I want to use a precision 5 volt reference as Vref for the adc on an Atmega16. My problem is that the Vcc uses a common 78L05 regulator, whose output can range from 4.75 to 5.25 volts. Can I tie the AVCC to the 5 volt reference instead of Vcc? Will I get accurate adc readings if Vcc is say 4.8 volts while AVCC is 5.00 volts? Thanks for any advice.
Mark
Check the datasheet. AVcc limits: (Vcc - 0.3, Vcc + 0.3) and (2.7, 5.5). ARef limits: (2.0, AVcc) single-ended, (2.0, AVcc - 0.2) differential. A 5 V AVcc looks OK to me.
On the mega16, you can program the ADMUX register to connect AVcc to ARef internally, in which case a capacitor can be hung off of ARef to soak up noise.
The successive approximation DAC only cares about its voltage reference, not the chip's Vcc, so as long as none of the recommended limits are exceeded and the precision voltage reference can source enough current, you should be OK.
-- Rich Webb Norfolk, VA
4.8
This is a very legal question. Of course, Atmel does not specify the maximum voltage between AVCC and VCC. One would expect it to be around 0,5V to avoid NP conduction.
An alternative may be to reference Vcc from your AVcc, using a series diode with your reference, and an emittor follower transistor as buffer. If you prefer using a regulator IC (lower output impedance than the emittor follower) you can use a LM337 (1.25V step) and reference it to your AVcc with a resistive divider.
Success Sander
The AtMega16L specs say AVcc must be within +/- 0.3 volts of digital Vcc. So you are just within spec at +/- 0.25 volts range. Therefore it should be ok. Personally I would use a closer tolerance device than the
78L05. My current favourite is the LP2951.Note also that if you go the other way, and elect to tie AVcc to Vcc, then Vref *cannot* exceed Vcc.
-- Regards, Adrian Jansen adrianjansen at internode dot on dot net Design Engineer J & K Micro Systems Microcomputer solutions for industrial control Note reply address is invalid, convert address above to machine form.
Although the AVR chips are broadly similar, there are differences in the details. On the mega16, the bits REFS1 and REFS0 in the ADMUX register control what voltage is used as the ADC reference.
REFS1 controls a mux that selects between AVcc and the 2.56 V bandgap reference. REFS0 controls a gate that connects the output of the mux to the approximation DAC.
If ADMUX is written with 01xx_xxxx then AVcc will be used.
See Figure 98 and Table 83.
-- Rich Webb Norfolk, VA
Perhaps you can use the internal reference if you divide down the voltages to be measured - however, full scale will be equal to Vint, about 2.56 volts, not AVcc.
Actually, the LM78L05 drift is smaller than some regulators, and may be acceptable for your application - (check datasheet) - the issue is you can't predict the voltage of an individual device. In that case, you can put a calibration factor in EE memory and multiply your ADC reading by that. You'll probably need calibration anyway.
Roger
Thanks for all the info and help. I think I have a handle on it now.
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