What is the lower limit of Vref+ (16F877)

[Guest]

I am using RA3 as Vref+ (and Vss as Vref-) and I'm finding that the 10-bit A/D result is garbage if Vref+ is lower than 2 V. Is this in fact how the 16F877 should be behaving? My guess is that this is normal, but I'd like to hear from someone else about this.

[Charlie U]

Table 15-12 in the data sheet lists 2.0 volts as the absolute minimum for Vref+ - Vref-, or in your case Vref.

[C Turner]

While the data sheet does specify a minimum Vref of 2 volts, I did some testing with the '876A and the '819 and found - as the data sheet implied - that accuracy began to suffer when Vref was <2 volts.

It didn't immediately go to "garbage" but the LSBs started to flake out first - which is typical with many SAR-type A/Ds.

If you *are* seeing garbage appear right at/below 2 volts Vref, I'd suspect that maybe the Vref isn't "stiff" enough. Keep in mind that a fair amount of current is pulled from Vref during conversion - and if you are simply using a resitive divider the source impedance may be too high. A relatively large capacitor (a 0.1 uf ought to do it) may help stiffen it up a bit.

(Then again, maybe the specs *really* are that tight...)