Minimum Vref Voltage?

[financecatalyst]

I am using 16F877A for a ADC conversion from a temprature sensor LM35.
If I use VDD as reference, I get 5/1024 = 4.8mv appx resolution. I am thinking of using Vref as a reference and I would like to know how much minimum voltage can I put on Vref pin? I am thinking if I am able to put 1.1 volt then I get appx 1mV resolution. Am I thinking around the right lines or something is wrong with my planning?

I would also like to know where to get to this info from the datasheet if possible. Thanks

[Acetronics2]

a nice drawing ?

( courtesy Mikroelektronika )
ADC_limits.pdf

Alain

[HenrikOlsson]

Hi,
1.1V won't work. Look at table 17-14 in the datasheet.
It says that the minimum voltage difference between VRef- and VRef+ is 2.0V. If VRef- is connected to GND then the minimum voltage at VRef+ would be 2.0V.

However, it ALSO says that VRef+ must be at least AVdd-2.5 so if you're powering the analog section of the PIC with 5V then the minimum voltage at VRef+ is 2.5V.

Running the PIC at 3.3V and using a 2.048V reference gives you a resolution of 2mV.

Other options is to add a front end to your signal, getting its amplitude up, or use an external ADC chip.

/Henrik.

[Charlie]

To express what Henrik said a bit differently, the device datasheet says the device output will change 2.05 V over the full temperature range. If you are able to match that perfectly with Vref, then you will have the maximum resolution possible, since the full scale of the encoder would exactly match the full swing of the source voltage. So if you are going to use a 10 bit ADC, it's simply not possible to get more than 2 mV accuracy, and with quantizing errors, and offset errors, 5 or 10 is more likely. Alternatives? There are some PICs with 12 bit ADC, or restrict the temperature range, or do something creative with op-amps and multiple pins.