I'm using the following circuitry to protect my microcontroller input pin from the dangers of the bad and ugly outside world (a trigger signal coming from an unknown source).
The signal comes in from the right, goes through some preliminary filtering/clamping, and then enters the MAX366CSA (a signal-line protector IC, see datasheet). The output pin of the MAX366 goes straight to the microcontroller pin (Atmega2560).
For the measurement I'm using a 10x/500MHz/8pF, properly compensated scope probe. The blue curve is measured at the MAX366 output pin, The yellow curve shows the 4.5V square wave that's being fed into the TRIG IN connector.
As you can see, the rising edge is fine, but the falling edge shows a strange double-decay characteristic. After a fast drop to about 1.6V, a very slow decay with a time constant of around 500µs follows. From the MAX366 datasheet I couldn't find an obvious reason for that behaviour, i.e. their application examples never show any special requirement for the output, like a pull-down to discharge the output or something. The signal at the MAX366 input is fine and has a fall time comparable to the rise time.
Can anyone shed light onto this behaviour? Is there a design flaw on my side, or something obvious I've overlooked in the datasheet?
To improve the falling edge, would you suggest a ~20k pulldown to do the job?