Voltage drop by reference voltage generated by zener diode
Rather than attempting to go directly into a digital terminated input you might be better off to  use an external comparator.

You can use a 2V zener for a reference, but I would suggest an active reference such as the LM4040-2.048, which is much more stable.

Alternatively you can use your current circuit with an active reference such as TLVH431 (with two resistors to set the 2.0V voltage) and run it at relatively high current such as 25mA (reduce R1), so the current from the input resistance has negligible effect on the reference voltage.