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I want to measure temperature of an oven using a thermocouple. The maximum change in the temperature will be 1oC/s. The amplifier section will already contain a low pass filter, but I want to put an extra passive RC filter very close to the ADC input pin of PIC32MX270F256B. However, I can't decide a suitable resistance value (R) because I can't find the input impedance of the ADC pin neither in the datasheet nor in the family reference manual. What range of resistance value can I use here?

schematic

simulate this circuit – Schematic created using CircuitLab

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The Reference Manual seems pretty clear on the subject:

The analog input model of the 10-bit A/D converter is shown in Figure 17-22. The total acquisition time for the A/D conversion is a function of the internal amplifier settling time and the holding capacitor charge time. For the A/D converter to meet its specified accuracy, the charge holding capacitor (CHOLD) must be allowed to fully charge to the voltage level on the analog input pin. The analog output source impedance (RS), the interconnect impedance (RIC), and the internal sampling switch (RSS) impedance combine to directly affect the time required to charge the CHOLD. The combined impedance of the analog sources must therefore be small enough to fully charge the holding capacitor within the chosen sample time. After the analog input channel is selected (changed), this acquisition function must be completed prior to starting the conversion. The internal holding capacitor will be in a discharged state prior to each sample operation.

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An external RC filter is sometimes added for anti-aliasing of the input signal. The R component should be selected to ensure that the acquisition time requirements are satisfied. Any external components connected (via high-impedance) to an analog input pin (capacitor, Zener diode, etc.) should have very little leakage current at the pin.

The Datasheet gives a recommended input impedance for the ADC:

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  • \$\begingroup\$ Yes, I found it in table 29-32 at page 275. Thank you. \$\endgroup\$ – hkBattousai Mar 23 '15 at 10:06

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