Arduino ADC usage

Question

Hey all, is anyone savvy with arduino boards? 
 I'm looking to use one to perform an analog to digital conversion on a measurement signal coming from a sensor. 
 The signal range is up to around 9V so I've potential divided this down by half so any value should be readable by the adc. 
 I've then used the analog read function on a0 and then done some cals to work out the source voltage and corresponding sensor pressure. 
 What I'm struggling with is the input to the arduino, because when using the serial monitor the ADC signal outputted is seemingly random, and goes up and up then begins at zero again. 
 Is this pin float? I feel like it is.

AJJewsbury · Accepted Answer

I've done a little with Arduinos (and Raspberry Pis) 
 Presuming nothing has mis-configured the A0 pin (e.g. https://docs.arduino.cc/learn/microcontrollers/analog-input ) and everything's connected correctly I'd expect it to work. What sort of calculations are you doing? - you can certainly get odd effects if you inadvertently use integer rather than floating point arithmetic with small or large values or forget to initialize things when looping. 
 - Andy.

mapj1 · Answer

Note that the arduino ADC inputs are high Z and capacitive, and will drift all over if not solidly connected. Also they will mis-read if the transient source impedance is not low enough, as the voltage then gets pulled about by the action of the internal ADC connecting its sampling capacitors to the pin during the conversion cycles. 
 So for tests to get stability to bit 12, use a 1k pot between Vref and GND or if for power saving reasons your source impedance is more than a few k ohms decouple it with 10n or more to ground, so that during the read event, the voltage is still held more or less constant while the internal logic of the ADC switches things in and out to get its conversion compartors sorted.

Andy Millar · Answer

The other thing I'm wondering about is "a sensor" - how noisy is the sensor (and as Mike suggests, what's its output impedance)? How noisy is the environment it's working in? E.g. if it's connected to a system involving a brushed motor (even if it's not directly connected to it) it won't be at all surprising if the signal bounces around like this. It's very typical that filtering, and possibly screening, is needed on the input and then averaging (effectively further filtering) needed on the ADC output. The sensor can, depending what and where it is, even pick up the clock frequencies from the arduino itself... 
 What's also critical is how fast you need your result to be, crudely you want a big enough capacitor on the input so that the input rate of change is slowed down to the point where it's just not too slow! 
 Welcome to the wonderful world of analogue 
 Thanks, 
 Andy

mapj1 · Answer

further to that watch the voltage gradient along the earthing- just because on the circuit it is all shown connected together, note that the sensor outputs a voltage between its pins, i.e relative to 'earth' at its location. The ADC measures between its pins, so relative to 'earth' at the arduino location. These may well be at opposite ends of an accidental wire antenna, and to anything that can measure a voltage in micro-seconds, then the ends of a few feet of wire can easily be some tens to hundreds of mV apart. So if adding capacitance to the line to filter, it needs to be at the ADC end, and with a definite resistance in series to give it something to chew on. Or better bring the earth along the same path as the signal, concentrically (coax) so that the two sides of the signal experience pretty much the same disturbances. There is a very good reason really delicate or fast signals are sent down lines as differential signals ;-) 
 Mike.

Arduino ADC usage

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