A/D Conversion Clock Select bits confussion ..

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  1. - 24th July 2015, 14:37 #1
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    tumbleweed
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    Default Re: A/D Conversion Clock Select bits confussion ..

    I wondered about that too. As you say, connecting it to a floating input would probably just leave the charge on Chold since all you have is the leakage current to discharge it.

    I noticed in figure 19-4 that they show a 2TAD "discharge" time between measurements, and they say this:
    19.2.3 DISCHARGE
    The discharge phase is used to initialize the value of
    the capacitor array. The array is discharged after every
    sample. This feature helps to optimize the unity-gain
    amplifier, as the circuit always needs to charge the
    capacitor array, rather than charge/discharge based on
    previous measure values.
    The PIC18 reference manual has the same sort of text, but is also has the statement
    The charge holding capacitor (CHOLD) is not discharged after each conversion
    so it sounds like they're talking about something to do with the internal ADC structure and not Chold.

    It WOULD be interesting to see the same measurements without any averaging and with alternating input voltages for the in between "idle" cycles.
  2. - 24th July 2015, 16:06 #2
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    Default Re: A/D Conversion Clock Select bits confussion ..

    Hi guys,
    OK, doing a dummy conversion on a floating input wasn't the greatest idea....

    I took out the averaging and set up my arb gen to generate 3.3Vpp noise and injected that into AN1 (the dummy channel).

    Code:
    25.6mV input, 10k source resistor, 3.3V noise on "dummy channel":
FOSC/2     2 TAD      Result: 512
FOSC/8     2 TAD      Result: 120
FOSC/32    2 TAD      Result: 23
FRC        2 TAD      Result: 6
FOSC/4     2 TAD      Result: 184
FOSC/16    2 TAD      Result: 54
FOSC/64    2 TAD      Result: 7
FOSC/2     4 TAD      Result: 190
FOSC/8     4 TAD      Result: 59
FOSC/32    4 TAD      Result: 14
FRC        4 TAD      Result: 13
FOSC/4     4 TAD      Result: 117
FOSC/16    4 TAD      Result: 21
FOSC/64    4 TAD      Result: 14
FOSC/2     6 TAD      Result: 127
FOSC/8     6 TAD      Result: 32
FOSC/32    6 TAD      Result: 11
FRC        6 TAD      Result: 11
FOSC/4     6 TAD      Result: 74
FOSC/16    6 TAD      Result: 7
FOSC/64    6 TAD      Result: 10
FOSC/2     8 TAD      Result: 512
FOSC/8     8 TAD      Result: 15
FOSC/32    8 TAD      Result: 15
FRC        8 TAD      Result: 12
FOSC/4     8 TAD      Result: 56
FOSC/16    8 TAD      Result: 10
FOSC/64    8 TAD      Result: 6
FOSC/2     12 TAD     Result: 512
FOSC/8     12 TAD     Result: 11
FOSC/32    12 TAD     Result: 16
FRC        12 TAD     Result: 13
FOSC/4     12 TAD     Result: 31
FOSC/16    12 TAD     Result: 11
FOSC/64    12 TAD     Result: 14
FOSC/2     16 TAD     Result: 512
FOSC/8     16 TAD     Result: 13
FOSC/32    16 TAD     Result: 11
FRC        16 TAD     Result: 11
FOSC/4     16 TAD     Result: 18
FOSC/16    16 TAD     Result: 11
FOSC/64    16 TAD     Result: 9
FOSC/2     20 TAD     Result: 49
FOSC/8     20 TAD     Result: 15
FOSC/32    20 TAD     Result: 10
FRC        20 TAD     Result: 8
FOSC/4     20 TAD     Result: 14
FOSC/16    20 TAD     Result: 10
FOSC/64    20 TAD     Result: 13

