PIC18F4520: What is Vref range?

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PIC18F4520: What is Vref range?

Postby vkj » Sun Dec 18, 2016 5:44 am

What is the max. value of Vref (ADC reference voltage)? Acc. to the datasheet, The low is Vss - 0.3V. That is clear. But what is the high value?

The datasheet for PIC18F4520 says (param. A21, pg 358), VREFH (reference voltage high) min. = Vss, VREFH max. = VREFH. I dont understand what that last figure (max.=VREFH) means.

The datasheet for PIC16F877A says: (param A21, Vref+): AVDD + 0.3V. Tho Im not sure what AVDD is, I guess that's the same as VDD + 0.3V.

Im wondering because the usual 5V supply to power the chip comes from an LM7805, which is in my case is about 4.95 V, while Vref is exactly 5V. Is this a problem?

Thanks,

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Re: PIC18F4520: What is Vref range?

Postby t1d » Mon Sep 16, 2019 2:00 am

Well, I don't know the answer to what is the greatest value that Vref can be, but I am pretty sure 5v is no problem. But, I do know that if your supply is less than your Vref chip, you have a problem...

The Vref chip will have its own power input requirements... Just like the LM7805, you will have to supply something more than the desired reference voltage, to cover whatever voltage drops the Vref IC might have. That information is in the Vref IC data sheet. It could be that the same voltage that you are supplying to the LM7805 would do to supply the Vref IC directly.

You probably knew these things. Please let use know what Vref IC you are using and what is your rectified supply voltage value. Maybe post the data sheet for the Vref IC.
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Re: PIC18F4520: What is Vref range?

Postby ric » Mon Sep 16, 2019 5:29 am

vkj wrote:What is the max. value of Vref (ADC reference voltage)? Acc. to the datasheet, The low is Vss - 0.3V. That is clear.

You're looking at the "Absolute maximum" ratings, which is what you have to stay well clear of to avoid device damage.
They are not the function specifications that tell you what levels it will work correctly at.

Spec "A20" tells you that at a supply voltage of 3.0V or greater, VREH-VREFL must be 3.0V or greater.
That means the minimum Vref value is 3V.

But what is the high value?

The datasheet for PIC18F4520 says (param. A21, pg 358), VREFH (reference voltage high) min. = Vss, VREFH max. = VREFH. I dont understand what that last figure (max.=VREFH) means.

Yes, that is a confusing an totally unhelpful spec. I think it's a typo.
The PIC18f4550 is a very similar chip, and in that datasheet the maximum is listed as "VDD", not "VREFH".

Im wondering because the usual 5V supply to power the chip comes from an LM7805, which is in my case is about 4.95 V, while Vref is exactly 5V. Is this a problem?

I suspect you won't get correct readings up close to 5V.
As t1d just queried, how are you getting a reference voltage higher than VDD?
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