AVR-Chat

Hi there;

I was just browsing the docs on the AVR ADC.  I was wondering if anyone
could tell me why I would want to use a ADC prescalar other than 2 
(the mimimum)?  Does it affect the accuracy or noise?  Thanks.

Craig

At 11:56 AM 9/1/2004, Craig Limber wrote:


>Hi there;
>
>I was just browsing the docs on the AVR ADC.  I was wondering if anyone
>could tell me why I would want to use a ADC prescalar other than 2 
>(the mimimum)?  Does it affect the accuracy or noise?  Thanks.

The adc clock has to be around 200-250kHz, or you loose accuracy.
How much you loose, or wether it even runs at all, depends on how far you push it.
I wouldn't push it out of spec without a very good reason, because they give no guidance on how badly things degrade outside the published limits.

Not to be too picky, but since my last name is Loose, I like to see the word
used correctly. You "lose" accuracy - "loose" is an adjective. ;)

Erik

-----Original Message-----
From: Dave VanHorn [mailto:dvanhorn@dvanhorn.org]
Sent: Wednesday, September 01, 2004 11:04
To: AVR-Chat@yahoogroups.com; avr-chat@yahoogroups.com
Subject: Re: [AVR-Chat] Why use the ADC pre-scalar bits


At 11:56 AM 9/1/2004, Craig Limber wrote:


>Hi there;
>
>I was just browsing the docs on the AVR ADC.  I was wondering if anyone
>could tell me why I would want to use a ADC prescalar other than 2
>(the mimimum)?  Does it affect the accuracy or noise?  Thanks.

The adc clock has to be around 200-250kHz, or you loose accuracy.
How much you loose, or wether it even runs at all, depends on how far you
push it.
I wouldn't push it out of spec without a very good reason, because they give
no guidance on how badly things degrade outside the published limits.





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The ADC clock needs to be in a fairly narrow range if the
accuracy specs are to be met. If its too slow, the
sample-hold cap will loose some of its charge and the
converter won't work right. If its too fast, you will loose
accuracy.

But, since the processor clock can vary over a rather wide
range, the ADC prescaler lets you select an ADC clock thats
in the specified window. 

If you have a slow processor clock, then, YES you want (and
need) to use the minimum prescale value. But, if the
processor clock is faster, then you need to adjust the
prescale value accordingly.

Jim
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At 01:21 PM 9/1/2004, Erik Loose wrote:

>Not to be too picky, but since my last name is Loose, I like to see the word
>used correctly. You "lose" accuracy - "loose" is an adjective. ;)

Sorry, I'm fighting a screaming headache today.