A/D Conversion Clock Select bits confussion ..

[richard]

if it were possible that the acquisition time could be too long then surely the optimum acquisition time would have to varied logarithmically according to the instantaneous sample voltage . the suggestion is not realistic ,unsubstantiated and unworkable

[tumbleweed]

Agreed.

Confused...

Don't be. You have it correct.

What you're computing is the minimum time it takes for the RC sample/hold network to charge to the input voltage assuming you're making a full-scale voltage change. Depending on the accuracy you're trying to reach that normally takes about 7T-8T. Once it's charged up (or down), you're good to go, but it does no harm in waiting longer (as long as nothing else changes).

The only thing to watch out for is if you change PIC's be sure to check the specs of the new device. The source resistance of the internal switch and the cap can be different for different families, so that can change the calculation.

[aratti]

Henrik, I am using a rather old pic (18F2620), I still have a couple of hundreds of them. But timing apart your and mine are very similar in the setting of the TAD value and the acquisition time.
As far as TAD value is concerned I can Select value up to Fosc/64 (minimum delay required for 1 TAD = 1.6 us)
As far as ACQUISTION TIME is concerned, I have a range selectable from 2 TAD to 20 TAD. (So you have a lower and upper limits)

I made some experimenting some years ago, using the manual acquisition time, so I could increase/decrease the acquisition time beyond the 2/20 TAD limits. Unfortunatly I was not able to find these data (I have been digging for the whole day without success) but I assure you they were very interesting. The experiment was done taking 100 reading at three specific points of the 10 bits range. First point was fixed to 10 ADC count (low end), second point in the middle at 512 ADC count and the third point at 1012 ADC count (high end). These reading were taken at different acquisition time going from 1 us up to 200 us in steps of 10 us each.
Once, all the data were collected ( they have been collected via RS232 connection and loaded directly into an Excel sheet), the statistic applied were : Average/Standard deviation/standard error.
Looking at the standard error, I noticed an increase in the two extreme regions when the acquisition time was too low or higher than the 2/20 TADs region , no significant variation of standard error were observed in the middle range.
I will continue to search these files (I am sure I still have them somewhere) and If I found them I will post them here in this thread.
But everybody, with some time to spare, can repeat the experiment on his own.
Cheers.
Al

[HenrikOlsson]

Hi,
OK, I did the test.

For this I was using a 18F25K20 running at 64MHz and 3.3V. Vref set to VDD/VSS and measured at 2.999V.
Actual voltages measured with a FLUKE189

The test program cycles thru all the available clock selections and all the available acquisition times (49 combinations). For each selection it takes 8 readings on AN0. Between each reading it makes a dummy conversion of another (floating) channel. After 8 readings it calculates the averages and presents the result.

The test was conducted at 3 different input voltages (25mV, 1.65V, 3.285V) with 3 different source impedences (1k, 10k, 39k). For the 18F25K20 10k is the max recommended source impedence.

Here's the code if anyone wants to repeat the test.
Please note the ADC clock and acquisition time may be different on other devices, this matches the 25K20 that I used:

'****************************************************************
'*  Name    : ADC Test.PBP                                      *
'*  Author  : Henrik Olsson                                     *
'*  Notice  : Copyright (c) 2015 Henrik Olsson                  *
'*          : All Rights Reserved                               *
'*  Date    : 2015-07-23                                        *
'*  Version : 1.0                                               *
'*  Notes   : Test program for verifying ADC clock selection    *
'*          : and acquisition times.                            *
'*          : For 18F25K20.                                     *
'****************************************************************
DEFINE OSC 64
DEFINE LOADER_USED 1                        ' We're using a bootloader.
DEFINE HSER_RCSTA 90h                       ' Enable serial port & continuous receive
DEFINE HSER_TXSTA 24h                       ' Enable transmit, BRGH = 1
DEFINE HSER_CLROERR 1                       ' Clear overflow automatically
DEFINE HSER_SPBRG 138                       ' 115200 Baud @ 64MHz, -0,08%

SPBRGH = 0
BAUDCON.3 = 1                               ' Enable 16 bit baudrate generator

