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I need to design a circuitry with at least 16 inputs used to capture low speed digital input signals (such as buttons with dry-contact). I'm also planning to use a multiplexer/demultiplexer (such as 74HC4051-Q100) to save on GPIO-s. Question: if I use a multiplexer with built-in ESD protection, do I still need additional protection on the inputs (opto-isolation or other)?

Please consider that I'd like to avoid the need for multiple dozens of independent components (like 16 x 2 resistors and 16 x 2 diodes, etc. added separately) but I prefer to use fewer components if possible.

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  • \$\begingroup\$ Your measures to protect the inputs should scale pretty well with how disastrous a failure would be, so this is kind of hard to answer. \$\endgroup\$ Feb 13, 2023 at 16:29
  • \$\begingroup\$ Also, what sorts of transients are you expecting? For example, is this a medical device that needs to survive a defibrillation event?? \$\endgroup\$ Feb 13, 2023 at 16:30
  • \$\begingroup\$ So I plan to have relatively long wires (20-50 meters) from an external button (door opener) to the PCB where I'd like to capture the button push signal. The PCB will be implemented in a fixed box in a building, so no "extreme conitions" are expected. \$\endgroup\$
    – balas
    Feb 13, 2023 at 16:38

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If I use a multiplexer with built-in ESD protection, do I still need additional protection on the inputs (opto-isolation or other)?

Depends on how much ESD protection you need, the 74HC4051-Q100 provides up to 2kV (the datasheet says it exceeds 2kV, by how much, who knows). So if the design needs to be tested or compatible to 8kV (you will send this in to an NRTL and they will test it with an ESD gun) you may want to put diodes that can handle the higher rating. If you have a chassis ground its best to shunt the ESD to chassis ground instead of PCB ground to keep the currents from the ESD events off the ground plane of the PCB.

Another option is if you have user accessible buttons, some buttons come with a pin that is connected to the case that can be used to shunt ESD events to ground. If you can find buttons like this you could potentially eliminate the ESD diodes as any ESD from user accesible buttons would be shunted directly to ground (also a good idea to connect the frame of the button to chassis ground and not pcb ground)

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  • \$\begingroup\$ Thank you! Would that mean in "normal circumstances" (like an office building with wires going in parallel to 230V power lines, or other low-voltage wirings) a 2kV ESD protection would be enough? Also if I'm adding extra diodes, is there a recommended type I should use? I guess also a line resistor (eg 1K) and a capacitor to ground would be benefitial? \$\endgroup\$
    – balas
    Feb 13, 2023 at 18:03
  • \$\begingroup\$ No, the human body ESD model is a 100pf cap charged up to a few kV and then discharged into the test device. 230V from AC mains would fry anything it touches until the breaker or fuse trips it. ESD diodes are not for noise immunity. If you need noise immunity use filtering and good layout practices (avoid loops that can be antennas, use a continuous ground plane). Yes a 1k with a capacitor to ground is a low pass filter and could be a cutoff for high frequency noise. \$\endgroup\$
    – Voltage Spike
    Feb 13, 2023 at 18:25
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There are dedicated ICs available for these. They are commonly used in industrial applications.

Commonly, they feature isolation (if you dont need it, just bridge it), over-current protection, short-circuit protection, reverse polarity protection, ESD and surge protection, status monitoring, interrupt generation, power path protection and a SPI interface.

Whatever 'low-speed' means too you: Have a look. They are the best value in terms of 'I have a One-Off with specialized requirements and need it to work - 15 bucks extra is not an issue'.

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    \$\begingroup\$ "There are ICs available" is not useful without giving a name or search terms. \$\endgroup\$
    – Ben Voigt
    Feb 13, 2023 at 17:46

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