Maximum External Vref for ADC (PIC)

[ThermalRunaway]

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

 

[ericgibbs]

hi,
Look at this extract for Vref compared to Vdd

 

[ThermalRunaway]

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

 

[ericgibbs]

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

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.

 

[ThermalRunaway]

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.