When you buffer a voltage with a capacitor like the above circuit you are measuring the capacitor voltage and so the input resistance is extremely low.
Mike.
Sorry, that's not correct at all. You're confusing DC and AC resistance. The AC resistance is low so the internal ADC sampling cap's charging time is low. But that's not "leakage", which is a DC loss. It applies even if the ADC is off and the pin is just configured as input. 1uA through a 10K resistor and 100uF capacitor is still a 10K DC input resistance and is a 10uV error. With 2.5v Vref and 10-bit ADC, that's 2.44uV/code, so the ADC accurate is degraded by the high input resistance.
IIRC the PIC came up with the 2.5K input rating based on a 1uA leakage assumption, 5v Vref, and a criteria of less the HALF a code of error on a 10-bit ADC. (5v/(1024*2))/1uA=2.441Kohms. This is the possible DC error. It won't matter if you have 100uF capacitance and an hourlong sampling interval, the DC leakage error is still present. With a 2.5v Vref, you'd need <1.2K input resistance to meet a less-than-half-a-code criteria.
In practice, your PIC will probably have <1uA leakage, but it's not guaranteed. The leakage is also not going to be independent of analog input. You will get the most + leakage when the analog voltage is most negative.
Be aware that the PIC pin is NOT the only form of leakage. Traces always have leakage to nearby pins- this is especially true if you didn't clean up the flux. And capacitors have a nonzero leakage. Electrolytics can be especially bad in this regard, and highly variable. As much as
5uA from a low voltage (15v), low capacitance( <6.8uF) electrolytic alone is plausible (higher voltage caps can have even more leakage). Tantalums are about 5x less, but it's still present. Ceramics are pretty good though.
So PIC leakage ain't the big issue if you were to include an electrolytic or even tantalum cap. Bottom line is that if you use a capacitor, the leakage is HIGHER, and the maximum acceptable DC input resistance will need to be LOWER, the opposite of what Pommie's saying.
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