Increase accuracy of ADC without device AGND

Question

I am using the ADC of the PIC16F18344 —

http://www.microchip.com/wwwproducts/en/PIC16F18344

It has neither Avdd nor Agnd, but it does have selectable Vref+ and Vref- pins. If I were to stop using chip Vdd and Vss for ADC, dedicate a regulator to Avdd and Avss, run these lines to the Vref pins, and select them in the firmware, would I see accuracy benefits equivalent to a device that had dedicated analogue supply pins? I think, for this application, noise immunity is more important than absolute accuracy. There is also calibration supported, as well.

Currently the ADC is connected in simple voltage divider/"ratiometric" configuration with a rheostat:

^{simulate this circuit – Schematic created using CircuitLab}

I am using all 10 bits of the result, although I can tolerate some error in the least-significant bits. I am using a software median filter to cancel noise and it works fairly well. The sampling rate is quite low, perhaps two or three times a second. There is a passive T filter before the ADC; the chokes are actually approximations of ferrite cores.

Answer 1

would I see accuracy benefits equivalent to a device that had dedicated analogue supply pins?

Well, the data sheet isn't that great. For instance it doesn't obviously tell you what would be the accuracies if you used the software selectable internal voltage reference so, ignoring that and basing my answer stricly on the question asked, the table on page 425 provides some insight: -

The basic accuracies are shown within the blue box and could be up to +/-6 LSBs. With a voltage reference of 3 volts (Vdd = 3) one LSb is 2.93 mV and so a reading could be accurate to +/- 17.6 mV.

If your input signal is such that it needs to use most of 3 volt span then you are stuck with an accuracy of +/- 17.6 mV. However, if your input signal might only range between 0 volts and 1.8 volts then one LSb is now 1.76 mV and you have an accuracy of 10.6 mV.

Generally it's better to use an external reference but, if that voltage reference is 3 volts then expect no improvement other than that due to it being stabler that the voltage regulator that produces Vdd.

However, if you are measuring a signal that is ratioed to Vdd then use Vdd as the reference because then you get the benefit of a ratiometric conversion (if needed).