- Is the input high, low, or indeterminate when the input signal is absent?
- How long from the first edge do you need to know that the signal has returned?
- How long after the last edge do you need to know that the signal has stopped?
Assuming the answers are something like low, about 10ms, about 10ms, then the following circuit should do the trick:
R2 should be about 100k. It is a "cheap" method of protecting the gate from voltages outside the supply.
D1 can be any diode, a 1N4148 is perfectly suitable.
The circuit will produce a high output as soon as the input goes high, and remain high for 1.1RC seconds. For about 10ms response, use R1 = 1MΩ and C1 = 10nF.
U1 and U2 can be obtained in a single SOT-23-6 package, which along with everything else should be pretty small, especially if you use 0603 or smaller resistors.
edit: oh, and the input impedance of the microcontroller is almost certainly high enough that a buffer should not be required for the circuit you posted above (The 10k figure you quoted is probably a recommended source impedance, not the impedance of the A/D itself). The problem is that it responds quite slowly. If that's OK, then that's OK