ADCON0 = %00000001                          ' Enable the ADC, select AN0
ADCON1 = %00000000                          ' Vref is Vdd/Vss respectively
ADCON2 = %10001000                          ' Right justified result, 2TAD, FOosc/2

TRISA.0 = 1                                 ' RA0/AN0 is input
ANSEL.0 = 1                                 ' Digital buffer disabled, analog enabled

Samples VAR BYTE
ACQT VAR BYTE
ADCS VAR BYTE
RESULT VAR WORD
Accumulator VAR WORD

GODONE VAR ADCON0.1

PAUSE 1000
HSEROUT["Start",10,13]

For ACQT = 1 to 7                           ' We have 7 different TAD settings ranging from 2TAD to 20TAD
    For ADCS = 0 to 6                       ' We hace 7 different conversion clock selections 
        ADCON2 = 128 + (ACQT * 8) + ADCS    ' "Build the ADCON2 word by combining the two settings.
        
        PAUSE 25
        
        Accumulator = 0
                
        For samples = 0 to 7                ' Take 8 samples
            
            ' First make a dummy conversion of a channel we're not really interested in
            ADCON0.2 = 1
            GODONE = 1
            WHILE GODONE : WEND
            
            ' Then switch to AN0 and make a conversion.
            ADCON0.2 = 0
            GODONE = 1
            WHILE GODONE : WEND            
            
            ' Get the result and add it to the accumulator
            Result.HighByte=ADRESH
            Result.LOWBYTE=ADRESL
            Accumulator = Accumulator + Result

        NEXT
        
        ' Divide accumulated result by 8 to get the average of our 8 readings.
        Result = Accumulator >> 3
        
        ' These selections matches the 18F25K20
        Select CASE ADCS
            CASE 0
                HSEROUT["FOSC/2     "]
            CASE 1
                HSEROUT["FOSC/8     "]
            CASE 2
                HSEROUT["FOSC/32    "]
            CASE 3
                HSEROUT["FRC        "]
            CASE 4
                HSEROUT["FOSC/4     "]
            CASE 5
                HSEROUT["FOSC/16    "]
            CASE 6
                HSEROUT["FOSC/64    "]
        END Select
        
        
        SELECT CASE ACQT
            CASE 1
                HSEROUT["2 TAD    "]
            CASE 2
                HSEROUT["4 TAD    "]
            CASE 3
                HSEROUT["6 TAD    "]
            CASE 4
                HSEROUT["8 TAD    "]
            CASE 5
                HSEROUT["12 TAD   "]
            CASE 6
                HSEROUT["16 TAD   "]
            CASE 7
                HSEROUT["20 TAD   "]        
        END SELECT
        
        ' Report the result.
        HSEROUT["  Result: ", #RESULT, 13]
        
    NEXT
NEXT

PAUSE 100

END

And here are the results:

*****************************************************
*****************************************************
VRef is VDD/VSS measured to 3.299V
1k resistor in series with input
*****************************************************
*****************************************************

