A/D Voltage Question

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#1
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I am working on an application that requires the use of a 3.3v regulated power supply, but I am also using an analog remote that I have set up for 5v. Is it possible to set the VCC to 3.3 volts and set the AVCC and AREF to 5V or will the chip limit this? I am using an atmega8L. Thanks.

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Well, neither. Check the datasheets on AREF range and AVcc range and Absolute Maximum Ratings and note that Mega8 has Vcc and AVcc tied together internally, and I think the answer becomes apparent.

Lee

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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If it's possible you can use:
http://focus.ti.com/lit/ds/symli...
IC that can connect 3,3V and 5V systems

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Use a voltage divider made of two resistors to take down ADC-input to max 3.3 V. And connect AREF and AVcc to Vcc (3.3 V).

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Quote:
And connect AREF and AVcc to Vcc (3.3 V).

I don´t recommend a hardware connection.
I think it´s more flexible and more safe to do the connection internally by selecting AVCC as the ADCRef.
Just connect the recommended cap to ARef.

Klaus
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Look at: www.megausb.de (German)
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Wouldn't using the voltage divider change the values that are stored for the a/d conversion? That is what I was trying to avoid, but doesn't sound like I will be able to.

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Quote:
Wouldn't using the voltage divider change the values that are stored for the a/d conversion? That is what I was trying to avoid, but doesn't sound like I will be able to.

??? What do you mean???

Klaus
********************************
Look at: www.megausb.de (German)
********************************

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when the a/d stores the value into the ADCH and ADCL registers, won't it be different for 3.3v than it would be for 5v

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Quote:
when the a/d stores the value into the ADCH and ADCL registers, won't it be different for 3.3v than it would be for 5v

surely!

the value allways follows the formula:

ADCValue = 1024 * V_IN/VRef. (for a 10 bit unipolar input ADC like in AVRs)

Therefore:
If you change ARef you surely change the ADCValue.

But:
if you use
* 3.3V as VREF and use a voltage devider with 5:3.3 ratio
you get the same results as
* when using 5V as VRef and no voltage devider.

Klaus
********************************
Look at: www.megausb.de (German)
********************************