how to set ADC reference voltage

[bharatwalia]

Hi,
I am using PIC16F886 which has 10-bit ADC resolution.
Currently the PIC's internal Vref is set to 5.0V, which reads 0-5V as 0-1023.

But i need to set Vref to 2.5V so that it reads 0-2.5V as 0-1023 bit resolution.

How to do that?

Thanks and Regards.

[Ttelmah]

You have to apply an external 2.5v reference to pin 5, and select: "VSS_VREF" in setup_adc_ports (together with the pin(s) you are using). The ADC then uses the Vss (GND) as it's zero point, and the voltage on Vref+ as it's top point.
There is no 'internal Vref'. It is currently using the _supply_ as it's reference, with all the noise etc., this introduces.....

The _comparator_ module has an fixed internal reference, but it is only 0.6v. It also has available a resistance 'tree' allowing the comparator to be operated from a fraction of the supply, but the impedance of this is too high to feed the ADC. What you can do is read the voltage 'on' this line, or on the fixed reference line with the ADC, and taking a reading from the fixed voltage reference, then allows you potentially to work out what voltage is feeding the ADC - can be useful for battery operation. However neither allow you to operate the ADC from them.

If you look at the ADC BLOCK DIAGRAM in the data sheet (Fig 9.1), you will see that the reference for the ADC, comes either from pin AN3, or AVdd only. Note also that the _minimum_ voltage that can be used for Vref, is 2.2v (AD06 in the data sheet).

Best Wishes

[bharatwalia]

Hi Ttelmah,
Thanks