reference voltage chip for AVref?

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Hi,

I used to connect AVref with AVcc (with LM2576) to get the reference voltage for the ADC in AT90CAN128 and I found the ADC output is not so stable (value changes all the time even when the input is constant). I am not sure whether it is better to use a reference voltage chip alone for this purpose. Please give me some hints.

Thank you
Senmeis

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A LM2576 is NOT a voltage reference. Instead, it is a switching regulator. Connecting this to your AVCC and AVref pins will certainly cause A/D issues.

For best results, use a REAL voltage reference. There are lots to choose from. If you are partial to National Semi, look at something like a LM4040CIM3-5.0. This is available in several packages and tolerances. If you want something else, go to Digi-Key and search for "voltage reference" and you will find plenty of them to choose from.

Also, good PCB layout is important. Place an appropriate decoupling cap as close to the AVref pin as possible along with the voltage reference. Skipping this step will result in "noisy" readings.

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Hmm, noise perhaps? How much is the change?

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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Thank you.

I think the Buck converter with LM2576 is OK for AVcc and for Vcc. As for AVref, maybe it is not so stable so a voltage reference is better. According to the data sheet, the reference can be set as AVcc:

Quote:
REFS1:REFS0 = 0:1 => AVcc with external capacitor on AREF pin.

Are they internally connected? Must I connect AVcc manually on AVref pin?

Thank you
Senmeis

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Quote:
Are they internally connected? Must I connect AVcc manually on AVref pin?

Look at the ADC diagram in the datasheet.

Embedded Dreams
One day, knowledge will replace money.

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If you wish to use Vcc as Vref, and Vcc is supplied by a switching regulator, I suggest feeding the Vref pin with Vcc through a RC filter. This gives you control over the time constant. Using the "internal" Vcc to Vref connection with the external cap on Vref could work as well, but the external resistor scheme gives you more options. Switching regulators put high frequency ripple on the Vcc bus which could certainly spoil your a/d readings if good filtering is not provided from Vcc to Vref.

Tom Pappano
Tulsa, Oklahoma

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Could also stick a 4.x V Zener diode, anode to ground, cathode to supply, as a precision shunt... Assuming a forward drop of .7v for the diode, it would clamp the supply at 4.7v and get rid of the load variations on the main supply... Be prepared to deal with the heat though depending on the supply voltage...

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Quote:
Be prepared to deal with the heat though depending on the supply voltage...

And a lot of it, plastered across a 3 amp regulator! 8-)

Tom Pappano
Tulsa, Oklahoma

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Well I am guessing you do not need 3 amps of current for a reference voltage, so adding a series resistor between the regulator and the shunt Zener should limit that somewhat... ;)

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The fact that you said:

Quote:
Could also stick a 4.x V Zener diode, anode to ground, cathode to supply, as a precision shunt...

Sort of caught my attention, especially when you metioned "heat".

Tom Pappano
Tulsa, Oklahoma