PIC ADC

Discussion in 'General Electronics Chat' started by mik3, Mar 2, 2009.

  1. mik3

    Thread Starter Senior Member

    Feb 4, 2008
    4,846
    63
    Hello guys,

    Does anyone knows what is the maximum sampling frequency for an analog port of a PIC uC working at 8MHz?

    I am using PIC18F2550 and I want to sample a 10KHz sinewave as to calculate its average.
    From the datasheet of the uC I calculated that the maximum achievable sampling rate is about 85KHz. This value is too low for my needs.

    Am i correct in my calculation or I misunderstood something?
     
  2. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    Not sure, but does not sound too far off to me. If you just want the average why not do that in an analog circuit?
     
  3. mik3

    Thread Starter Senior Member

    Feb 4, 2008
    4,846
    63
    My bright ideas to reduce components :p
     
  4. ElVale

    New Member

    Mar 3, 2009
    2
    0
    Why is 85KHz not enough for a 10 KHz signal? anyway you have to check out the adquisition time in the datasheet at the A/D conversion requirements table. If I'm not wrong, maximum sampling frequency posible is about 130KHz.
     
  5. jut

    Senior Member

    Aug 25, 2007
    224
    2
    I see you're using a USB PIC. I ventured there before, but got scared and went back to RS232. Have you got the USB communications working?

    In table 28-29, page 400 of the most current 2550 data sheet, I'm seeing a

    acquisition time of...
    1.4 us

    conversion time of...
    11*Tad = 11(.7 us) = 7.7 us

    discharge time of...
    0.2 us

    Total is 9.3 us. Which is 107 kHz.
     
  6. mik3

    Thread Starter Senior Member

    Feb 4, 2008
    4,846
    63
    Jut,

    I won't use the USB function and I got almost the same calculations as you. However, this a very low sampling frequency to sample a 10KHz signal and obtain good results thus I will use analog components to find the average value of the 10 KHz signal.
     
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