ATmega8 Voltage Reference Problem

[1843]

Hi all,

I tested some ATmega8 today. Their ADC reference is set to 2.56V and the voltage reference is externally decoupled at the AREF pin by a 22uF cap.
But in actual I saw that ATmega8's read 1023 of ADC value only for 2.63V input voltage and not 2.56V...!?

I think that they are not original parts... Maybe are produced by a bad factory.

Any idea?

Best regards,
1843

[MaverickITA]

ATmega8 Voltage Reference Problem

Hi !

I have the same problem.
I am sure that the chip are originals.
I bought many from different seller.

I used a cap of 100nF connected to AREF for decoupling.
I saw that without cap the voltage is different.

I solved the problem using an external 2.048V voltage reference.

Best regards,
Maverick ITA

[Carl-NC]

Did you actually measure the AREF pin?

[bugwhiskers]

Hi 1843,

The following was cut and pasted from page 248 in the ATMEGA8 documentation.

V

INT Internal Voltage Reference 2.3 2.56 2.7 V

About 17.39% tolerance.

regards
bugwhiskers

[1843]

Did you actually measure the AREF pin?

No, I measured the ADC input signal.

[1843]

Hi 1843,

The following was cut and pasted from page 248 in the ATMEGA8 documentation.

V

INT Internal Voltage Reference 2.3 2.56 2.7 V

About 17.39% tolerance.

regards
bugwhiskers

Yes you are right! I didn't read that!!

[Carl-NC]

So if you measure the AREF pin, you will likely find it is very close to 2.63V. If you want a precision full scale range, then you should be able to provide an external precision reference voltage from, say, a factory-trimmed bandgap chip. Or from a pot adjustment.