I'm making a board with an AVR. Between the MCU and the connector I would have multiple lines for SPI communication. Since the environment of this board is expected to be very dry, it is needed to protect the MCU pins from ESD events.
My solution is to use a combination of a ferrite bead, a series resistor and TVS diodes as shown in the image below. But this solution cannot reduce the clamp voltage which can be 10V~30V depending on the TVS diodes. If the I/O pin was an open-drain with a large pull-up resistor, then one can place a series resistor behind the TVS diodes. But I'm not sure what I could do for SPI lines. Is there a simple way to reduce the clamped voltage? Or is there any suggestions to protect the line better?
Would it be better to use an array of TVS diodes (or an IC that contains them)?
I don't want to use Zenor diodes, unless they are better than TVS diodes for ESD protection purpose.