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INA821: Strain gauge

Zhonghui Xu
Expert 5445 points
Part Number: INA821
Other Parts Discussed in Thread: INA333

Hi team,

we are planning to use this to read strain gauge. we purchased development board for initial testing. our gain settings is 50 ( RG= 1K). while there is no input the opamp is outputting 3.3V. vref value in our case is 3.3V questions -- can this be used to read strain gauge. strain gauge output is 0.027mV/V -- 25um and max range is 1mm. what should be the output when no input is connected.

Thanks

Best regards,

  • 0
    Guru 140200 points

    Hi Zhonghui,

    with correct supply voltage and 3.3V referene voltage the INA821 will output 3.3V with vanishing input signal. What supply voltages do you use?

    Kai

  • 0
    TI__Guru* 77541 points

    Hi Zhonghui,

    Q: strain gauge output is 0.027mV/V -- 25um and max range is 1mm. what should be the output when no input is connected.

    If 25um displacement represents 0.027mV/V, then 1mm displacement will represent 0.027mV/V* 1mm/25um = 1.08 mV/V, assuming it is linear. 

    I believe that the strain gauge's sensitivity is a function of excitation voltage. If 5V excitation reference voltage is used, then 25um displacement will generate a differential input voltage of 5Vdc*0.027mV/V = 0.135mV. At 1mm displacement, the transducer will generate 5Vdc*1.08mV/V = 5.4mV.

    The transducer's output sensitivity are 0.135mV is corresponding to 25um displacement, and 5.4mV is corresponding to 1mm displacement. 

    Assuming Gain=50, 25um will have output 50V/V*0.135mV + Vref = 6.75mV + Vref; and 1mm displacement will have output 50V/V*5.4mV + Vref = 270mV + Vref.

    I think that INA821's gain could be increased to 100 or higher, where CMRR will be improved. This also depends on your excitation voltage and supply voltage rails (single or dual options). 

    Below is the INA821's transfer function. 

    Enclosed is an example of INA821 simulation that may be a help in simulating the application. 

    INA821 Strain Gauge 05112021.TSC

    If you have additional questions, please let us know. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    TI__Guru* 77541 points

    Hi Zhonghui,

    I am going to close this inquiry. If you have additional questions, please let us know. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    TI__Expert 5445 points

    Hi Raymond,

    Thanks a lot for your reply.

    In our setup the excitation voltage is 3.3v and vref is also 3.3v. i am using the EVM board. but the Vout is always around 0.

    Please help to analyze it.

    Thanks,

    Best regards,

  • 0 in reply to Zhonghui Xu
    TI__Guru* 77541 points

    Hi Zhonghu,

    You are unable to operate INA821 using 3.3V single supply rail. The minimum single supply voltage is 4.5Vdc. or you can operate in dual supply voltage, which minimum need to be +/-2.25V or Vcc-Vee >=4.5V. 

    In addition, the input common mode Vcm has to be V- + 2V = 2V per this configuration. 

    If you want to use low voltage INA, you may consider INA333 or similar ones. 

    If you have additional questions, please let me know. 

    Best,

    Raymond