable, like this one.

So, is there any advantage placing the multiplexer chips on one board or the other (ie positioning the chips closer to the pots or closer to the microcontroller)? Or should I assume that either way should work fine?

To be clear, the pots and switches will be biased with the 3.3V coming from the microcontroller.

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    \$\begingroup\$ If the multiplexer is on the Tiny board, wouldn't you have to carry (many, many) more analog signals across the IDC? \$\endgroup\$
    – Reinderien
    Commented Jan 21, 2021 at 20:50
  • \$\begingroup\$ Yes, unfortunately. \$\endgroup\$
    – Matthew O'Connell
    Commented Jan 22, 2021 at 12:33

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One problem with IDC cables is grounding and noise issues, they need to be accounted for or there could be problems with the design.

IDC cables can have cross talk between conductors, so sending the digital signals over IDC could potentially bleed into the next pin if the signals are switching and create noise (I've had this happen before).

With IDC, grounding can become an issue, the IDC is something like 26 or 28AWG which means at least ~50mΩ per foot (plus a few more 10mΩ's for the connectors on each end). If you send a 10mA signal that is switching digitally (or an IC that swtiches 10mA of current on board 2), then the ground could move ~500uA and that is almost 1bit for a 12bit ADC at 3.3V. So make sure the ground is solid by using more than one conductor (use a lot more than one pin).

Unless cost is a major factor, I'd just put one or more cheap ADC's on the other board like the ADS7952S

The best option would be to put the processor on the same board and make the IDC short as possible

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Since the multiplexers require both digital select and analog signals coming from the pots, place the multiplexers on the microcontroller board. Hence only analog signals pass through the IDC cable. Use more than one wire in the IDC cable for the ground so as to reduce common ground impedance and resulting cross-talk.

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    \$\begingroup\$ There are many more analog signals than digital signals to select the multiplexers so I would do it the other way round or as @Voltage Spike suggests. \$\endgroup\$
    – Kevin White
    Commented Jan 21, 2021 at 22:05
  • \$\begingroup\$ Kevin that is a good point. The multiplexers could even be nested so that only one analog signal return is required. In which case I would run the analog signal via a separate twisted shielded cable. Hence, use the IDC only for data/line select. The analog signal could also be driven by a differential ADC driver before going through the cable. \$\endgroup\$
    – GLK
    Commented Jan 22, 2021 at 23:32

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