I am using a CD4051 analog multiplexer to interface with ADC.
The system multiplexes from 8 identically setup inputs.
The voltage at an Ch X pin of multiplexer appears exactly as it is on COM pin until it is below about 1V.
However, as it increases above 1 V, the offset(voltage drop) starts to gradually increase with input voltage.
2V at Ch X pin leads to 1.98V at COM pin.
The supply voltage for both ADC and analog multiplexer is 3.3V
The Ron of analog multiplexer is 125 Ohms but there is virtually no current flowing through it, since the input impedance of the ADC is 6M Ohms.
I tried removing 1K resistor connecting COM pin and ADC input, but the drop voltage increased to 30mV from 20mV.
The result is that There is about 1 % nonlinearity. What is wrong and How should I improve it?
Best Answer
Pay attention to Andy's comments on the dynamic input impedance of your ADC, which is almost surely an issue given your measurements but there is another important issue.
You state the Ron to be 125 ohms, but that's the typical value with a Vss-Vee= 15V. Your situation is completely different, and if you are actually using a CD4051, it will only barely operate properly from 3.3V and Ron is not guaranteed at all (it's only guaranteed to be less than 1050 ohms at 5V, and 1.2K at 85°C). Nor is even a typical figure given. You are well into here be dragons territory.
I suggest you add a buffer (op-amp voltage follower) after the multiplexer to isolate the dynamic loading of the ADC (typically it's a linear function of sampling rate) and substitute a 74HC4051 for the CD4051.