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
    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
    Is the buffer enabled in the AD7708? Also, how stable is your Vref?
  3. SgtWookie


    Jul 17, 2007
    Is CHOP\ = 0?

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

    Thread Starter Active Member

    Mar 24, 2008
    Thanks for your replies.

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