analogdigital-logicgpiomicrocontroller
A general question that I can't find an answer to so easily. Why is it that each digital input pin to a microcontroller doesn't require an extra pin for ground? Each analogue input requires two inputs , + and -. I would have thought that all inputs, be it digital and analogue would need a reference voltage (ground).
I am talking specifically about using a development prototyping board. The one I am using Atmel is Xplained pro for SAMD21 chip.
Thanks in advance.
This is very simple:
Since both the voltage into the A/D input and the A/D's measuring range is relative to the PWR voltage, the PWR voltage cancels out. The A/D pin voltage will be non-linear with light intensity. Make R2 the value R1 has in the center of its range or where you want the most resolution.
The idea behind using a multiplexer is that you would read all of the inputs in a sequence very quickly and then loop around and do it again. It's a trade-off of bandwidth and latency vs. port count. It lets you divide the available bandwidth and required port count down by the same factor. If you have an ADC that can sample at 10 KSps and an 8 port mux, then with 1 input you can sample at 10 KSps on that input, with 2 inputs you can sample at 5 KSps on both inputs, with 4 inputs you can sample at 2.5 KSps on each input, etc. One side-effect is that the samples are interleaved in time as you can't look at all of the inputs at exactly the same time. With 2 inputs, there will be an offset of 1 sample period between readings on alternate channels where the first sample from input 1 will occur at 'T=0', the second sample will come from input 2 and will occur at T=100us, the third sample will come from input 1 and will occur at T=200us, etc.
If you're just trying to read from POTs, you should be fine as the signals coming out of them will change very slowly. All you need to do is write a for loop that will cycle through all of the mux settings and read in the corresponding analog value.
Best Answer
The digital inputs all share the same common ground as the microcontroller.
Not all analog inputs are differential (i.e. having + and - inputs), many microcontrollers allow you to configure them as single-ended or differential for example.
The differential inputs allow you to be more accurate for very low level signals by rejecting common mode signals and ground errors.
This is not as necessary for ordinary low speed digital signals as they have the built in noise margin from the logic level specifications.