How to select Vref?

[Sid2286]

Hi,

I'm using PIC18F4520 for my project. My input for ADC is 200mV to 1.2V.
Now I want to give vref to my microcontroller. It has to pin RA3/AN3/
VREF+ and RA2/AN2/VREF-/CVREF pins.

So how do I give Vref? Should I apply 1.5v to only VREF+ and keep open VREF- or should I ground the Vref- pin??

Please help.

Rgds,
Sid

[Mike Walne]

The data sheet shows how to do it.

You have to set the code and hardware to match.

To get maximum resolution you could connect a source of a little over 1.2V to Vref+, and one of just under 200mV to Vref-.

Mike

[Ttelmah]

First read the data sheet.
What is the _minimum_ Vref the ADC can use (Table 26-24 Parameter A20).

So, your 1.5v Vref, won't work.

Then understand that the Vref output on the PIC, is _not_ suitable to drive the ADC Vref input. Note the Vref Input current (A50 in the same table). Then look at the Vref output, and how it is generated (resistor tree). To use this to drive the Vref input, you must buffer it.

The internal reference is also derived directly by division from the power supply. No better accuracy.

This is why the internal reference module is called the _comparator_ voltage reference, not a generic voltage reference....

For best accuracy, use an external 3v bandgap reference to drive the ADC Vref, and an op-amp buffer to feed your voltage source into the chip. You can perform the operation (Vin-0.2)*3, with an op-amp, and a feed from the same reference voltage.

Normally feed Vref-, from the connection at the bottom of your reference source. Using the processor 'ground', may be perfectly adequate, depending on what noise sources are present, and how good the system 'ground' is.

Best Wishes

Best Wishes

[Ttelmah]

[Mike Walne]

[Ttelmah]

Yes, it's the highest span requirement I've seen on a PIC.

However, given a low level signal like this probably won't meet the impedance requirements of the ADC, it makes a buffer amplifier the 'way to go'.

Best Wishes