AW: [Icc-avr] Qry :AVR AVREF Value?

[Johannes Assenbaum]

AREF is derived from the bandgap voltage, that drifts by temperature and Vcc mainly. See figure 166 in mega8 datasheet for typical characteristics.

Re initial precision, I think 2,605V @ 5V vcc is fair for that very low cost solution.

For higher precision, actual AREF must be known to adjust ADC results by software. To get the actual AREF voltage, both measuring AREF at AREF pin and sampling a known voltage from distinct ADC input are common, depending on lot size.

Regards, Johannes


> Hi Gerhard,

> I am sure you checked the correct setting of: ADMUX - REFS1:0: Reference
> Selection Bits ?

> ...
> But I just checked the datasheet (mega8 for example) and found

> 	VINT Internal Voltage Reference min 2.3 typ 2.56 max 2.7 V

> but I did not found any note on the causing parameters (temp, .....)
> hmmm....

> Best, Stephan

> ________________________________

> Von: icc-avr-bounces at imagecraft.com
> [mailto:icc-avr-bounces at imagecraft.com] Im Auftrag von AMS Solutions -
> GJ Laubscher
> Gesendet: Dienstag, 4. September 2007 10:14
> An: ImageCraft UserGroup
> Betreff: [Icc-avr] Qry :AVR AVREF Value?


> Hi All,
> Using a mega8 with the ADC enabled selecting AVREF= internal 2.56vdc -
>  Sampling rate at 125k, which is within the 20-200khz as per atmel

> -circuit -k

>   Vcc=5v
>     connected to
>   AVCC via 10uH,
>       connected with a 100nF cap to GND (as per recomm atmel)
> &
>    external cap 100nF poly connected to Pin21 (AVREF)

> I meassure 2.605Vdc on AVREF.

>    I expected to read 2.56Vdc......
> .........Using  recently calibrated Fluke 77

> I wonder whether this V-value on AVREF is in fact correct or should it
> have been 2.56V?
>  Maybe FLuke out on low V?

> hmmm...  maybe one of you guys have had the same experience or can point
> me to more reading/appl
> material.

> I really appreciate your feedback!

> Regards,
> Gerhard Laubscher
> TeleFax : +27-21-557 0160    Mobile : 082 366 1950
> email : amsol at amssolutions.co.za   website:
> http://www.amssolutions.co.za


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