Input voltage 25.7mV
FOSC/2     2 TAD      Result: 8
FOSC/8     2 TAD      Result: 8
FOSC/32    2 TAD      Result: 7
FRC        2 TAD      Result: 7
FOSC/4     2 TAD      Result: 8
FOSC/16    2 TAD      Result: 7
FOSC/64    2 TAD      Result: 7
FOSC/2     4 TAD      Result: 9
FOSC/8     4 TAD      Result: 7
FOSC/32    4 TAD      Result: 7
FRC        4 TAD      Result: 7
FOSC/4     4 TAD      Result: 8
FOSC/16    4 TAD      Result: 7
FOSC/64    4 TAD      Result: 7
FOSC/2     6 TAD      Result: 8
FOSC/8     6 TAD      Result: 7
FOSC/32    6 TAD      Result: 7
FRC        6 TAD      Result: 7
FOSC/4     6 TAD      Result: 7
FOSC/16    6 TAD      Result: 7
FOSC/64    6 TAD      Result: 7
FOSC/2     8 TAD      Result: 8
FOSC/8     8 TAD      Result: 7
FOSC/32    8 TAD      Result: 6
FRC        8 TAD      Result: 7
FOSC/4     8 TAD      Result: 7
FOSC/16    8 TAD      Result: 7
FOSC/64    8 TAD      Result: 7
FOSC/2     12 TAD     Result: 8
FOSC/8     12 TAD     Result: 8
FOSC/32    12 TAD     Result: 7
FRC        12 TAD     Result: 7
FOSC/4     12 TAD     Result: 7
FOSC/16    12 TAD     Result: 7
FOSC/64    12 TAD     Result: 7
FOSC/2     16 TAD     Result: 8
FOSC/8     16 TAD     Result: 7
FOSC/32    16 TAD     Result: 7
FRC        16 TAD     Result: 7
FOSC/4     16 TAD     Result: 7
FOSC/16    16 TAD     Result: 7
FOSC/64    16 TAD     Result: 7
FOSC/2     20 TAD     Result: 8
FOSC/8     20 TAD     Result: 7
FOSC/32    20 TAD     Result: 7
FRC        20 TAD     Result: 7
FOSC/4     20 TAD     Result: 7
FOSC/16    20 TAD     Result: 7
FOSC/64    20 TAD     Result: 7

Input Voltage: 1.650V
FOSC/2     2 TAD      Result: 497
FOSC/8     2 TAD      Result: 509
FOSC/32    2 TAD      Result: 511
FRC        2 TAD      Result: 511
FOSC/4     2 TAD      Result: 508
FOSC/16    2 TAD      Result: 511
FOSC/64    2 TAD      Result: 511
FOSC/2     4 TAD      Result: 504
FOSC/8     4 TAD      Result: 511
FOSC/32    4 TAD      Result: 511
FRC        4 TAD      Result: 511
FOSC/4     4 TAD      Result: 512
FOSC/16    4 TAD      Result: 511
FOSC/64    4 TAD      Result: 511
FOSC/2     6 TAD      Result: 507
FOSC/8     6 TAD      Result: 511
FOSC/32    6 TAD      Result: 511
FRC        6 TAD      Result: 511
FOSC/4     6 TAD      Result: 512
FOSC/16    6 TAD      Result: 511
FOSC/64    6 TAD      Result: 511
FOSC/2     8 TAD      Result: 509
FOSC/8     8 TAD      Result: 511
FOSC/32    8 TAD      Result: 511
FRC        8 TAD      Result: 511
FOSC/4     8 TAD      Result: 512
FOSC/16    8 TAD      Result: 511
FOSC/64    8 TAD      Result: 511
FOSC/2     12 TAD     Result: 511
FOSC/8     12 TAD     Result: 511
FOSC/32    12 TAD     Result: 511
FRC        12 TAD     Result: 512
FOSC/4     12 TAD     Result: 512
FOSC/16    12 TAD     Result: 511
FOSC/64    12 TAD     Result: 511
FOSC/2     16 TAD     Result: 511
FOSC/8     16 TAD     Result: 511
FOSC/32    16 TAD     Result: 511
FRC        16 TAD     Result: 511
FOSC/4     16 TAD     Result: 512
FOSC/16    16 TAD     Result: 511
FOSC/64    16 TAD     Result: 511
FOSC/2     20 TAD     Result: 511
FOSC/8     20 TAD     Result: 511
FOSC/32    20 TAD     Result: 511
FRC        20 TAD     Result: 511
FOSC/4     20 TAD     Result: 512
FOSC/16    20 TAD     Result: 512
FOSC/64    20 TAD     Result: 511

