XMEGA ADC unsigned transfer function

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hooverphonique
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Posted by hooverphonique: Tue. Mar 23, 2010 - 11:05 AM
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According to the xmega a datasheet, the unsigned transfer function is

RES = 4095 * VIN - (-dV) / VREF

where

dV = VREF * 0.05

Substituting yields

RES = 4095 * ( VIN/VREF + 0.05 )

which means that the offset (5% of 4095) is independent of VREF. Now, I know that the 5% are not written in stone, and that's fine, but what puzzles me is, that the offset seems to depend on VREF :shock:

Here a chart that plots RES vs. VREF on a grounded input pin (hope the document is accessible to everyone).
http://spreadsheets.google.com/pub?key=tw2kUVFCqJW-uKfyqZmiXzQ&output=pdf

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AVR Microcontrollers, megaAVR and tinyAVR
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Bronze Marktheusch
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Posted by theusch: Tue. Mar 23, 2010 - 01:12 PM
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As I recall from Xmega threads the unsigned mode doesn't work that well. Try repeating with signed mode, and search for "xmega adc" or similar to uncover those past discussions.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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steve17
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Posted by steve17: Thu. Apr 1, 2010 - 01:25 PM
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Apparently the delta V is dependent on a lot of things. App note AVR1300 says this:

Attachment(s): 

Image icon how_to_measure2.png
Image icon Sometimes delta V can be useful according to the appnote.
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hooverphonique
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Posted by hooverphonique: Tue. Dec 14, 2010 - 09:47 AM
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yes.. in order to measure it *accurately*, but comparing e.g. for Vref 1.0V and 2.5V, the offset is not even in the same ballpark for same chip, and Atmel keeps saying in the datasheets/app notes, that the offset should be around 200.. 150 is not around 200 in my book, and I'm not sure I really trust their transfer function that says that the offset is independent of Vref.

Sorry for returning here, but hadn't seen the last answer before now and just ran into this issue again (didn't even remember submitting the original post :-)

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