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XTR108: xtr108 strain gauge bridge sensor application

Part Number: XTR108
Other Parts Discussed in Thread: XTR115, PGA308, XTR106

Hi,

On the XTR108 datasheet, it  mainly explains about the RTD resistor and briefly explains on bridge sensor. I am wondering if there is any application notes or full instruction on how we can connect bridge sensor such as load cell to the XTR108 like XTR115? Also wondering if the XTR108 Evaluation board works with the bridge sensor, if it is then can you please provide me the drawing how to connect the bridge sensor on the evaluation board?

Many Thanks

Umesh 

  • HI Umesh,

    Most of our customers use the XTR108 for RTD applications; where the built-in excitation currents are used to excite the RTD and therefore the Evaluation board  primarily documents the interface of the XTR108 with an RTD.   The XTR108 linearization circuit is able to perform a second order non linearity correction by scaling the excitation current using the linearization DAC.

    Bridge sensors tend to require a voltage excitation, and as the datasheet mentions, it is possible to interface a bridge sensor with the XTR108.

    On figure 12,  a fixed excitation voltage for the bridge can be generated using the reference voltage (1.193V nominal).  The 1.193V reference voltage is gained up using the integrated OPA amplifier and external resistors R1, R2 to obtain the desired bridge fixed excitation voltage.   The voltage at the output of the OPA becomes VEX =VREF(1+R1/R2).   In this case, the bridge sensor connects between the IRET pin and the OPA OUT pin, and the bridge differential outputs connects through the multiplexer inputs to the inputs of the XTR108 PGA.


    On Figure 13, the excitation voltage is generated using the excitation currents from the XTR108.  The excitation voltage is the product of the external resistor RI and the 2x IREF current VEXC= 2*IREF*RI.  The bridge sensor is connected between the OPA OUT pin and the IRET pin and the differential bridge output connects through the multiplexer inputs of the PGA.

    Thank you and Regards,

    Luis

  • Thank you for the reply. This means I can use the evaluation board for the bridge sensor right? 

    Best Regards

    Umesh

  • HI Umesh,

    From the hardware perspective, it is possible to connect a bridge sensor using the XTR108 EVM hardware using the configurations above. However, the EVM hardware and software is designed for RTD configurations, where the hardware includes a RTD emulator, and the jumper connections are set up for 2-wire or 3-wire RTD sensor configurations. In addition, if you take a look at the software description on the XTR108EVM user guide, all the software GUI the calibration routines and settings are written for RTD sensors for temperature measurement applications. The EVM software is not intended to support Bridge sensors.

    Another option you could explore are the PGA308/309 which are voltage out conditioners for bridge sensors. Depending on the current consumption of the bridge sensor, you could interface the PGA308/309 with a 4-20mA current transmitter. Another alternative is the XTR106 which is a bridge sensor 4-20mA transmitter, however no evaluation is available for this older device.

    Best Regards,

    Luis