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

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

FOSC/2     2 TAD      Result: 511
FOSC/8     2 TAD      Result: 861
FOSC/32    2 TAD      Result: 1008
FRC        2 TAD      Result: 1022
FOSC/4     2 TAD      Result: 812
FOSC/16    2 TAD      Result: 963
FOSC/64    2 TAD      Result: 1014
FOSC/2     4 TAD      Result: 511
FOSC/8     4 TAD      Result: 964
FOSC/32    4 TAD      Result: 1021
FRC        4 TAD      Result: 1023
FOSC/4     4 TAD      Result: 888
FOSC/16    4 TAD      Result: 1004
FOSC/64    4 TAD      Result: 1015
FOSC/2     6 TAD      Result: 511
FOSC/8     6 TAD      Result: 994
FOSC/32    6 TAD      Result: 1019
FRC        6 TAD      Result: 1023
FOSC/4     6 TAD      Result: 911
FOSC/16    6 TAD      Result: 1018
FOSC/64    6 TAD      Result: 1019
FOSC/2     8 TAD      Result: 511
FOSC/8     8 TAD      Result: 1006
FOSC/32    8 TAD      Result: 1019
FRC        8 TAD      Result: 1021
FOSC/4     8 TAD      Result: 964
FOSC/16    8 TAD      Result: 1022
FOSC/64    8 TAD      Result: 1016
FOSC/2     12 TAD     Result: 511
FOSC/8     12 TAD     Result: 1015
FOSC/32    12 TAD     Result: 1019
FRC        12 TAD     Result: 1019
FOSC/4     12 TAD     Result: 991
FOSC/16    12 TAD     Result: 1023
FOSC/64    12 TAD     Result: 1015
FOSC/2     16 TAD     Result: 511
FOSC/8     16 TAD     Result: 1014
FOSC/32    16 TAD     Result: 1018
FRC        16 TAD     Result: 1018
FOSC/4     16 TAD     Result: 1005
FOSC/16    16 TAD     Result: 1016
FOSC/64    16 TAD     Result: 1023
FOSC/2     20 TAD     Result: 983
FOSC/8     20 TAD     Result: 1020
FOSC/32    20 TAD     Result: 1017
FRC        20 TAD     Result: 1023
FOSC/4     20 TAD     Result: 1010
FOSC/16    20 TAD     Result: 1017
FOSC/64    20 TAD     Result: 1019
    Then I connected the dummy channel to GND:
    Code:
    25.6mV input, 10k source resistor, "dummy channel" connected to GND:

FOSC/2     2 TAD      Result: 8
FOSC/8     2 TAD      Result: 6
FOSC/32    2 TAD      Result: 8
FRC        2 TAD      Result: 7
FOSC/4     2 TAD      Result: 6
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: 8
FRC        4 TAD      Result: 7
FOSC/4     4 TAD      Result: 6
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: 6
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: 8
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: 8
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: 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: 8
FOSC/16    20 TAD     Result: 8
FOSC/64    20 TAD     Result: 7

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

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

FOSC/2     2 TAD      Result: 511
FOSC/8     2 TAD      Result: 714
FOSC/32    2 TAD      Result: 996
FRC        2 TAD      Result: 1019
FOSC/4     2 TAD      Result: 610
FOSC/16    2 TAD      Result: 892
FOSC/64    2 TAD      Result: 1018
FOSC/2     4 TAD      Result: 511
FOSC/8     4 TAD      Result: 862
FOSC/32    4 TAD      Result: 1018
FRC        4 TAD      Result: 1018
FOSC/4     4 TAD      Result: 687
FOSC/16    4 TAD      Result: 991
FOSC/64    4 TAD      Result: 1019
FOSC/2     6 TAD      Result: 511
FOSC/8     6 TAD      Result: 955
FOSC/32    6 TAD      Result: 1020
FRC        6 TAD      Result: 1019
FOSC/4     6 TAD      Result: 761
FOSC/16    6 TAD      Result: 1010
FOSC/64    6 TAD      Result: 1019
FOSC/2     8 TAD      Result: 511
FOSC/8     8 TAD      Result: 983
FOSC/32    8 TAD      Result: 1020
FRC        8 TAD      Result: 1018
FOSC/4     8 TAD      Result: 836
FOSC/16    8 TAD      Result: 1018
FOSC/64    8 TAD      Result: 1019
FOSC/2     12 TAD     Result: 511
FOSC/8     12 TAD     Result: 1010
FOSC/32    12 TAD     Result: 1020
FRC        12 TAD     Result: 1019
FOSC/4     12 TAD     Result: 944
FOSC/16    12 TAD     Result: 1019
FOSC/64    12 TAD     Result: 1020
FOSC/2     16 TAD     Result: 511
FOSC/8     16 TAD     Result: 1017
FOSC/32    16 TAD     Result: 1019
FRC        16 TAD     Result: 1018
FOSC/4     16 TAD     Result: 981
FOSC/16    16 TAD     Result: 1020
FOSC/64    16 TAD     Result: 1019
FOSC/2     20 TAD     Result: 511
FOSC/8     20 TAD     Result: 1017
FOSC/32    20 TAD     Result: 1019
FRC        20 TAD     Result: 1019
FOSC/4     20 TAD     Result: 998
FOSC/16    20 TAD     Result: 1021
FOSC/64    20 TAD     Result: 1020
    And finally, dummy channel to Vdd (3.3V):
    Code:
    25.6mV input, 10k source resistor, "dummy channel" connected to Vdd:

