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The input circuit has some threshold. If the input is below this voltage, it will be seen as low (active), above it will be seen as high (inactive). I'd have to see a schematic for the RC to know precisely what this threshold is (or run an experiment), but many common logic systems switch at about 1.4V.
It is possible that the input has "hysteresis" where the high>low threshold is lower than the low>high one.
You should also realize that the threshold is not precise, it can vary from part to part and can change with temperature, voltage, etc.
The digital inputs have some pull up circuit, probably a resistor. This could affect the output of your analog circuit depending on its design.
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Don Reid
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