Atmega16 adc question

M

Mark

Guest
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
 
"Mark" <mlerman@ix.netcom.com> wrote in message
news:qOqrd.726$Va5.308@newsread3.news.atl.earthlink.net...
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.
Click to expand...
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
 
Mark wrote:
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


The AtMega16L specs say AVcc must be within +/- 0.3 volts of digital
Click to expand...
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.
 
On Wed, 01 Dec 2004 21:35:18 GMT, "Mark" <mlerman@ix.netcom.com> wrote:

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.
Click to expand...
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
 
Thanks for all the info and help. I think I have a handle on it now.

"Mark" <mlerman@ix.netcom.com> wrote in message
news:qOqrd.726$Va5.308@newsread3.news.atl.earthlink.net...
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
Click to expand...
 
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