ADC signal conditioning circuit
From your circuit it seems that the signal to be read out, can swing from GND to 24V. From the left to the right, I would say the following: 


Schottky diode acts as a reverse voltage protection mechanism.
Bidirectional TVS diode protects against spikes and surges in the signal line.
Inductor, capacitor and 100Ohm resistor form a RLC low pass filter
The 10KOhm resistor along with the Zener diode clamps the maximum voltage, protecting the opamp.
The 10nF will take large dV/dt, which are too fast for the zener to start to operate
Opamp configured as a buffer, decouples the signal to be read out from the ADC by providing respectively a high input and low output impedance
1kOhm and 10nF form an additional RC low pass filter.