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Single-Chip, loop-powered 4-20mA RTD Sensor Transmitter Reference Design (TIDA-00851)

Wojciech Sobol
Prodigy 90 points
Other Parts Discussed in Thread: TIDA-00851, PGA900EVM, PGA900

I build the board based on TIDA-00851 design to measure RTD and testing it using PGA900EVM board. I am following all steps from the design guide but I am getting the negative voltage readings.

The PGA900 is measuring the voltage between VINTP (positive?) and VINTN (negative?), but on the schematic and everywhere in the design guide the positive signal from the RTD is connected to VEXTN and the negative one to VEXTP.

SHOULD'T BE INVERSE (positive RTD signal to VEXTP and negative to VEXTN)?

  • 0
    TI__Intellectual 2845 points

    Hi,

    The default configuration of this design is for a 3-wire PT100 configuration. VINTP provides the excitation voltage for the PT100 and the output is measured across VEXTP and VEXTN. Those pins are Muxed to the same ADC as VINTP/VINTN, as well as the internal temp sensor on the PGA900. 

    I am not seeing in the document where is specifies that the VEXTN should be tied to the positive signal of the PT100. Yes, the inverse should be the case for positive voltages being read.