RC filter before ADC for weighing scale application

Discussion in 'General Electronics Chat' started by gotumal, Jul 24, 2008.

  1. gotumal

    Thread Starter Active Member

    Mar 24, 2008
    99
    0
    Dear reader,

    I have a weighing scale board with AD623 and AD7708 as an ADC. After instrumentaion amplifier, I have RC filter. Initially I used 1K and 0.uF. But because of fluctuations, I kept on increeasing RC valuse until I reach 100k and 0.33uF. With this fluctuations seems to be less without ham[ering responce. Now when I kept 1Kg. weight, it drifts by 20 gms in 20 mins :confused:.

    I suspected capacitor discharge votage, so I checked waveforms on CRO. To my surprise, DC line was pretty clear before RC filter. So I opened ADC pin and knew that noice was introduced by ADC itself. Now stuck up since I have al;ready taken all the care including bypass capacitor, grounding plane, ripple in poer supply, stable refernce volateg etc.

    Need a direction!

    Could anybody send link for filter calculation for various sensors?

    Thanks in advance.
     
  2. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    Is the buffer enabled in the AD7708? Also, how stable is your Vref?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Is CHOP\ = 0?

    What's the gain of your instrumentation amp?
     
  4. gotumal

    Thread Starter Active Member

    Mar 24, 2008
    99
    0
    Thanks for your replies.

    But my reference is pretty constant. The gain is around 110 an chop is enabled.
     
Loading...