user37273: I m trying to give ext ref. To Arduino using lm317 1.25 ref.
I  tried to do it. Iswt this voltage in programme too 
But when I run it it was just displaying 1.25
And der was no variation as I varie the resistance of potentiometer which is connected to A0
So plz help me.....
My Arduino chip is Atmega 8A

user37274: yes but you have to use the analogReference(EXTERNAL); command otherwise it will short the default VCCref volatage with the external (i assume this will cause damage.. dunno never done that. yet...)
also if you use a 1.25v reference just remember that that will be the maximum voltage you can measure up to. 
theres a couple of things you can do 
1 use a voltage devider circuit to drop the max voltage out of your sensor(potentiometer) down to about 1.25v
2 put a series resistor betweent vcc and the positive terminal of your potentiometer (creating a voltage devider between the resistor and the full resistance of the potentiometer)
3 and probably the easiest connect the positive of the potentiometer to the lm317 /reference pin so that its output will be between Aref and GND this is the best option as it will be the most stable as its output will be in propotion to any fluctuation of the lm317 
as in 3. the same can be done if using the internal Vref conect the pot to Aref (but remember that it has a limited current output so be carefull not to draw too much by using low resistance pots or many in parallel)

37273: <p>I m trying to give ext ref. To Arduino using lm317 1.25 ref.
I  tried to do it. Iswt this voltage in programme too 
But when I run it it was just displaying 1.25
And der was no variation as I varie the resistance of potentiometer which is connected to A0
So plz help me.....
My Arduino chip is Atmega 8A</p>
 37274: <p>yes but you have to use the analogReference(EXTERNAL); command otherwise it will short the default VCCref volatage with the external (i assume this will cause damage.. dunno never done that. yet...)
also if you use a 1.25v reference just remember that that will be the maximum voltage you can measure up to. 
theres a couple of things you can do </p>

<p>1 use a voltage devider circuit to drop the max voltage out of your sensor(potentiometer) down to about 1.25v</p>

<p>2 put a series resistor betweent vcc and the positive terminal of your potentiometer (creating a voltage devider between the resistor and the full resistance of the potentiometer)</p>

<p>3 and probably the easiest connect the positive of the potentiometer to the lm317 /reference pin so that its output will be between Aref and GND this is the best option as it will be the most stable as its output will be in propotion to any fluctuation of the lm317 </p>

<p>as in 3. the same can be done if using the internal Vref conect the pot to Aref (but remember that it has a limited current output so be carefull not to draw too much by using low resistance pots or many in parallel)</p>
