Realistic ADC measurement capabilities

Discussion in 'General Electronics Chat' started by JeremyB, Sep 28, 2012.

  1. JeremyB

    Thread Starter New Member

    Jul 22, 2012
    8
    0
    Hi guys,

    I'm using PIC18F87J72 energy metering device and attempting to measure AC power by sensing across a 0.0005 ohm shunt resistor. Am I crazy to think that I will be able to see a 1 uV difference on the Current +/- differential inputs?Settings: 24-bit, Gain of 32 (maximum), OSR 256, DRCLK 3.9KHz, Dithering on. Both input signals use up the majority of the allowed input range (+/- 600mV). So far all of the readings just looks the same and I think it's because I just can't detect that small of a voltage in the presence of noise. Thoughts would be greatly appreciated!!

    Thanks,
    Jeremy
     
  2. nigelwright7557

    Senior Member

    May 10, 2008
    487
    71
    Use vref divided by the max a2d converter output number to get the minimum voltage change you can detect.
     
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Welcome to the real world! :)

    You are always going to get many mV of noise on your ADC line. You can low pass filter to remove some of the noise, then average readings in software to remove more of it. Both of those work better when you are measuring something real slow, so you can be heavy handed with the filtering.

    But ultimately the best solution comes from having a larger signal in comparison to the noise. Getting extreme with grounding and shielding etc will help a bit, but even your PIC micro itself will be generating a few mV or HF noise in all the electrics around it.

    Like most questions people ask, it would be so much easier to help if you gave more info. Like if you said exactly what you are measuring, how often, circuit sepcifics, and desired data rate out. :)
     
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