AREF pin vs. voltage divider

Forum 2005-2010 (read only) General Frequently-Asked Questions
1

I want to use Arduino in my car, which means that I have to convert 0-14V signals to 0-5V level for the input pins. I use a voltage divider on each pin to drop the voltage to about 4V.

I thought it would be a good idea to use the same voltage divider for the AREF pin and use EXTERNAL analogue reference, but a strange thing happened. The voltage on the AREF pin is about 1V lower that the voltage on the input pin, even though they use identical voltage dividers and the same input voltage. Why is this happening?

I have the voltage dividers and the power supply on a shield. When I power up the shield by itself, without mounting it on Arduino, the voltages are equal, but when Arduino is connected, as I said, the voltage on the AREF pin is lower. What am I missing?

2

I thought it would be a good idea to use the same voltage divider for the AREF pin and use EXTERNAL analogue reference

You'll never know what the voltage is, because your reference is variable...

Stick with the regulated 5V as a reference.

-j

3

You are right, now that I think about it, the voltages of various signals in the car certainly stay the same no matter what the battery voltage is at the time, so I better not use it as a reference.

And I probably also found the answer to my question - I am using 100K and 47K resistors as the voltage divider - maybe the resistance between the AREF pin and GND is lower and that's why the voltage divider doesn't work properly?

4

That resistance (147k) sounds kinda high to me.

I'd look at the actual values of the resistors in your circuit, too. Typical resistors have 5% or 10% tolerances, and you may be simply unlucky enough for the tolerances to stack up that way.

-j

5

No no, the voltage actually changes in the moment I connect the voltage divider output to the AREF pin. (and the error is 1% but that's irrelevant in this case)

6

The input impedance of the Aref input is low (about 5K) so this loads the potential divider and gives you a lower reading.

The analogue input input impedance is much greater so it dosn't load the potential divider, although you should reduce it to about 10K to minimise noise.

Loading the potential divider is reducing the effective value of the bottom resistor by having two resistors in parallel, the bottom resistor and the load. When determining the absolute value of a potential divider it is a good rule to make the lower resistor 10 times smaller than the load to minimise the loading effect.

7

Thank you, this is exactly what I was looking for :sunglasses:

Just to be sure - by "LOADING the potential divider" and "smaller then the LOAD" you mean the input impendance of the pin? So for the AREF pin, the lower resistor should be 0.5K? Sorry, I am new at this :-[

8

you mean the input impendance of the pin?

In this case yes. But it also applies to any other use of a voltage divider.

So for the AREF pin, the lower resistor should be 0.5K

About that size yes, it's not too critical though.

Sorry, I am new at this

No worries we all have to start some time. :wink: