Choosing the right ADC Tad

I'm trying to make my ADC as accurate as possible - in the datasheet for the PIC18F2550 it seems to indicate that the Tad value must be as small as possible, and then later there's a table (page 273 https://www.electro-tech-online.com/custompdfs/2010/01/39632e.pdf) that gives the maximum Fosc for each given Tad. I was using a 2 Tad speed with a 8Mhz oscillator speed and yet everything still works... does it really matter what Tad setting I use? Which should I be using?

Also with ADC accuracy, is it more accurate to use the Vref+ and Vref- pins even if all I do is connect the Vdd and Vss rails to them?

Hi,

You can use this handy little utility to calculate your adc values for you.
**broken link removed**

As for ADC accuracy, well thats a subject and a half that I will leave to other more knowledgeable members, however look at Nigels ADC tutorial where he uses a voltage reference diode which is very simple but much better than the pics voltages.