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I am designing a PCB for analog multiplexing. I want to protect the signal lines against ESD and overvoltage.

The voltage in the signal lines shouldn't exceed the limits of the two supply voltage levels of the circuit by more than 0,5 V to prevent damage to the multiplexer IC. The supply voltage levels are in normal operation +/-5 V, but in case a lower voltage is supplied or the supply is completely disconnected, the signal line should still be protected. I already have a reverse polarity protection for the supply voltage input lines.

Since the wanted clamping voltage is dependent on the actual supplied voltage, an ESD or TVS diode with a fixed working voltage doesn't seem suitable. Instead I thought of using Schottky diodes, but I am not sure which Schottky is suitable for handling electrostatic discharge.

Maybe you have a suitable diode for this use case in mind, or even a better idea for protecting the signal lines.

Edit: The Multiplexer is part of a voltage divider. So I do not want to introduce a series resistance in the multiplexer signal lines.The ESD rating of the Multiplexer IC is for HBM +/-1,5kV and for CDM +/- 500V.

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  • \$\begingroup\$ Why do you say 5V TVS diodes (back-to-back) should not be suitable for protection at ±5V working voltages ? What does the device manufacturer say about the multiplexer's susceptibility to ESD discharges ? \$\endgroup\$
    – citizen
    Nov 10, 2021 at 12:40
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    \$\begingroup\$ If the absolute value of the supply voltages are lower than 5V or the supply is disconnected then the Multiplexer signal line can raise to a value exceeding the supply voltage limits by more than 0.5V before even reaching the working voltage of a TVS diode of +/-5.5V. The ESD rating of the Multiplexer IC is for HBM +/-1,5kV and for CDM +/- 500V. \$\endgroup\$
    – Ludwig S
    Nov 10, 2021 at 12:59
  • \$\begingroup\$ Yes I would agree for over-voltage protection due to somebody connecting an overvoltage on the signal line. As for suggesting that an ESD discharge around the signal line(s) would not also raise de value of the supply voltage in the vicinity (talking about very fast pulses) that is not so sure... \$\endgroup\$
    – citizen
    Nov 10, 2021 at 14:08

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You are correct: two Schottky diodes, one to each supply rail. If the supplies are present, then the input is clamped to -5.5 V to 5.5 V. The the power is off, they are clamped to -0.5 V to 0.5 V. However, you also need a 5 V clamp across each power supply, to prevent the input from driving the supply voltages to in excess of 5 V. Also, a 100 nF capacitor on each supply to absorb ESD. Do not forget a resistor in series with the input, to absorb the voltage and limit the current.

schematic

simulate this circuit – Schematic created using CircuitLab

which Schottky is suitable for handling electrostatic discharge.

Any Schottky diode will work, as long as you limit the current with a series resistor.

For DC lines, ESD is easily clamped with a 100 nF capacitor to ground. For signal lines, you can't put 100 nF across the input, so you use the Schottky diodes to isolate the signal from the DC across the 100 nF capacitor.

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    \$\begingroup\$ Thank you for the very detailed answer and the good ideas! But since my analog multiplexer is part of a voltage divider I shouldn't introduce any series resistance to the signal line. It's also possible to put current limiting resistors after the Schottky diodes to ground, but the on-resistance of my analog multiplexer is only 4 Ohms, so I wouldn't protect the multiplexer IC if the multiplexer signal line is conducting, because the signal line to the IC has a much lower impedance than the protection line. Seems like I need to find a Schottky diode, which can handle this without a resistor. \$\endgroup\$
    – Ludwig S
    Nov 10, 2021 at 11:04
  • \$\begingroup\$ > I shouldn't introduce any series resistance -- Well, how about a ferrite bead then? practically 0 Ohm. However, it will only work for ESD and a bit or RF. If DC or AC is applied to the input, it will blow up components. There are circuits that disconnect the input if the voltage is excessive, but they are very complicated. \$\endgroup\$ Nov 10, 2021 at 16:18
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    \$\begingroup\$ That is also a very good idea. But actually there is ac and dc on these multiplexer signal lines, so I will try a combined solution: Ferrite bead and Schottky Diodes. Thank you for the input. \$\endgroup\$
    – Ludwig S
    Nov 15, 2021 at 11:48
  • \$\begingroup\$ @Ludwig S -- >Thank you for the input -- feel free to up-vote all of the people who helped you, as I up-voted you. That's how it works in stack exchange \$\endgroup\$ Nov 15, 2021 at 14:54
  • \$\begingroup\$ Wow! I didn't realize that you needed 15 credits to vote up. Thank you for making me aware of it. I hope it helps that I upvoted you. \$\endgroup\$ Nov 16, 2021 at 15:48

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