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INA122: wheastone bridge issues

Yannick Yapo
Prodigy 70 points
Part Number: INA122
Other Parts Discussed in Thread: INA333,

Hello everybody, I have a big problem with my INA122P amplifier. I put force sensor (CZL204E) to determine the mass on it. However, the voltage was too low. So, I decided to use a wheatstone bridge to amplify this voltage. (the sensor is commonly describe as a variable resistance).

I use the circuit (in attached files) with a gain resistor (Rg) = 150 ohms so the gain must have been 1300 but actually the gain is only 150. I don't understand why , is my circuit false or did I do mistake in my solding ? Sorry for my english.

https://www.casimages.com/i/210503041747500210.png.html

https://www.active-robots.com/3137-button-load-cell-0-200kg-czl204e.html#specifiaction

  • 0
    TI__Guru* 77541 points

    Hi ,

    I have several questions about your circuit. 

    1. What is your excitation voltage for the load cell?

    2. What is INA122P's supply voltage? Could you share the circuit with us?

    3. What is your load range per the application from low to high? or calibration range?

    4. Are you open to use other instrumentation amplifier, if this is a low voltage application?

    Once I have the above information, I maybe able to simulate the circuit and answer your question. 

    Enclosed is a Tina simulation for Gain=150 where R11 changes from 350Ω to 353Ω or load cell changes up to 10.67mV in R11. . 

    INA122 E2E 05032021.TSC

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Prodigy 70 points

    Raymond, thanks for your reply ,

    1) the excitation voltage is 5V

    2) the INA122P supply voltage is also 5V ans V- is 0V, I have already shared the circuit in my previous message (attached files)

    3) the load is beetween 0-200kg -> 0 -20 mV

    4)I am open to use an other amplifier even if i'd rather like use this one... but if it's the only solution...

    Best,

    Yannick

  • 0
    Guru 140200 points

    Hi Yannick,

    it's impossible to help without a complete schematic Relaxed

    Kai

  • 0 in reply to Yannick Yapo
    TI__Guru* 77541 points

    Hi Yannick,

    I use the circuit (in attached files) with a gain resistor (Rg) = 150 ohms so the gain must have been 1300 but actually the gain is only 150.

    Due to lack of schematic in the design, I am unable to answer the above question as Kai pointed out. 

    From the latest information in "the load is beetween 0-200kg -> 0 -20 mV", Gain=1338.3 with Rg=150Ω may have some issues, where 20mV*1338.3 = 26.77V as output with 5V supply voltage rail. 

    In the following, I am going to take a guess in what you may have. Please share your schematic with us, so that I am able to address your question promptly.  

    a. Assumed Gain=150 V/V

    b. 0-200kg -> 0 -20 mV, based on the figure above, the application's output swing is 20mV*150 = 3V approximately. 

    c. I would recommend INA333 or similar low voltage instrumentation amplifiers for the 5V operation; thermal drift figure is also better in INA333. You may make your own selection. 

    d. 350Ω to 356Ω @ R_loadcell is equivalent to 20mV change. INA122 has higher Voffset than INA333, but the INA122's offset voltage can be tare it out in weight scale application (so it is not an issue). 

     

    8540.INA122 E2E 05032021.TSC

    If you have other questions, please let us know. 

    Best,

    Raymond