Input Voltage: 3.286V
FOSC/2     2 TAD      Result: 511
FOSC/8     2 TAD      Result: 1013
FOSC/32    2 TAD      Result: 1019
FRC        2 TAD      Result: 1019
FOSC/4     2 TAD      Result: 995
FOSC/16    2 TAD      Result: 1018
FOSC/64    2 TAD      Result: 1019
FOSC/2     4 TAD      Result: 511
FOSC/8     4 TAD      Result: 1019
FOSC/32    4 TAD      Result: 1019
FRC        4 TAD      Result: 1020
FOSC/4     4 TAD      Result: 1011
FOSC/16    4 TAD      Result: 1020
FOSC/64    4 TAD      Result: 1019
FOSC/2     6 TAD      Result: 1006
FOSC/8     6 TAD      Result: 1019
FOSC/32    6 TAD      Result: 1019
FRC        6 TAD      Result: 1020
FOSC/4     6 TAD      Result: 1016
FOSC/16    6 TAD      Result: 1019
FOSC/64    6 TAD      Result: 1019
FOSC/2     8 TAD      Result: 1014
FOSC/8     8 TAD      Result: 1020
FOSC/32    8 TAD      Result: 1019
FRC        8 TAD      Result: 1019
FOSC/4     8 TAD      Result: 1018
FOSC/16    8 TAD      Result: 1019
FOSC/64    8 TAD      Result: 1019
FOSC/2     12 TAD     Result: 1017
FOSC/8     12 TAD     Result: 1019
FOSC/32    12 TAD     Result: 1019
FRC        12 TAD     Result: 1019
FOSC/4     12 TAD     Result: 1020
FOSC/16    12 TAD     Result: 1019
FOSC/64    12 TAD     Result: 1019
FOSC/2     16 TAD     Result: 1019
FOSC/8     16 TAD     Result: 1019
FOSC/32    16 TAD     Result: 1019
FRC        16 TAD     Result: 1019
FOSC/4     16 TAD     Result: 1020
FOSC/16    16 TAD     Result: 1019
FOSC/64    16 TAD     Result: 1019
FOSC/2     20 TAD     Result: 1020
FOSC/8     20 TAD     Result: 1020
FOSC/32    20 TAD     Result: 1019
FRC        20 TAD     Result: 1019
FOSC/4     20 TAD     Result: 1020
FOSC/16    20 TAD     Result: 1020
FOSC/64    20 TAD     Result: 1019

*****************************************************
*****************************************************
VRef is VDD/VSS measured to 3.299V
10k resistor in series with input
*****************************************************
*****************************************************

Input voltage 25.7mV
FOSC/2     2 TAD      Result: 9
FOSC/8     2 TAD      Result: 8
FOSC/32    2 TAD      Result: 7
FRC        2 TAD      Result: 7
FOSC/4     2 TAD      Result: 8
FOSC/16    2 TAD      Result: 7
FOSC/64    2 TAD      Result: 7
FOSC/2     4 TAD      Result: 8
FOSC/8     4 TAD      Result: 7
FOSC/32    4 TAD      Result: 7
FRC        4 TAD      Result: 7
FOSC/4     4 TAD      Result: 8
FOSC/16    4 TAD      Result: 7
FOSC/64    4 TAD      Result: 7
FOSC/2     6 TAD      Result: 8
FOSC/8     6 TAD      Result: 7
FOSC/32    6 TAD      Result: 7
FRC        6 TAD      Result: 7
FOSC/4     6 TAD      Result: 7
FOSC/16    6 TAD      Result: 7
FOSC/64    6 TAD      Result: 7
FOSC/2     8 TAD      Result: 8
FOSC/8     8 TAD      Result: 7
FOSC/32    8 TAD      Result: 7
FRC        8 TAD      Result: 7
FOSC/4     8 TAD      Result: 7
FOSC/16    8 TAD      Result: 7
FOSC/64    8 TAD      Result: 7
FOSC/2     12 TAD     Result: 8
FOSC/8     12 TAD     Result: 7
FOSC/32    12 TAD     Result: 7
FRC        12 TAD     Result: 7
FOSC/4     12 TAD     Result: 7
FOSC/16    12 TAD     Result: 7
FOSC/64    12 TAD     Result: 7
FOSC/2     16 TAD     Result: 8
FOSC/8     16 TAD     Result: 8
FOSC/32    16 TAD     Result: 7
FRC        16 TAD     Result: 7
FOSC/4     16 TAD     Result: 8
FOSC/16    16 TAD     Result: 7
FOSC/64    16 TAD     Result: 7
FOSC/2     20 TAD     Result: 8
FOSC/8     20 TAD     Result: 7
FOSC/32    20 TAD     Result: 7
FRC        20 TAD     Result: 7
FOSC/4     20 TAD     Result: 7
FOSC/16    20 TAD     Result: 7
FOSC/64    20 TAD     Result: 7


