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INA333 wheatstone bridge

pradeesh thomas
Prodigy 70 points
Other Parts Discussed in Thread: INA333, TIPD170

i have a circuit with INA333 to measure temperature using RTD. i am using 3.3V single ended supply and i fixed my gain 11.

The problem is my gain is varying when i change the resistance(RTD) from 820 to 1290 ohms, i included the chart. Why this is happening, Any help?

  • 0
    TI__Mastermind 25795 points

    Hello Pradeesh,

    The impedance of a PT1000 at 25C is ~1.1kohms. Therefore the voltage on the Vin+ node is approximately 825mV.

    Vin+=3.3*(1.1k/(1.1k+3.3k))=825mV

    The voltage at the Vin- node is approximately 660mV.

    Vin-=3.3*(820/(820+3300))=660mV

    The common-mode voltage is the average of these two voltages, which is approximately 743mV.

    The output swing of an instrumentation amplifier depends on the common-mode voltage (among other things). Please refer to the following link for more information:

    http://e2e.ti.com/support/amplifiers/precision_amplifiers/w/design_notes/3373.understanding-instrumentation-amplifier-vcm-vs-vout-plots

    You will find that you do not have enough output swing given your circuit configuration.  The INA-CMV-CALC tool will be especially helpful in understanding this.

    Ultimately my recommendation is to change the value of the resistors in the bridge to set the common-mode voltage to mid-supply (1.65V). This will maximize your output swing.  You could also add a resistor at the bottom of the bridge to pedestal the common-mode voltage (I recommend ~1.2kohms). Please understand that this will affect your overall transfer function. Please refer to TIPD170 for more information.