Noises in Crane scale

Discussion in 'The Projects Forum' started by ruchirvn, May 17, 2012.

  1. ruchirvn

    Thread Starter New Member

    May 17, 2012
    2
    0
    Hi,

    I am building a wireless crane scale for my project. My receiver side is finished but at the transmitter side I am using load cell of 2 ton capacity and I am exciting it with 5V DC voltage. I am using 3 op27 op-amps to built instrumentation amplifier and then a LPF stage after that I am using ADC 7109 to convert into digital value but I am getting noise in the circuit at the filter stage itself .Could you please tell me how can I suppress this noise ?

    Thanks and Regards
    Ruchir
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,036
    Without seeing a schematic? Are you kidding? Adding lots of capacitors might help, but it's hard to say more without a schematic.
     
  3. ruchirvn

    Thread Starter New Member

    May 17, 2012
    2
    0
    Hi,
    I have attached schematic in pdf format with this post.
    please go through it once...
     
  4. BSomer

    Member

    Dec 28, 2011
    433
    106
    I noticed that you do not have any values for the resistors and capacitors in your schematic. I think that knowing what these values are can greatly help in troubleshooting your problems.
     
  5. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    Every power pin of every opamp (and every other chip) should have two decoupling/bypass capacitors in parallel, from right at the pin to (usually) ground (or the load's ground). Without any other information, just use a 0.1 uF X7R ceramic in parallel with a 10 uF electrolytic (with a voltage rating that's some margin above the supply rail voltage). There should also be a larger electrolytic capacitor from each rail to ground where the power rails enter the PCB.

    Your digital and analog power rails and grounds should run separately, all the way back to the power supply. And the analog signal grounds should be completely separate (until the star ground point at the power supply) from the analog power decoupling grounds, as should any digital signal grounds versus digital power decoupling ground.

    All conductor pairs should be kept as close together as possible, so they don't create any enclosed loop area, which would be an antenna. Pairs would include AC mains, rectified mains and gnd, signal and gnd, power rail and gnd, output and gnd, etc. Do not route one member of a pair apart from the other one, at all. Unless you have power and ground planes, best is directly on top of each other, on opposite sides of PCB. Otherwise, right next to each other with absolute-minimum spacing. Wire pairs should be tightly twisted together, all the way to the ends.

    Are you using ground and power planes?

    A photo or two of the actual circuit would be extremely helpful.
     
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