Input voltage 1.650V
FOSC/2     2 TAD      Result: 445
FOSC/8     2 TAD      Result: 471
FOSC/32    2 TAD      Result: 508
FRC        2 TAD      Result: 511
FOSC/4     2 TAD      Result: 448
FOSC/16    2 TAD      Result: 494
FOSC/64    2 TAD      Result: 511
FOSC/2     4 TAD      Result: 457
FOSC/8     4 TAD      Result: 493
FOSC/32    4 TAD      Result: 511
FRC        4 TAD      Result: 511
FOSC/4     4 TAD      Result: 470
FOSC/16    4 TAD      Result: 507
FOSC/64    4 TAD      Result: 511
FOSC/2     6 TAD      Result: 461
FOSC/8     6 TAD      Result: 503
FOSC/32    6 TAD      Result: 511
FRC        6 TAD      Result: 511
FOSC/4     6 TAD      Result: 485
FOSC/16    6 TAD      Result: 510
FOSC/64    6 TAD      Result: 511
FOSC/2     8 TAD      Result: 472
FOSC/8     8 TAD      Result: 507
FOSC/32    8 TAD      Result: 511
FRC        8 TAD      Result: 511
FOSC/4     8 TAD      Result: 494
FOSC/16    8 TAD      Result: 511
FOSC/64    8 TAD      Result: 511
FOSC/2     12 TAD     Result: 478
FOSC/8     12 TAD     Result: 510
FOSC/32    12 TAD     Result: 511
FRC        12 TAD     Result: 511
FOSC/4     12 TAD     Result: 510
FOSC/16    12 TAD     Result: 511
FOSC/64    12 TAD     Result: 511
FOSC/2     16 TAD     Result: 493
FOSC/8     16 TAD     Result: 511
FOSC/32    16 TAD     Result: 511
FRC        16 TAD     Result: 511
FOSC/4     16 TAD     Result: 512
FOSC/16    16 TAD     Result: 511
FOSC/64    16 TAD     Result: 511
FOSC/2     20 TAD     Result: 503
FOSC/8     20 TAD     Result: 511
FOSC/32    20 TAD     Result: 511
FRC        20 TAD     Result: 511
FOSC/4     20 TAD     Result: 512
FOSC/16    20 TAD     Result: 511
FOSC/64    20 TAD     Result: 511


Input voltage 3.286V
FOSC/2     2 TAD      Result: 511
FOSC/8     2 TAD      Result: 929
FOSC/32    2 TAD      Result: 1012
FRC        2 TAD      Result: 1019
FOSC/4     2 TAD      Result: 880
FOSC/16    2 TAD      Result: 981
FOSC/64    2 TAD      Result: 1019
FOSC/2     4 TAD      Result: 511
FOSC/8     4 TAD      Result: 980
FOSC/32    4 TAD      Result: 1018
FRC        4 TAD      Result: 1019
FOSC/4     4 TAD      Result: 926
FOSC/16    4 TAD      Result: 1011
FOSC/64    4 TAD      Result: 1019
FOSC/2     6 TAD      Result: 511
FOSC/8     6 TAD      Result: 1001
FOSC/32    6 TAD      Result: 1019
FRC        6 TAD      Result: 1020
FOSC/4     6 TAD      Result: 959
FOSC/16    6 TAD      Result: 1017
FOSC/64    6 TAD      Result: 1019
FOSC/2     8 TAD      Result: 511
FOSC/8     8 TAD      Result: 1010
FOSC/32    8 TAD      Result: 1019
FRC        8 TAD      Result: 1019
FOSC/4     8 TAD      Result: 979
FOSC/16    8 TAD      Result: 1019
FOSC/64    8 TAD      Result: 1019
FOSC/2     12 TAD     Result: 511
FOSC/8     12 TAD     Result: 1017
FOSC/32    12 TAD     Result: 1020
FRC        12 TAD     Result: 1020
FOSC/4     12 TAD     Result: 1000
FOSC/16    12 TAD     Result: 1019
FOSC/64    12 TAD     Result: 1019
FOSC/2     16 TAD     Result: 923
FOSC/8     16 TAD     Result: 1019
FOSC/32    16 TAD     Result: 1019
FRC        16 TAD     Result: 1019
FOSC/4     16 TAD     Result: 1010
FOSC/16    16 TAD     Result: 1019
FOSC/64    16 TAD     Result: 1019
FOSC/2     20 TAD     Result: 990
FOSC/8     20 TAD     Result: 1019
FOSC/32    20 TAD     Result: 1019
FRC        20 TAD     Result: 1020
FOSC/4     20 TAD     Result: 1015
FOSC/16    20 TAD     Result: 1019
FOSC/64    20 TAD     Result: 1019


