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!
Best Answer
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:
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.