Weird Temperature Reading Fluctuations, LabVIEW
from web ...


  The limitation of the 328p's ADC speed is not related to the clock speed or a sw driver, but it is related to the internal ADC sample and hold system (S/H). 


The ADC must sample the incoming signal such it charges an internal S/H capacitor (~14pF) through an internal resistor (up to 100kohm) to the exact voltage as it is at the analog input pin.

When the S/H cap is "charged" enough, ADC disconnects the S/H cap from the analog input and does the ADC conversion of the internal capacitor's voltage. 

The value of this internal S/H component (its RC constant) is too big for a fast 10bit conversion (as you cannot charge the internal S/H capacitor fast enough via the internal resistor because the resistor is large in its value).

So even when you make the fastest conversion in your code, the internal S/H capacitor is not being fully charged (it means charged up to the level of the voltage at the analog input's pin) and you do the ADC conversion with much less precision (ie. 7bit only).


you could try two successive readings and toss the first to see the change, but that slows the sample rate.


if  true, then your source impedance is too high and 0.01uF cap across Vin to port will eliminate this glitch.