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Capacitive Wheatstone Bridge

Darren DeBattista
Prodigy 200 points

Other Parts Discussed in Thread: INA331, TEST2

Hello,

I am experimenting with an RC Wheatstone Bridge with the intention of measuring the change in capacitance in a sensor. The bridge is excited using a sine wave with a dc offset of 2.5v. Attached is the schematic of my implementation so far.

 

I am reasoning out that at the output of the INA331 should be the difference of nodes A and B multiplied by 5. However I am getting a totally unexpected result. In fact at present the bridge is balanced and the output should be zero... Does anyone here notice anything wrong with the circuit. I am guessing that I am missing something!! 

 1538.Test2.TSC

 

Thanks for your help.

  • 0
    TI__Mastermind 25865 points

    Hello Darren,

    It appears as though you have a typo. The jumper for VS2 should be labeled "Vref", not "Vrerf".

    Once that is fixed, the simulation seems to work well.

    However, I am concerned about your input bias current return path. Please ensure that a bias current return path exists for the sensor you're using.  There is a discussion of this in the INA331 data sheet.

    Also, please note that you're exceeding the input common-mode voltage range when VG1=4V (2.5Vdc+1.5Vp). For your circuit configuration the maximum value should be 3.8V for linear operation.