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Load cell problem

Nicholas Cinti
Prodigy 20 points
Other Parts Discussed in Thread: LM324, INA333, INA331

Im using a LM 324 quad op amp to design an instrumentational amp. I would like a gain of 1000. For the Rgain value i am using 2000 ohm resistor and for the R value i am using 1M ohm resistor. When i connect the 5V power supply and the laod sensors i get an output of 2.6V constant, regardless of whether ipush down on the load cells or not.. I am wondering what I am doing wrong. The voltage output should change in correspondence with pushing down on the load cell?

I am using a SEN-10245 load cell. 

The equation i am using for the gain is Av = (1 +2R/Rgain)

I am trying to build a digital scale.

I loaded the circuit diagrams i am following 

Any help on the subject is appriciated. 

  • 0
    Expert 1095 points

    Hi Nicholas,

    I guess you are trying to make your own instrumentation amplifier by using LM324. Why don't you go for pre-made INA(Instrumentation amplifiers) for better results. And try avoiding to use large resistances like 1 M Ohm as they will probably contribute to noise. Also give more details about the circuit like what is the range of input voltage so that some one will analyse your circuit and reply. The probable reason in your current circuit is saturation of output in first stage of OP-AMPs itself I guess.

  • 0
    TI__Mastermind 25805 points
    Hello Nicholas,

    Though I don't support the LM devices, I do support TI's instrumentation amplifiers (INAs). For a plethora of reasons (e.g. common-mode range, noise, offset voltage, drift, PCB area, etc.) INAs are the preferred solution for this application. Given a single 5V supply, I recommend evaluating the INA333 for precision or the INA331 for cost vs. precision.
  • 0
    Prodigy 20 points

    Im using a LM 324 quad op amp to design an instrumentational amp. I would like a gain of 1000. For the Rgain value i am using 2000 ohm resistor and for the R value i am using 1M ohm resistor. When i connect the 5V power supply and the laod sensors i get an output of 2.6V constant, regardless of whether ipush down on the load cells or not.. I am wondering what I am doing wrong. The voltage output should change in correspondence with pushing down on the load cell?

    I am using a SEN-10245 load cell. 

    The equation i am using for the gain is Av = (1 +2R/Rgain)

    I am trying to build a digital scale.

    I loaded the circuit diagrams i am following 

    Any help on the subject is appriciated. 

  • 0
    TI__Mastermind 25805 points
    Hello Nicholas,

    It appears as though you cross-posted this question in the Precision Amplifiers forum. I will merge the two threads.
  • 0
    TI__Guru*** 148116 points
    Nicholas,

    The signal is amplified by 1000 and the VIO error of both op-amps are amplified by 501.
    That is a lot of potential error.

    INA has much less error, but this comes at a monetary price.