If you need to acquire a signal into a I/O board which exceeds its voltage range (10V in this example), a resistive voltage divisor is required and then a 10V bidirectioanl transil before the board analog input. When voltage is more than 10V the two zener diodes start to conduct discharging current to ground.
In the second version I put a buffer made with an OpAmp configured as voltage follower to prevent any possible leakage current into the transil which can turn it on accidentally.