*****************************************************
*****************************************************
VRef is VDD/VSS measured to 3.299V
39k resistor in series with input
*****************************************************
*****************************************************

Input voltage: 25.7mV
FOSC/2     2 TAD      Result: 11
FOSC/8     2 TAD      Result: 7
FOSC/32    2 TAD      Result: 7
FRC        2 TAD      Result: 7
FOSC/4     2 TAD      Result: 8
FOSC/16    2 TAD      Result: 7
FOSC/64    2 TAD      Result: 7
FOSC/2     4 TAD      Result: 9
FOSC/8     4 TAD      Result: 7
FOSC/32    4 TAD      Result: 7
FRC        4 TAD      Result: 7
FOSC/4     4 TAD      Result: 7
FOSC/16    4 TAD      Result: 7
FOSC/64    4 TAD      Result: 7
FOSC/2     6 TAD      Result: 8
FOSC/8     6 TAD      Result: 8
FOSC/32    6 TAD      Result: 7
FRC        6 TAD      Result: 7
FOSC/4     6 TAD      Result: 8
FOSC/16    6 TAD      Result: 7
FOSC/64    6 TAD      Result: 7
FOSC/2     8 TAD      Result: 8
FOSC/8     8 TAD      Result: 7
FOSC/32    8 TAD      Result: 7
FRC        8 TAD      Result: 7
FOSC/4     8 TAD      Result: 8
FOSC/16    8 TAD      Result: 7
FOSC/64    8 TAD      Result: 7
FOSC/2     12 TAD     Result: 9
FOSC/8     12 TAD     Result: 7
FOSC/32    12 TAD     Result: 7
FRC        12 TAD     Result: 7
FOSC/4     12 TAD     Result: 7
FOSC/16    12 TAD     Result: 8
FOSC/64    12 TAD     Result: 7
FOSC/2     16 TAD     Result: 8
FOSC/8     16 TAD     Result: 7
FOSC/32    16 TAD     Result: 7
FRC        16 TAD     Result: 7
FOSC/4     16 TAD     Result: 7
FOSC/16    16 TAD     Result: 7
FOSC/64    16 TAD     Result: 7
FOSC/2     20 TAD     Result: 8
FOSC/8     20 TAD     Result: 7
FOSC/32    20 TAD     Result: 7
FRC        20 TAD     Result: 7
FOSC/4     20 TAD     Result: 7
FOSC/16    20 TAD     Result: 7
FOSC/64    20 TAD     Result: 7

