Maximum External Vref for ADC (PIC)

Morning all,

I am implementing data logging functionality into one of my projects and for this I am using the analog input capabilities of a PIC Microcontroller. I've done this sort of thing before, but in my previous experience I always used the internal voltage reference or set it to VDD.

This time I want to use an external voltage reference. Ideally, I'd like to use a 5V reference for the ADC, but the maximum recommended supply voltage for my PIC Micro is 4V (mine will actually be powered at 3V3).

Question:

Is it possible to use an external reference with a higher voltage than the supply voltage for the micro?

I thought that there would be a limit defined for the external voltage reference in the datasheet for the device, but unfortunately there appears to be no mention of any such limit. There must be a limit, so I'm suprised that this is not defined. Perhaps they assume I should already know that the limit will be VDD for the Micro?

Has anyone on the board got advice on this matter?

I suppose I could always try it out practically....

Brian

hi,
Look at this extract for Vref compared to Vdd

Hi Eric,

I was about to retort that this information is not stated in the datasheet for my device. Then I decided to take one last look and, to my embarrasment, it is there in black and white for all to see!!!

I don't know how I missed that. I was in an irritable mood yesterday (for some reason) so perhaps that was it...

Okay, so I'm limited to VDD then. In that case I'll be needing an op-amp stage to divide down the output of my sensor, which has a span of 0-5V.

Thanks Eric,

Brian

ThermalRunaway said:

Hi Eric,

I was about to retort that this information is not stated in the datasheet for my device. Then I decided to take one last look and, to my embarrasment, it is there in black and white for all to see!!!

I don't know how I missed that. I was in an irritable mood yesterday (for some reason) so perhaps that was it...

Okay, so I'm limited to VDD then. In that case I'll be needing an op-amp stage to divide down the output of my sensor, which has a span of 0-5V.

Thanks Eric,

Brian

Click to expand...

Hi Brian
The input impedance of the adc input pin is VERY high, so if your driving impedance is lowish''', say less than 10K, you could use a simple resistor divider.

I just found out why I missed it yesterday.

The voltage reference pins are named Vref- and Vref+ for most of the datasheet literature, but in the electrical specifications table on my datasheet they are labelled slightly differently (VrefL and VrefH). Unfortunately I used the lazy tactic of using the PDF Search function to find all instances of Vref+ when I was looking for the electrical specifications for it. Therefore, I missed the specification in that table.

That'll teach me to read the document more carefully next time.