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I am now designing a circuit looks like this:

schematic

simulate this circuit – Schematic created using CircuitLab

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?

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Rather than attempting to go directly into a digital terminated input you might be better off to use an external comparator.

enter image description here

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.

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