I am now designing a circuit looks like this:
My goal with this circuit is to compare the input digital signal with a reference signal generated by zener diode D1, so that the result could be identified by the differential I/O of MicroLabBox. The minimal difference between those differential ports is 100mV.
The problem is, when the input signal goes to LOW, some current must be drawn from R1 due to the diverse current flowing through Ri5, which leads to the drop of reference voltage at D1. The more input signals are referring the voltage at D1 at the same time, the more the reference voltage would drop.
Is there a solution to this problem?