Err, do you understand how a voltage divider works?
Get 2 resistors and make a voltage divider. At the centre point of the divider measure the voltage with a multimeter. Now do the same, but put a 22R in between the centre point and the multimeter. The voltage will be as near to the same as it makes no difference.
This is due to the fact that the multimeter should have a fairly large impedance.
Looking at the Atmel 640/1280/2560 docs, there is a whole section on the circuitry around the analog inputs: Section 26.6.1 Analog Input Circuitry (from [www.atmel.com] ). From a brief look over the SAM3X datasheets, it uses the same store-and-hold mechanism, and therefore should have similar input circuitry.
The way the circuitry works is to connect an internal capacitor to the input through an internal series resistor. The time it takes the capacitor (which on the Mega is about 14pF) to charge is the minimum sample time.
From the Atmel 640/1280/2560 docs:
So by adding a series resistor in there, the only thing you really reduce is the time it takes to charge the internal capacitor. Since the internal resistance between the input pin and the capacitor is between 1k-100k, 22R and a PTC should not noticeably increase the time it takes to charge the S/H cap much at all.
The main issue I suspect will be the standard input leakage of the pin, which I can't see looking over the docs for either the Atmel 640/1280/2560 (Mega) or the SAM3X8E (Due). It is something that I think would need to be tried to see if it has any effect.
Get 2 resistors and make a voltage divider. At the centre point of the divider measure the voltage with a multimeter. Now do the same, but put a 22R in between the centre point and the multimeter. The voltage will be as near to the same as it makes no difference.
This is due to the fact that the multimeter should have a fairly large impedance.
Looking at the Atmel 640/1280/2560 docs, there is a whole section on the circuitry around the analog inputs: Section 26.6.1 Analog Input Circuitry (from [www.atmel.com] ). From a brief look over the SAM3X datasheets, it uses the same store-and-hold mechanism, and therefore should have similar input circuitry.
The way the circuitry works is to connect an internal capacitor to the input through an internal series resistor. The time it takes the capacitor (which on the Mega is about 14pF) to charge is the minimum sample time.
From the Atmel 640/1280/2560 docs:
Quote
The analog input circuitry for single ended channels is illustrated in Figure 26-8. An analog source applied to ADCn is subjected to the pin capacitance and input leakage of that pin, regard- less of whether that channel is selected as input for the ADC. When the channel is selected, the source must drive the S/H capacitor through the series resistance (combined resistance in the input path).
The ADC is optimized for analog signals with an output impedance of approximately 10kΩ or less. If such a source is used, the sampling time will be negligible. If a source with higher impedance is used, the sampling time will depend on how long time the source needs to charge the S/H capacitor, which can vary widely. The user is recommended to only use low impedant sources with slowly varying signals, since this minimizes the required charge transfer to the S/H capacitor."
So by adding a series resistor in there, the only thing you really reduce is the time it takes to charge the internal capacitor. Since the internal resistance between the input pin and the capacitor is between 1k-100k, 22R and a PTC should not noticeably increase the time it takes to charge the S/H cap much at all.
The main issue I suspect will be the standard input leakage of the pin, which I can't see looking over the docs for either the Atmel 640/1280/2560 (Mega) or the SAM3X8E (Due). It is something that I think would need to be tried to see if it has any effect.
