That is my circuit:

Input: 400V DC

The circuit behavior is:

1: resistive partitor with high precision resistor to decrease the input voltage.

2: zener diode protect from eventually overvoltage.

Now the question:

In point 3 is useful to use a low pass filter for clean the DC on ADC input from eventually high frequency noise? If yes, how can I connect it for a proper functioning?

• A capacitor across pins 2 and 3 will do the job. The governing resistance is the 1k, so the low pass filter will start to cut-off when $\omega = \frac{1}{RC}$ – Andy aka Jul 24 '14 at 11:46