ATtiny861A: ADC reference voltage should be 1 Volt under AVcc

adcatmelattinyavrdatasheet

I just faced with that ATtiny861A datasheet forcing me to use analog reference voltage to be at least 1 Volt under analog supply voltage:

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Let me doubt this datasheet statement for two reasons:

  1. ADC should (and will as I tested) work with reverence voltage equal to Vcc (this is one of the reference options available through ADC control registers).

  2. The same table for ATmega48A shows more reasonable ADC reference voltage range: from 1.0V to AVcc:

ATtiny48A datasheet page 310

Actually I used to believe to the info stated into datasheet. But this time I need to call this statement to question as I NEED reference voltage to be almost 5V (or at least 4.096V which in turn also out of range).

Of course one of the possibility would be to put the schematic to the real world test. But: a) if it will work under test condition it could not work under different circumstances like high temperature or slightly lower/higher Vcc b) this will take significant time for my case.

Can anyone put some words to confirm my doubts or maybe I should contact Atmel support to clarify this point?

Best Answer

Let me start by "Small chip die has specifications X and large die from same company has better specifications" is not an argument for any manufacturer. Of course the larger, more expensive die will have better stuff in many dimensions.

But, are you using bipolar or single ended channels? The page you are looking on is the limitation for bipolar channels, possibly due to some internal stage.

Two pages higher, it says for single ended channels "Aref External = 2.0V to AVcc".

Whether an internal reference track has a certain option is also not a valid argument, as you do not know all the paths internally on the chip itself. So you cannot just say "If I have it switch over internally it can do this, but externally it can't, that's bullshit, I will just do it externally", because internally it may also switch over some stages, that work better, but need to be hard-related to VCC to be able to do that. It may not be likely, but it is possible.

So if you do try it with bipolar channels and nowhere else you can find a limitation to their use that also implies a lower Vref and this is something you want to produce more than one of, then you should still at least test it to further limits than it will ever go in the real world.

For example if you make it for -20deg C to +50 deg C, it needs to be verified to work correctly for extended periods of -25deg C to +60 deg C. As such also with main incoming supply voltage. If the board is designed for 11V to 13V, it needs testing for 10V to 15V, or some such. And not just for a minute, but at the very, very least for the better part of a day at each extreme and possibly several points in between.

And for proper testing, you should test each parameter with each possible setting for each other parameter. I.e. 10V to 15V in 1V or 0.5V increments, half a day each at every temperature between -25 and +60 at 5 degree increments, in the example above.

And that needs to be done with several randomly picked items from each production run.

EDIT:

Doing tests like that with slightly less rigour would be wise, even if you stay inside the datasheet's parameters for all parts. But then you can probably stick to "Test all voltages for a couple of minutes at a temperature below zero, one above room temp and at room temp". Unless it is a very critical part, of course.

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