FOSC/2     2 TAD      Result: 512
FOSC/8     2 TAD      Result: 344
FOSC/32    2 TAD      Result: 29
FRC        2 TAD      Result: 8
FOSC/4     2 TAD      Result: 432
FOSC/16    2 TAD      Result: 192
FOSC/64    2 TAD      Result: 9
FOSC/2     4 TAD      Result: 512
FOSC/8     4 TAD      Result: 262
FOSC/32    4 TAD      Result: 9
FRC        4 TAD      Result: 8
FOSC/4     4 TAD      Result: 383
FOSC/16    4 TAD      Result: 46
FOSC/64    4 TAD      Result: 8
FOSC/2     6 TAD      Result: 512
FOSC/8     6 TAD      Result: 180
FOSC/32    6 TAD      Result: 8
FRC        6 TAD      Result: 7
FOSC/4     6 TAD      Result: 350
FOSC/16    6 TAD      Result: 17
FOSC/64    6 TAD      Result: 7
FOSC/2     8 TAD      Result: 512
FOSC/8     8 TAD      Result: 99
FOSC/32    8 TAD      Result: 8
FRC        8 TAD      Result: 7
FOSC/4     8 TAD      Result: 312
FOSC/16    8 TAD      Result: 10
FOSC/64    8 TAD      Result: 7
FOSC/2     12 TAD     Result: 512
FOSC/8     12 TAD     Result: 25
FOSC/32    12 TAD     Result: 7
FRC        12 TAD     Result: 8
FOSC/4     12 TAD     Result: 229
FOSC/16    12 TAD     Result: 8
FOSC/64    12 TAD     Result: 7
FOSC/2     16 TAD     Result: 512
FOSC/8     16 TAD     Result: 12
FOSC/32    16 TAD     Result: 7
FRC        16 TAD     Result: 8
FOSC/4     16 TAD     Result: 147
FOSC/16    16 TAD     Result: 7
FOSC/64    16 TAD     Result: 7
FOSC/2     20 TAD     Result: 512
FOSC/8     20 TAD     Result: 9
FOSC/32    20 TAD     Result: 7
FRC        20 TAD     Result: 8
FOSC/4     20 TAD     Result: 66
FOSC/16    20 TAD     Result: 7
FOSC/64    20 TAD     Result: 7

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


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

FOSC/2     2 TAD      Result: 1022
FOSC/8     2 TAD      Result: 1021
FOSC/32    2 TAD      Result: 1020
FRC        2 TAD      Result: 1020
FOSC/4     2 TAD      Result: 1022
FOSC/16    2 TAD      Result: 1020
FOSC/64    2 TAD      Result: 1019
FOSC/2     4 TAD      Result: 1022
FOSC/8     4 TAD      Result: 1019
FOSC/32    4 TAD      Result: 1019
FRC        4 TAD      Result: 1019
FOSC/4     4 TAD      Result: 1019
FOSC/16    4 TAD      Result: 1021
FOSC/64    4 TAD      Result: 1020
FOSC/2     6 TAD      Result: 1023
FOSC/8     6 TAD      Result: 1021
FOSC/32    6 TAD      Result: 1019
FRC        6 TAD      Result: 1020
FOSC/4     6 TAD      Result: 1020
FOSC/16    6 TAD      Result: 1020
FOSC/64    6 TAD      Result: 1019
FOSC/2     8 TAD      Result: 1022
FOSC/8     8 TAD      Result: 1020
FOSC/32    8 TAD      Result: 1019
FRC        8 TAD      Result: 1019
FOSC/4     8 TAD      Result: 1020
FOSC/16    8 TAD      Result: 1021
FOSC/64    8 TAD      Result: 1019
FOSC/2     12 TAD     Result: 1022
FOSC/8     12 TAD     Result: 1020
FOSC/32    12 TAD     Result: 1019
FRC        12 TAD     Result: 1020
FOSC/4     12 TAD     Result: 1020
FOSC/16    12 TAD     Result: 1021
FOSC/64    12 TAD     Result: 1020
FOSC/2     16 TAD     Result: 1021
FOSC/8     16 TAD     Result: 1021
FOSC/32    16 TAD     Result: 1019
FRC        16 TAD     Result: 1021
FOSC/4     16 TAD     Result: 1021
FOSC/16    16 TAD     Result: 1019
FOSC/64    16 TAD     Result: 1019
FOSC/2     20 TAD     Result: 1020
FOSC/8     20 TAD     Result: 1021
FOSC/32    20 TAD     Result: 1019
FRC        20 TAD     Result: 1021
FOSC/4     20 TAD     Result: 1018
FOSC/16    20 TAD     Result: 1019
FOSC/64    20 TAD     Result: 1020
    Henrik.
  3. - 25th July 2015, 12:20 #3
    tumbleweed's Avatar
    tumbleweed
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    Default Re: A/D Conversion Clock Select bits confussion ..

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