Verify changed analog reference voltage on Arduino

Question

I want a reference analog voltage of 2.0 volts on a 3.3 V Arduino.

Since there is an internal resistance of 32k on the Arduino, I hooked up a 21k resistor to Vcc.

How can I confirm that I now have an analog voltage range from 0 to 2.0 V? Can I do this via code? Or just use a voltmeter?

• Andy aka: It probably took longer to ask this than test it. If you are confident about the 32k internal (I'm not sure it will be reliable) use a voltmeter or use code.
• Connor Wolf: The "32KΩ" number for the aRef impedance in the documentation is a number of things. First, you don't know if the 32K is the effective impedance to ground, VCC, or an unknown arbitrary voltage between the two. Second, the 32K number is a typical value, and I really really doubt that Atmel bother to make sure the aRef input is held to close tolerances between parts. You really are going to need a resistive divider at minimum, and a capacitor from the aRef pin to ground is a good idea.

Answer 1

With a sketch, read and print a known test voltage of a little bit less than your reference voltage, say 1.5V in this case. The printed result will be the same fraction of 1024 that your test voltage is of the reference voltage, or:

V_test x
------ = ----
V_ref 1024
 1024
V_ref = ---- V_test
 x

Edit: The intent here was to supply a presumed 2.0V external analog reference, say it's your 20K resistor. Now supply a known voltage of ~80% the 2V reference to one of the MCU's analog inputs. Run a sketch to print what the MCU reads as that voltage as. Calculate the actual analog reference voltage as I did above. If it is 2.0V, you're done.

This is how I'd make a 2.0V ARef. An AtMega328p's ARef only draws ~100μA at Vcc = 3.3V, so this should still be pretty close:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

As above, get A/D reading of the known voltage input and calculate what the reference voltage must be, to confirm it.

• l3win: by test voltage you mean applying a response from my sensor device to attain that test voltage?
• JRobert: I'd make a known, constant voltage with a battery or a voltage divider from your Vcc. That way you can calculate what the reference voltage must be.
• l3win: hmmm...I hooked a Voltmeter to GND and the AREF pin, but it still shows approximately 3.3 V with my 21k resistor between Vcc and AREF
• JRobert: The ARef input is high impedance so, yes, you shouldn't see much drop across the 21K. See my edited answer for creating the analog reference voltage and confirming it with a short sketch.
• l3win: thx for your help JRobert. But what about the internal 32k resistance in the Aref pin? According to the text on analogreference arduino.cc/en/Reference/AnalogReference, I should be able to use only a single resistor (22k in my case) to convert to 2.0 V. In your schematic, won't the Aref voltage be dropped due to that internal 32 k resistor?