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Pic temp sensor

Discussion in 'Electronic Design' started by Pj, Feb 22, 2008.

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  1. Pj

    Pj Guest

    What would probably be the likely cause of the output bouncing on the
    lower 4 bits, bits 0,1,2,3, but has good stability on the upper bits.
    I'm using a junk box Pic 16f807 with one input line to AN0 which is
    being fed from the output of a LM324 op amp wired as a non-inverting
    dc gain. The signal to the op amp is coming directly from a
    thermocouple.

    If it's do to noise why does it only bounce on the lower bits?

    Pj
     
  2. Fred Bloggs

    Fred Bloggs Guest

    Lower bits are the smaller voltage detection bits, right...
     
  3. John Larkin

    John Larkin Guest


    It's analog noise from the opamp; digitized, the lower bits wobble.

    An LM324 is a terrible opamp for this. But expect noise anyhow, since
    thermocouples make tiny voltages.

    Use a better opamp, and take lots of samples and average.

    John
     
  4. Arlet Ottens

    Arlet Ottens Guest

    Or use a better ADC. For instance, the AD7792 has an integrated
    amplifier, and 50/60 Hz filters. With some careful layout, it will get
    you 16 stable bits.
     
  5. Guest

    A low pass filter to reduce the bandwidth will reduce the noise, you
    only need dc to 0.5Hz at most.
     
  6. Maybe he only needs 8 or 9 bits... not everything has to be a frick'n
    gold plated design y'know.
    Best regards,
    Spehro Pefhany
     
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