Reading ADC with external Vref Mega32C1

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Hi,

 

I have a Mega32C1 board where I read from the ADC. Having the internal Vref on without connecting the external Vref (AREFEN = 0, REFS1 = 1, REFS0 = 1, datasheet page 210) gives me a correct reading.

If I instead want to use the external Vref I have select it as refenrence source instead (AREFEN = 1, REFS1 = 0, REFS0 = 0), I always read maximum ADC value. The external Vref is 1.25V and the internal is 2.56V, but my measurement signal is roughly 0.9V +- 0.3V.

 

Is there something I am missing? Is there a misstake often done that gives this symptoms?

 

Greatful for any help!

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Last Edited: Wed. Nov 2, 2016 - 11:31 AM
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Try Vcc as Vref and see if the problem persists.

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AgwanII wrote:
Is there something I am missing?

...

See the "Min" value?

 

So, what can you do?  Dunno.  That model has an "amplifier", with one of the selections [apparently--I'm not a user of this model] being x5.  I suppose you could use x5 and a 5V reference (if your Vcc is 5V) for your ~1V signal.

 

This model doesn't have differential ADC, either. 

 

If it is really important for your app to get more than ~250 counts of resolution reading your input signal, I'd say you might need to consider an external op amp circuit to amplify the area of interest?

 

 

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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Ah, thats it. I did not imagine that there would be a minimum voltage for the ref that high. Thanks theusch!