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I have a board which has its microcontroller pins directly breaking out to a breakout header. I am fairly certain the board won't be in ESD safe environment when being tested.

I decided to get fairly standard ESD diodes and placed it across all Digital pins. They have capacitance in the pF's so shouldn't cause any distortion there.

My question is if it is okay to place ESD diodes on analog inputs to the micro. Is there any way i reduce acuracy of these inputs?

I have been told in the past to avoid using ESD diodes on very high speed signals (in my case it was MIPI-DSI), as the very low capacitance can become significant. I am just wondering if there are other cases (analog inputs being one) where this concern should be taken?

Thanks!

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3 Answers 3

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First I'd like to stress that all microcontroller chips already have on-chip ESD protection, without that they would become almost impossible to handle.

But it is indeed a good idea to add extra protection to pins which can be touched from the outside and/or interface with other boards etc.

You can just treat the Analog pins the same way as the Digital pins and add the same protection. Under normal operation the extra ESD devices will not conduct and therefore cannot harm the signal. You will not reduce accuracy at all. Again let me point out that the pins will already have ESD protection inside the chip. Typically that is a circuit like this:

enter image description here

The 2 diodes on the left are additional external diodes, not how they are in parallel with 2 of the on-chip diodes.

Only for very high frequency signals either analog or digital (like MIPI) ESD protection can influence the operation. But very often cables and PCB traces add more capacitance that the small ESD protection devices.

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  • \$\begingroup\$ Would it not be influenced by the really small analog signals either? Leakages in the diodes? \$\endgroup\$
    – Hasman404
    Jul 19, 2018 at 10:31
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    \$\begingroup\$ Define "really small", in general you would not feed a "really small" and easily disturbed signal directly to an analog input of an MCU. You would first amplify the signal. Also diode leakage is a current and generally MCUs have voltage inputs. Also leakage is DC, not signal. If the noise of an ESD diode is an issue then your signal is too small and/or at a too high impedance level so it should be amplified and/or buffered first. \$\endgroup\$ Jul 19, 2018 at 10:35
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    \$\begingroup\$ The on-chip ESD protection is generally only intended to protect from ESD events that might happen during manufacturing. It's almost never sufficient to handle the kind of events that happen in the field when a circuit node is accessible to human contact or is having cables plugged in or out of it. \$\endgroup\$
    – The Photon
    Jul 19, 2018 at 16:21
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If the signals fed to your analog inputs are really small, you should be aware that ESD diode will have its leakage current (nano amps or micro amps) and that will create shot noise, especially when there is a high DC bias on the input.

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    \$\begingroup\$ This is generally solved by amplifying and/or buffering the small/weak signal before it reaches any long wires and/or ESD protection. Feeding small/weak signals directly (without buffering) into a microcontroller is asking for trouble. Experienced circuit designers just don't do this because it is a recipe for disaster. Once buffered the diode leakage is irrelevant as the buffer will force the voltage whatever the leakage or noise current from the ESD diode. \$\endgroup\$ Jul 19, 2018 at 10:41
  • \$\begingroup\$ @Bimpelrekkie ESD protection needs to be on the circuit node that is accessible to the outside world. Putting it after an amplifier defeats the purpose. \$\endgroup\$
    – The Photon
    Jul 19, 2018 at 14:51
  • \$\begingroup\$ @ThePhoton This is how I envisioned the setup: sensor => amplifier => connector => cable => connector to other PCB => ESD protection => MCU. i.e. the amplifier is on a different board \$\endgroup\$ Jul 19, 2018 at 17:48
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Actually, there are low-capacitance ESD diodes for high speed signals. But TVS Diodes are maybe what you need.

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