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AD7715 Datasheet(PDF) 38 Page - Analog Devices |
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AD7715 Datasheet(HTML) 38 Page - Analog Devices |
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38 / 41 page  Data Sheet AD7715 Rev. E | Page 37 of 40 TEMPERATURE MEASUREMENT Another application area for the AD7715 is in temperature measurement. Figure 15 outlines a connection from a thermo- couple to the AD7715. In this application, the AD7715 is operated in its buffered mode to allow large decoupling capacitors on the front end to eliminate any noise pickup that there may have been in the thermocouple leads. When the AD7715 is operated in buffered mode, it has a reduced common-mode range. To place the differential voltage from the thermocouple on a suitable common-mode voltage, the AIN(−) input of the AD7715 is biased up at the reference voltage, 2.5 V. Figure 16 shows another temperature measurement application for the AD7715. In this case, the transducer is an resistive tem- perature device (RTD), a PT100. The arrangement is a 4-lead RTD configuration. There are voltage drops across the lead resistances RL1 and RL4, but these simply shift the common- mode voltage. There is no voltage drop across lead resistances RL2 and RL3 as the input current to the AD7715 is very low. The lead resistances present a small source impedance so it would not generally be necessary to turn on the buffer on the AD7715. If the buffer is required, the common-mode voltage should be set accordingly by inserting a small resistance between the bottom end of the RTD and AGND of the AD7715. In the application shown in Figure 16, an external 400 μA current source provides the excitation current for the PT100 and it also generates the reference voltage for the AD7715 via the 6.25 kΩ resistor. Varia- tions in the excitation current do not affect the circuit as both the input voltage and the reference voltage vary ratiometrically with the excitation current. However, the 6.25 kΩ resistor must have a low temperature coefficient to avoid errors in the reference voltage over temperature. SERIAL INTERFACE MCLK IN MCLK OUT REF IN(+) REF IN(–) AGND DGND DOUT DIN SCLK AD7715 AIN(+) AIN(–) A = 1 TO 128 PGA BUFFER CLOCK GENERATION GND VOUT REF192 THERMOCOUPLE JUNCTION +5V +VIN R R CC AUTO-ZEROED Σ-∆ MODULATOR CHARGE BALANCING ADC DIGITAL FILTER AVDD REGISTER BANK CS DRDY RESET DVDD 5V Figure 15. Thermocouple Measurement Using the AD7715 SERIAL INTERFACE MCLK IN MCLK OUT DOUT DIN SCLK AD7715 A = 1 TO 128 PGA BUFFER CLOCK GENERATION AUTO-ZEROED Σ-∆ MODULATOR CHARGE BALANCING ADC DIGITAL FILTER AVDD REGISTER BANK CS DRDY RESET DVDD +5V 400µA RL4 RL3 RL2 RL1 RTD 6.25kΩ REF IN(+) REF IN(–) AGND DGND AIN(+) AIN(–) Figure 16. RTD Measurement Using the AD7715 |
Similar Part No. - AD7715_17 |
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Similar Description - AD7715_17 |
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