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Help configuring A/D parameters on 18F87K22

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  • Andy Wood
    replied
    Thanks Henrik!

    I think sometimes my brain over complicates simple things...

    Andy

    Leave a comment:


  • Henrik Olsson
    replied
    That's correct Andy.

    Leave a comment:


  • Andy Wood
    replied
    Hi Dave,

    Thanks for replying. I think I am over thinking this and maybe the answer to my question is simple and obvious:

    MCU running at 16Mhz
    ADCS bits are set to 101 (16Tosc)

    16MHz/16 = 1Mhz; 1/1MHz = 1uS... so in this example, Tad =1uS

    Am I correct?

    Andy

    Leave a comment:


  • DavidP
    replied
    Andy, It all depends on the frequency you have selected for the A/D clock. Attached is the data sheet relevant to the 18F87K22.
    18F87K22.pdf

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  • Andy Wood
    replied
    Hi Henrik,

    Thanks for your reply. I think I am missing something very basic here - what is the relationship between uS and Tad? Or, to ask a different way - how may uS is one Tad?

    Andy

    Leave a comment:


  • Henrik Olsson
    replied
    There are several things that needs to be taken into consideration. If you want to calculate it the complete procedure is outlined in section 23.3 in the datasheet.

    In the electrical specifications for the device it says that MINIMUM acquistion time, with a source impedence of 50ohm or less, is 1.4us (table 31-28). The example calculation uses 2.5k (which is the recommended MAXIMUM source impedence) and arrives at 2.5us

    IME there's no such thing as too long acqusition time so if you're not hunting instruction cycles and don't want to bother with calculating it, just go with something like 6-8Tad in your case.

    /Henrik.

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  • Andy Wood
    started a topic Help configuring A/D parameters on 18F87K22

    Help configuring A/D parameters on 18F87K22

    MCU running at 16Mhz. ADCS bits are set to 101 (16Tosc). If I wish to use a programmable acquisition time, how do I calculate the values for the ACQT bits?

    Regards,

    Andy
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