Input voltage: 1.650V
FOSC/2     2 TAD      Result: 406
FOSC/8     2 TAD      Result: 420
FOSC/32    2 TAD      Result: 470
FRC        2 TAD      Result: 507
FOSC/4     2 TAD      Result: 407
FOSC/16    2 TAD      Result: 441
FOSC/64    2 TAD      Result: 496
FOSC/2     4 TAD      Result: 408
FOSC/8     4 TAD      Result: 439
FOSC/32    4 TAD      Result: 495
FRC        4 TAD      Result: 511
FOSC/4     4 TAD      Result: 418
FOSC/16    4 TAD      Result: 469
FOSC/64    4 TAD      Result: 509
FOSC/2     6 TAD      Result: 415
FOSC/8     6 TAD      Result: 457
FOSC/32    6 TAD      Result: 505
FRC        6 TAD      Result: 511
FOSC/4     6 TAD      Result: 428
FOSC/16    6 TAD      Result: 485
FOSC/64    6 TAD      Result: 511
FOSC/2     8 TAD      Result: 418
FOSC/8     8 TAD      Result: 469
FOSC/32    8 TAD      Result: 509
FRC        8 TAD      Result: 512
FOSC/4     8 TAD      Result: 442
FOSC/16    8 TAD      Result: 495
FOSC/64    8 TAD      Result: 511
FOSC/2     12 TAD     Result: 427
FOSC/8     12 TAD     Result: 486
FOSC/32    12 TAD     Result: 511
FRC        12 TAD     Result: 511
FOSC/4     12 TAD     Result: 456
FOSC/16    12 TAD     Result: 506
FOSC/64    12 TAD     Result: 511
FOSC/2     16 TAD     Result: 450
FOSC/8     16 TAD     Result: 496
FOSC/32    16 TAD     Result: 511
FRC        16 TAD     Result: 512
FOSC/4     16 TAD     Result: 469
FOSC/16    16 TAD     Result: 509
FOSC/64    16 TAD     Result: 511
FOSC/2     20 TAD     Result: 453
FOSC/8     20 TAD     Result: 502
FOSC/32    20 TAD     Result: 511
FRC        20 TAD     Result: 511
FOSC/4     20 TAD     Result: 478
FOSC/16    20 TAD     Result: 510
FOSC/64    20 TAD     Result: 511

Input voltage: 3.286V
FOSC/2     2 TAD      Result: 768
FOSC/8     2 TAD      Result: 826
FOSC/32    2 TAD      Result: 931
FRC        2 TAD      Result: 1010
FOSC/4     2 TAD      Result: 801
FOSC/16    2 TAD      Result: 873
FOSC/64    2 TAD      Result: 986
FOSC/2     4 TAD      Result: 511
FOSC/8     4 TAD      Result: 867
FOSC/32    4 TAD      Result: 984
FRC        4 TAD      Result: 1019
FOSC/4     4 TAD      Result: 824
FOSC/16    4 TAD      Result: 929
FOSC/64    4 TAD      Result: 1014
FOSC/2     6 TAD      Result: 511
FOSC/8     6 TAD      Result: 904
FOSC/32    6 TAD      Result: 1006
FRC        6 TAD      Result: 1019
FOSC/4     6 TAD      Result: 847
FOSC/16    6 TAD      Result: 963
FOSC/64    6 TAD      Result: 1019
FOSC/2     8 TAD      Result: 511
FOSC/8     8 TAD      Result: 930
FOSC/32    8 TAD      Result: 1014
FRC        8 TAD      Result: 1019
FOSC/4     8 TAD      Result: 874
FOSC/16    8 TAD      Result: 985
FOSC/64    8 TAD      Result: 1019
FOSC/2     12 TAD     Result: 511
FOSC/8     12 TAD     Result: 963
FOSC/32    12 TAD     Result: 1019
FRC        12 TAD     Result: 1019
FOSC/4     12 TAD     Result: 901
FOSC/16    12 TAD     Result: 1006
FOSC/64    12 TAD     Result: 1019
FOSC/2     16 TAD     Result: 847
FOSC/8     16 TAD     Result: 985
FOSC/32    16 TAD     Result: 1019
FRC        16 TAD     Result: 1019
FOSC/4     16 TAD     Result: 927
FOSC/16    16 TAD     Result: 1015
FOSC/64    16 TAD     Result: 1019
FOSC/2     20 TAD     Result: 847
FOSC/8     20 TAD     Result: 999
FOSC/32    20 TAD     Result: 1019
FRC        20 TAD     Result: 1019
FOSC/4     20 TAD     Result: 950
FOSC/16    20 TAD     Result: 1017
FOSC/64    20 TAD     Result: 1019

I haven't analyzed any of it yet and I don't have any more time just now but I wanted to post the results. If anyone wants to graph the results in Excel or whatever to better see what's going on, please feel free to do so.

