Hi,
If the resolution of the A-D is 12 bits then the capacitor needs to be at least 4096 X 5pf to get 1 bit resolution which is just over 20nF I would suggest the use of a 47nF capacitor. As for the leakage you may want to add a pot to trim the results or do it in software.
I don't agree.
Resolution is not influenced by input impedance, the capacitor will influence accuracy and precision.
And about leakage current:
With an ADC you have two types of input current:
* one is the low static input current (usually no big problem)
* the other is the sample frequency dependent current (to charge the S/H capacitor)
This is the more problematic current. It makes the reading dependent of sampling rate...especially with high source resistance and it does not care what size of capacitor you use. The capacitor just changes the time constant tau.
Example: 5pF S/H capacitor. Discharged to 0V during conversion. It takes about 8 tau to get less than 1LSB error. This means 400ns @ 10kOhms
As long as the sampling time is longer than 400ns the error will be less than 1LSB.
Now add an external capacitor. Let's say 470nF.
Now let's assume the voltage is 3V. With every sample the 5pF capacitor needs to be charged from 0V to 3V.
This causes an average current of 150nA @ 10ksmpl/s.
This means a voltage drop of 1.5mV @10kOhms. (Or 15mV at 100kOhms)
And this error can't be compensated by a capacitor.
Klaus