I’ve just spent a bit of time looking for something suitable and my conclusion is:
a) 3.3V
b) SOT23 (or anything similar shape)
c) Low capacitance
Pick 2.
The only part that’s anywhere near is the RClamp0502 but that’s 5V, it’s the wrong size and I have no idea where to get one.
So…I was thinking: How about a hybrid solution?
Put a PESD3V3U1UT where the existing diode is, it gives 3.3V clamping on both channels plus ESD protection on one of them. Then add a 2-pin ESD protection diode across the input connector on the other channel (where the connector is soldered to the board). It doesn’t matter really what the voltage is for this diode so it’s no problem finding something with single picofarad capacitance (or less).
Any thoughts?