/Henrik.

[tumbleweed]

You certainly coverted all the bases! Good job.

Instead of bothering to graph all the data and try and make sense of it, Let's start with this:

The 25K20 has a TAD spec of 0.7us min.

At 64MHz, that means all ADCS settings except for FOSC/64 and FRC violate minimum times (Table 19-1).
Using the FOSC/64 setting gives a TAD of 1us

Computing Tc from equation 19-1 for each of the three source resistances gives:
1K = 0.27us
10K = 1.2us
39K = 4.2us

Computing TACQ = Tamp + Tc + Tcoff, given Tamp = 5us and Tcoff = 0 (25degC) produces:
1K = 5.27us
10K = 6.2us
39K = 9.2us

So, since TAD = 1us, TACQ must be a min of ~6-9 TAD for the three different resistances.

Removing all of the measurements that don't fit the above you get:

*****************************************************
1k resistor in series with input (TACQ=5.27us)
*****************************************************
Input voltage 25.7mV
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

Input Voltage: 1.650V
FRC        6 TAD      Result: 511
FOSC/64    6 TAD      Result: 511
FRC        8 TAD      Result: 511
FOSC/64    8 TAD      Result: 511
FRC        12 TAD     Result: 512
FOSC/64    12 TAD     Result: 511
FRC        16 TAD     Result: 511
FOSC/64    16 TAD     Result: 511
FRC        20 TAD     Result: 511
FOSC/64    20 TAD     Result: 511

Input Voltage: 3.286V
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: 1019
FOSC/64    12 TAD     Result: 1019
FRC        16 TAD     Result: 1019
FOSC/64    16 TAD     Result: 1019
FRC        20 TAD     Result: 1019
FOSC/64    20 TAD     Result: 1019


*****************************************************
10k resistor in series with input  (TACQ=6.2us)
*****************************************************

Input voltage 25.7mV
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


Input voltage 1.650V
FRC        6 TAD      Result: 511
FOSC/64    6 TAD      Result: 511
FRC        8 TAD      Result: 511
FOSC/64    8 TAD      Result: 511
FRC        12 TAD     Result: 511
FOSC/64    12 TAD     Result: 511
FRC        16 TAD     Result: 511
FOSC/64    16 TAD     Result: 511
FRC        20 TAD     Result: 511
FOSC/64    20 TAD     Result: 511


Input voltage 3.286V
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: 1019
FRC        16 TAD     Result: 1019
FOSC/64    16 TAD     Result: 1019
FRC        20 TAD     Result: 1020
FOSC/64    20 TAD     Result: 1019


*****************************************************
39k resistor in series with input  (TACQ=9.2us)
*****************************************************

Input voltage: 25.7mV
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

Input voltage: 1.650V
FRC        6 TAD      Result: 511
FOSC/64    6 TAD      Result: 511
FRC        8 TAD      Result: 512
FOSC/64    8 TAD      Result: 511
FRC        12 TAD     Result: 511
FOSC/64    12 TAD     Result: 511
FRC        16 TAD     Result: 512
FOSC/64    16 TAD     Result: 511
FRC        20 TAD     Result: 511
FOSC/64    20 TAD     Result: 511

Input voltage: 3.286V
FRC        6 TAD      Result: 1019
FOSC/64    6 TAD      Result: 1019
FRC        8 TAD      Result: 1019
FOSC/64    8 TAD      Result: 1019
FRC        12 TAD     Result: 1019
FOSC/64    12 TAD     Result: 1019
FRC        16 TAD     Result: 1019
FOSC/64    16 TAD     Result: 1019
FRC        20 TAD     Result: 1019
FOSC/64    20 TAD     Result: 1019

Pretty consistent. It shows that waiting longer than the minimum doesn't get you anything, but it also doesn't change anything either.
The interesting thing is as you look through the raw data you can see the effect of not using the proper TAD setting for the clock freq you're using, or not waiting/setting TACQ to at least the minimum time.