AVR-Chat

Im using the adc on a mega8 with a 5v LM4050 precision shunt as an external reference. I am able to read the full scale of 0 to 1023, the problem is that 0 is returned for any voltage under about 0.84v. Is this a limitation of the adc or am I doing something wrong? thanks

--Aaron


       
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Absolutely, you did something wrong. Plz check you schematic and PCB.

Regards.
Dao

A F <afaupell@yahoo.com> wrote:                                  Im using the adc on a mega8 with a 5v LM4050 precision shunt as an external reference. I am able to read the full scale of 0 to 1023, the problem is that 0 is returned for any voltage under about 0.84v. Is this a limitation of the adc or am I doing something wrong? thanks
 
 --Aaron
 
 __________________________________________________________
 Looking for a deal? Find great prices on flights and hotels with Yahoo! FareChase.
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How are your "grounds" Aaron?  If your return lines are at some 
different potential you'll have errors such as this.

Sometimes it's better to have a differential amplifier on the input to 
alleviate this problem.


REB



A F wrote:

>Im using the adc on a mega8 with a 5v LM4050 precision shunt as an external reference. I am able to read the full scale of 0 to 1023, the problem is that 0 is returned for any voltage under about 0.84v. Is this a limitation of the adc or am I doing something wrong? thanks
>
>--Aaron
>  
>

hai..
1. Try to capture the noise in the oscilloscope ,the
noise depend on the design of acd circuit ,so it may
have chance to show zero,
2. make ground for  unused channels of ADC.it may stop
 
oscillation in output. 

--- A F <afaupell@yahoo.com> wrote:

> Im using the adc on a mega8 with a 5v LM4050
> precision shunt as an external reference. I am able
> to read the full scale of 0 to 1023, the problem is
> that 0 is returned for any voltage under about
> 0.84v. Is this a limitation of the adc or am I doing
> something wrong? thanks
> 
> --Aaron
> 
> 
>        
>
____________________________________________________________________________________
> Looking for a deal? Find great prices on flights and
> hotels with Yahoo! FareChase.
> http://farechase.yahoo.com/
> 
> [Non-text portions of this message have been
> removed]
> 
> 



      Download prohibited? No problem! To chat from any browser without download, Click Here: http://in.messenger.yahoo.com/webmessengerpromo.php

Some devices allow interrnal pullup to remain on in ADC
mode. Check that pullups, etc, are off on those pins.

Jim


On Sun, 3 Jun 2007 20:17:17 -0700 (PDT)
 A F <afaupell@yahoo.com> wrote:
> Im using the adc on a mega8 with a 5v LM4050 precision
> shunt as an external reference. I am able to read the
> full scale of 0 to 1023, the problem is that 0 is
> returned for any voltage under about 0.84v. Is this a
> limitation of the adc or am I doing something wrong?
> thanks
> 
> --Aaron
> 
> 
>        
>
____________________________________________________________________________________
> Looking for a deal? Find great prices on flights and
> hotels with Yahoo! FareChase.
> http://farechase.yahoo.com/
> 
> [Non-text portions of this message have been removed]
> 

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I can agree with David on this one, I recently used a A2D on a Atmel
ARM7 microcontroller and got really noisy results. Looked at the
analog waveform with my scope and noticed that everytime the A2D did a
sample-and-hold, the voltage dropped by around 0.2V on a 2V signal,
all due to the output impedance of the analog driver. 

Buffering your analog signal could solve that problem ! 

Cheers
Hein B
Auckland, New Zealand.


--- In AVR-Chat@yahoogroups.com, "David VanHorn" <microbrix@...> wrote:

>
> On 6/4/07, Jim Wagner <jim_d_wagner@...> wrote:
> > Some devices allow interrnal pullup to remain on in ADC
> > mode. Check that pullups, etc, are off on those pins.
> 
> That could well be it, or the sourcing impedance is too high on
> whatever is driving the pins.
> There is a certain amount of leakage current possible, that might be
> giving you anomalously high voltages.  IIRC they want the source
> impedance below 10k.
>

On 6/4/07, Jim Wagner <jim_d_wagner@applelinks.net> wrote:
> Some devices allow interrnal pullup to remain on in ADC
> mode. Check that pullups, etc, are off on those pins.

That could well be it, or the sourcing impedance is too high on
whatever is driving the pins.
There is a certain amount of leakage current possible, that might be
giving you anomalously high voltages.  IIRC they want the source
impedance below 10k.

> Buffering your analog signal could solve that problem !

I always R/c filter analog inputs, with a knee just a bit faster than
I expect the signal to rationally change. This happens right at the
chip, so the AC impedance is rather low. I try HARD to have the DC
sourcing resistance as low as possible too.