# It seems as if there is a 220K pull-down resistor on analog pin 0

So I am building a project that uses the analog pins for precise measurements of resistance through a voltage divider and I have been having some seemingly unsolvable problems with nonlinear response to linearly changing resistances when using the A0 pin to measure.

I first realized this was the problem when I had a voltage divider consisting of a 76.8K and 100K 1% resistors with Vout connected to A0, and I was consistently getting wrong values. I measured the output voltage of the voltage divider and got 1.7V, instead of the ~2.7 volts I should be getting with my supply. When I disconnected the wire to A0, the voltage rose to the expected level.

I disconnected power and measured resistance between the analog pins of the Atmega328 I was using and ground and found that A0 had 220K of resistance, while all other pins measured as infinite. When using the other pins, the voltage divider returned the expected values and was able to measure a wide range of resistors with values just as accurate as my multimeter. I know I have changed a lot of low-level things with code (analog reference, the comparator, ADC clock prescalers, using analog pins as outputs, etc.) but I don't think any of this could persist while the chip is not powered on. Does doing any of these things connect a resistor in the way I was describing, and is there anything that could cause this to stay "stuck" this way?

• The datasheet says: `The ADC is optimized for analog signals with an output impedance of approximately 10 kΩ or less` - your voltage divider of 76k and 100k does not appear to satisfy those requirements. Apr 28, 2016 at 10:39
• Try adding a small capacitor between the voltage you try to measure and ground. Apr 28, 2016 at 13:32
• @NickGammon The voltage divider's impedance does not seem to be an issue, as it works perfectly on pins A1-A5 with a wide range of resistor values as the one being measured. The question is asking why there is the internal 220K resistance to ground that is causing it to work incorrectly on analog pin 0. Apr 28, 2016 at 17:23
• This is a raw chip is it? Not on some board? Try another chip, maybe this one is faulty. Apr 28, 2016 at 20:40
• Yes, it is a raw chip. I will try another one soon. Apr 28, 2016 at 21:38