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Sensor AFE with 4-wire RTD interface

Posted: 22 Oct 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Sensor AFE? resistive temperature device? four-wire?

This application note tackles the use of the LMP90100 /LMP90098 (24 bit) and LMP90080/ LMP90078 (16 bit) Sensor AFEs together with a resistive temperature device (RTD) using a four-wire configuration.

The four-wire RTD approach provides the highest level of accuracy because it isolates the excitation path of the RTD from the sensing path. The current source IB1 or IB2 is connected to one of the RTD line terminals by setting the appropriate bits in the ADC AUXCN: RTD_CUR_SEL. Line 1 of the RTD is tied to a programmable current source (IB1 or IB2) of the device and is not a sensing line, thus saves an input channel in the device for other sensors. The current value can be chosen between 100uA to 1mA in steps of 100uA. The voltage developed across the RTD is measured by connecting the RTD through the PGA to the ADC. The voltage measured across the RTD is proportional to temperature (determined by the characteristics of the RTD). The calculations below explain the relationship. The result is independent of the bias current of RTD. Hence any variation in the Bias current does not affect the final readings as they tend to cancel each other out. The value of RREF is selected according to the IBx current source setting. The RREF can be connected across VREFP1 & VREFN1 terminals as well. This will free up VIN6 & VIN7 pins for connection to other sensors. The user can choose between REF1 (VREFP1-VREFN1) and REF2 (VREFP2-VREFN2) by programming CHx_INPUTCN: VREF_SEL bit.

View the PDF document for more information.

Originally published by Texas Instruments at www.ti.com as "LMP90100 / LMP90098(24bit) and LMP90080 / LMP90078(16bit) 4 Wire RTD Interface".





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