A/D Conversion Clock Select bits confussion ..

[tumbleweed]

Thanks Henrik. So again, removing the ones where the ADCS setting is invalid/too fast gives:

25.6mV input, 10k source resistor, 3.3V noise on "dummy channel":

FRC        2 TAD      Result: 6
FOSC/64    2 TAD      Result: 7
FRC        4 TAD      Result: 13
FOSC/64    4 TAD      Result: 14
FRC        6 TAD      Result: 11
FOSC/64    6 TAD      Result: 10
FRC        8 TAD      Result: 12
FOSC/64    8 TAD      Result: 6
FRC        12 TAD     Result: 13
FOSC/64    12 TAD     Result: 14
FRC        16 TAD     Result: 11
FOSC/64    16 TAD     Result: 9
FRC        20 TAD     Result: 8
FOSC/64    20 TAD     Result: 13

*********************************************

3.285V, 10k source resistor 3.3V noise on "dummy channel":

FRC        2 TAD      Result: 1022
FOSC/64    2 TAD      Result: 1014
FRC        4 TAD      Result: 1023
FOSC/64    4 TAD      Result: 1015
FRC        6 TAD      Result: 1023
FOSC/64    6 TAD      Result: 1019
FRC        8 TAD      Result: 1021
FOSC/64    8 TAD      Result: 1016
FRC        12 TAD     Result: 1019
FOSC/64    12 TAD     Result: 1015
FRC        16 TAD     Result: 1018
FOSC/64    16 TAD     Result: 1023
FRC        20 TAD     Result: 1023
FOSC/64    20 TAD     Result: 1019

*********************************************

25.6mV input, 10k source resistor, "dummy channel" connected to GND:

FRC        2 TAD      Result: 7
FOSC/64    2 TAD      Result: 7
FRC        4 TAD      Result: 7
FOSC/64    4 TAD      Result: 7
FRC        6 TAD      Result: 7
FOSC/64    6 TAD      Result: 7
FRC        8 TAD      Result: 7
FOSC/64    8 TAD      Result: 7
FRC        12 TAD     Result: 7
FOSC/64    12 TAD     Result: 7
FRC        16 TAD     Result: 7
FOSC/64    16 TAD     Result: 7
FRC        20 TAD     Result: 7
FOSC/64    20 TAD     Result: 7

*********************************************

3.285V input, 10k source resistor, "dummy" channel connected to GND:

FRC        2 TAD      Result: 1019
FOSC/64    2 TAD      Result: 1018
FRC        4 TAD      Result: 1018
FOSC/64    4 TAD      Result: 1019
FRC        6 TAD      Result: 1019
FOSC/64    6 TAD      Result: 1019
FRC        8 TAD      Result: 1018
FOSC/64    8 TAD      Result: 1019
FRC        12 TAD     Result: 1019
FOSC/64    12 TAD     Result: 1020
FRC        16 TAD     Result: 1018
FOSC/64    16 TAD     Result: 1019
FRC        20 TAD     Result: 1019
FOSC/64    20 TAD     Result: 1020

*********************************************

25.6mV input, 10k source resistor, "dummy channel" connected to Vdd:

FRC        2 TAD      Result: 8
FOSC/64    2 TAD      Result: 9
FRC        4 TAD      Result: 8
FOSC/64    4 TAD      Result: 8
FRC        6 TAD      Result: 7
FOSC/64    6 TAD      Result: 7
FRC        8 TAD      Result: 7
FOSC/64    8 TAD      Result: 7
FRC        12 TAD     Result: 8
FOSC/64    12 TAD     Result: 7
FRC        16 TAD     Result: 8
FOSC/64    16 TAD     Result: 7
FRC        20 TAD     Result: 8
FOSC/64    20 TAD     Result: 7

*********************************************

3.285V input, 10k source resistor, "dummy channel" connected to Vdd:

FRC        2 TAD      Result: 1020
FOSC/64    2 TAD      Result: 1019
FRC        4 TAD      Result: 1019
FOSC/64    4 TAD      Result: 1020
FRC        6 TAD      Result: 1020
FOSC/64    6 TAD      Result: 1019
FRC        8 TAD      Result: 1019
FOSC/64    8 TAD      Result: 1019
FRC        12 TAD     Result: 1020
FOSC/64    12 TAD     Result: 1020
FRC        16 TAD     Result: 1021
FOSC/64    16 TAD     Result: 1019
FRC        20 TAD     Result: 1021
FOSC/64    20 TAD     Result: 1020

Probably the ones of most interest are where Chold has to swing the full range between samples:

3.285V input, 10k source resistor, "dummy" channel connected to GND:

FRC        2 TAD      Result: 1019
FOSC/64    2 TAD      Result: 1018
FRC        4 TAD      Result: 1018
FOSC/64    4 TAD      Result: 1019
FRC        6 TAD      Result: 1019
FOSC/64    6 TAD      Result: 1019
FRC        8 TAD      Result: 1018
FOSC/64    8 TAD      Result: 1019
FRC        12 TAD     Result: 1019
FOSC/64    12 TAD     Result: 1020
FRC        16 TAD     Result: 1018
FOSC/64    16 TAD     Result: 1019
FRC        20 TAD     Result: 1019
FOSC/64    20 TAD     Result: 1020

*********************************************

25.6mV input, 10k source resistor, "dummy channel" connected to Vdd:

FRC        2 TAD      Result: 8
FOSC/64    2 TAD      Result: 9
FRC        4 TAD      Result: 8
FOSC/64    4 TAD      Result: 8
FRC        6 TAD      Result: 7
FOSC/64    6 TAD      Result: 7
FRC        8 TAD      Result: 7
FOSC/64    8 TAD      Result: 7
FRC        12 TAD     Result: 8
FOSC/64    12 TAD     Result: 7
FRC        16 TAD     Result: 8
FOSC/64    16 TAD     Result: 7
FRC        20 TAD     Result: 8
FOSC/64    20 TAD     Result: 7

For no averaging those numbers seem pretty reasonable.