Low Impedance on Analog Inputr Causes Voltage Drop

Hello all. I am trying to use my Arduino Mega to measure voltage across a temperature diode. I need to read the voltage to four decimal places to get an accurate reading. When I use a voltmeter, the voltage across AB is .5662V. This voltage is exactly accurate according to my calibration curve, so I need this voltage to be displayed on my Arduino. But when I connect my analog input, The voltage on the Arduino and on the voltmeter drops to .5441V. I measured about a 100K ohm impedance across the analog input pin, which is high, but not high enough for my application. I need a way to increase the impedance of the parallel path the arduino is in. I tried a 10M ohm resistor in series between the arduino and A and it keeps the voltage from dropping; however, all the voltage in that path is across the resistor and barely any is across the arduino, which gives me an even more inaccurate reading. Any ideas are appreciated. Thanks!

You are trying to measure the resistance of an analog input pin with an Ohmmeter? Did you think about the effect of the protection diodes? The input impedance of an analog input is probably > 100 Megs if you haven't damaged it. What is the impedance of your volt meter?