AVR ADC internal reference voltage query

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munish4u911ex
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Posted by munish4u911ex: Thu. May 13, 2010 - 01:04 PM
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Hi all,
whenever we use internal reference voltage of 2.56V in AVR (specifically mega8), does it depend on supply voltage? No probs for 5V Vcc but I want to run m8 on 3.7V & I am unsure if it would work. Datasheet just says:

Quote:
The internal 2.56V reference is generated from the internal bandgap reference (VBG) through an internal amplifier.

Internal Voltage Reference is specified as (2.3, 2.56, 2.7) V (min,typ,max) but 2.3V under what Vref??

Above what threshold should I maintain Vcc so that Internal 2.56V is accurate?
I want my battery operated voltmeter to report accurate ADC conversions regardless of the dip in battery voltage. :roll:

-Regards,
Munish

-
Regards,
Munish

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Learning and Information, General Electronics, ATmega8
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alwelch
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Posted by alwelch: Thu. May 13, 2010 - 04:39 PM
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It will probably be OK at 3.7 but I would put a DVM on the reference pin and vary the VCC and see what happens.
Also you could go look up some stand alone band gap reference parts. There is usually a supply rejection spec. Most do not show a spec for this. So just lower the VCC and measure what you get at different VCC settings. Probably the variance will be 10 millivolts max.

EDIT: This Atmel appnote says it is "slightly dependent" on supply voltage but nothing more specific..

http://www.atmel.com/dyn/resources/prod_documents/doc2559.pdf

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