hi elmick...
Just to add to your personal database.
As stated you don't need in this instance a semiconductor solution, just resistors.
If in future if you have the same type of problem, where you must use a zener. I'll use your present problem as an example.
You want to drop 0.4V and clamp with a 3V3Z, if your zener requires say 4mA
to operate correctly you would use ohms law to calculate a series resistor
Rs= 0.4/ 0.004 = 100R
Connect one end of the resistor to the output of your opa and the other end to the anode of the zener and the cathode of the zener to ground.
The anode is the 'bar' and the cathode is the 'triangle' of the zener.
The 'clamped' voltage is at the junction of the resistor and zener.
You can apply this method to most zeners, just look at the zener spec, see how much current it requires, subtract the zener voltage from the supply voltage and apply ohms law to find the resistance value.
Regards
EricG