Why does the ADC overflow when using an external Aref?

Question

I'm using an LM35 temperature sensor, and it outputs 190 mV (which agrees with 19°C). I'm using an external reference voltage which I measure as 417mV. (This is set by the pin "aRefCntrl"). So I should have a reading around 467, but I get 1023, i.e. the ADC overflows. When I use the internal 1.1V reference I get a correct reading. Any ideas?

This is my code (Arduino waits for a request from the PC to send the data):

#define lm35 A0
#define aRefCtrl 10
void setup() { 
 pinMode(aRefCtrl, OUTPUT);
 digitalWrite(aRefCtrl, 0);
 analogReference(EXTERNAL);
 analogRead(lm35); // dummy read
 Serial.begin(9600); 
} 
void loop() {
 if ( Serial.available() ) {
 Serial.read(); // PC's request is a single byte
 int temperature = analogRead(lm35);
 Serial.println(temperature); 
 }
} 

follow-up on Ignacio's comment: would a decoupling capacitor do? (I can't test for the moment, I have to order some)

• Ignacio Vazquez-Abrams: How much current is available from the reference?
• Joris Groosman: @Ignacio: not much, the reference voltage comes from a 10k/1k divider on 5V. Isn't the voltage I measure at the aRef pin not the real reference voltage?

Answer 1

The minimum reference voltage is 1.0 V according to the atmega328 datasheet. See table 29-15. ADC Characteristics.

• Joris Groosman: Thanks a lot! I was looking for that information in the datasheet, scanning it for all occurrences of "Aref", but it seems to appear as "Vref" :-). (Page 310, FTW.)
• aaa: Hmm, Aref as in Analog Reference and Vref as in Voltage reference I think? But indeed 1.1V bandgap is a reliable (small voltage) source.