16f684 adc

[Guest]

I'm new to working with the built in ADC, and I have a few questions before I run my program on my 16f684 PIC.

First, when setting up the ADC, I noticed in other peoples code, they use:

[Ttelmah]

The place to start, is the processor .h file, where the constants used in such functions are defined. This, first shows you what options are available, and secondly, usually has a 'comment', to explain what the options do. So, for your 'VSS_VREF' definition, if you look in this file, you find the comment:

// Range 0-VrefH

While VSS_VDD, has the comment:

// Range 0-Vdd

So, the answer is 'yes'.


The 'reason' for the nomenclature chosen, actually has to do with other chips, where both VrefH, and VrefH are settable, so on these, you will have more options, like:

VSS_VDD
VREF_VDD
VSS_VREF
VREF_VREF

Allowing the four possible permutations, using either the corresponding VREF pin, or the supply, for each 'end' of the ADC range. On your chip, these are not available (the processor's data sheet, shows what options can be selected).

If you are using an external Vref, be sure to look at the DC characteristics, table 15-9, items A20, and A20A, in the data sheet.

Best Wishes

[Guest]

Great, that was exactly what I needed to know! Thanks.

Which would bring up my next question: My operating voltage range for the analog input is 1.6V-2.4V, and the VDD pin on the pic is being powered by 5V. Would it be correct to use the VDD pin as my VREF? Or would it be recommended to use the VERF pin powered @ 2.5V?

[Ttelmah]

You would potentially double the resolution using 2.5v, and 2.5v, is the minimum useable on this chip to give full 10bit resolution. A good choice.
Also the 'odds' are that a 2.5v bandgap reference will be more stable/accurate than the supply, improving things further.
You could obviously improve things further, by 'preprocessing' the signal with an op-amp.

Best Wishes