Thank you rlogiacco and peter_n for your replies. I am starting to get a clearer picture now. I have read atmega128RFa1 ADC section now. Its starting to make sense now.
So let me just type out what was able to understand.
ADCs input circuitry has a sample and hold circuit(series RC network) with a switch. So whatever is the input impedance which the Analog pin sees(implying the output impedance of the voltage) has a direct impact on the charging cycle of that RC sample and hold network. Smaller the impedance, faster charging of the capacitor. If its a larger impedance, longer charging times and hence when we read the values it will be not stable (since it takes a larger settling time) Is this correct?
Now if we have a larger impedance at the input, if we read any other analog pin, the value read out might have some errors since the capacitor has not discharged fast enough from the previous read? Can some please confirm on this statement?
So if I have a large impedance at the analog input it should be ok if I have some time delay between the readings? So the big question is what is the optimum waiting time I must wait before I read from a any other pin to get an accurate enough result? Is there some formula or rule of thumb for this? (Assuming the system cant spend too much idle time by putting a delay of some random number)
There is formula mentioned in Pg 424 of Atmega128RFA1 (ADC section) which talks about this, but I have understood that formula completely. Reference: http://www.atmel.com/Images/Atmel-8266-MCU_Wireless-ATmega128RFA1_Datasheet.pdf Page 424. Analog input circuitry section.
Can someone please take a look at this section?
Can you explain the input impedance limitation is around 3k for the ADC ? Is that in the datasheet of the ATmega128RFA1 ? or in an Application Note ?
Its mentioned in Page 424 of the datasheet linked above
A capacitor of 10nF (say 1nF...47nF) parallel with R2 (from ATmega analog input to GND) lowers the noise. It also keeps the value stable during the ADC conversion.
I still dont understand this section though. Is it lowering the input noise? But impedance is still high right? hence the problem would still persist. Right?
Atmega328 reference for the same: Smart | Connected | Secure | Microchip Technology Page 257 Analog input circuitry
So suppose my input impedance is around 100k(which cannot be reduced) and I want the best possible accuracy in a single read(Reading at 10 bit accuracy, I am OK with around 8 bit accuracy,last 2 LSB bits can toggle which would correspond to around 12mV(2-bit change) change if reference voltage as around 3.3V(3.3/1023~ 3mV)) whats the best way to go about it? Is it the capacitor method or wait some time before reading etc..