Datasheet AD7705, AD7706 (Analog Devices) - 39

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AD7705/AD7706 TEMPERATURE MEASUREMENT
AD7705 is very low. The lead resistances present a small source impedance; therefore, it is not generally necessary to use the Another application of the AD7705 is temperature measure- buffer of the AD7705. If the buffer is required, the common- ment. Figure 26 outlines a connection between a thermocouple mode voltage should be set accordingly by inserting a small and the AD7705. For this application, the AD7705 is operated in resistance between the bottom end of the RTD and the GND buffered mode to allow large decoupling capacitors on the front of the AD7705. In the application shown, an external 400 μA end to eliminate any noise pickup from the thermocouple leads. current source provides the excitation current for the PT100 When the AD7705 operates in buffered mode, it has a reduced and generates the reference voltage for the AD7705 via the common-mode range. To place the differential voltage from the 6.25 kΩ resistor. Variations in the excitation current do not thermocouple on a suitable common-mode voltage, the affect the circuit, because both the input voltage and the AIN1(−) input of the AD7705 is biased up at the reference reference voltage vary ratiometrically with the excitation voltage, 2.5 V. current. However, the 6.25 kΩ resistor must have a low
5V
temperature coefficient to avoid errors in the reference
VDD
voltage over temperature.
5V THERMOCOUPLE VDD JUNCTION AIN1(+) 400
μ
A MCLK IN AIN1(–) REF IN(+) 5V AD7705 6.25k
Ω
REF IN(–) MCLK IN R REF IN(+) L1 MCLK OUT AD7705 REF192 OUTPUT R AIN1(+) L2 REF IN(–) RESET RTD GND MCLK OUT GND DRDY R AIN1(–) L3 RESET RL4 DOUT DIN CS SCLK
01166-026
DRDY
Figure 26. Temperature Measurement Using the AD7705
GND
Figure 27 shows another example of a temperature measure-
DOUT DIN CS SCLK
01166-027 ment application for the AD7705. In this case, the transducer is Figure 27. RTD Measurement Using the AD7705 a resistive temperature device (RTD), a PT100, and the arrangement is a 4-lead RTD configuration. There are voltage drops across lead resistances RL1 and RL4, which shift the common-mode voltage. There is no voltage drop across lead resistances RL2 and RL3, because the input current to the Rev. C | Page 39 of 44 Document Outline FEATURES GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM REVISION HISTORY PRODUCT HIGHLIGHTS SPECIFICATIONS TIMING CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS OUTPUT NOISE (5 V OPERATION) OUTPUT NOISE (3 V OPERATION) TYPICAL PERFORMANCE CHARACTERISTICS ON-CHIP REGISTERS COMMUNICATION REGISTER (RS2, RS1, RS0 = 0, 0, 0) SETUP REGISTER (RS2, RS1, RS0 = 0, 0, 1); POWER-ON/RESET STATUS: 01 HEXADECIMAL CLOCK REGISTER (RS2, RS1, RS0 = 0, 1, 0); POWER-ON/RESET STATUS: 05 HEXADECIMAL DATA REGISTER (RS2, RS1, RS0 = 0, 1, 1) TEST REGISTER (RS2, RS1, RS0 = 1, 0, 0); POWER-ON/RESET STATUS: 00 HEXADECIMAL ZERO-SCALE CALIBRATION REGISTER (RS2, RS1, RS0 = 1, 1, 0); POWER-ON/RESET STATUS: 1F4000 HEXADECIMAL FULL-SCALE CALIBRATION REGISTER (RS2, RS1, RS0 = 1, 1, 1); POWER-ON/RESET STATUS: 5761AB HEXADECIMAL Calibration Sequences CIRCUIT DESCRIPTION ANALOG INPUT Ranges Sample Rate BIPOLAR/UNIPOLAR INPUT REFERENCE INPUT DIGITAL FILTERING Filter Characteristics Postfiltering ANALOG FILTERING CALIBRATION Self-Calibration System Calibration Span and Offset Limits Power-Up and Calibration THEORY OF OPERATION CLOCKING AND OSCILLATOR CIRCUIT SYSTEM SYNCHRONIZATION RESET INPUT STANDBY MODE ACCURACY DRIFT CONSIDERATIONS POWER SUPPLIES SUPPLY CURRENT GROUNDING AND LAYOUT EVALUATING THE PERFORMANCE DIGITAL INTERFACE CONFIGURING THE AD7705/AD7706 MICROCOMPUTER/MICROPROCESSOR INTERFACING AD7705/AD7706-to-68HC11 Interface AD7705/AD7706-to-8051 Interface AD7705/AD7706-to-ADSP-2103/ADSP-2105 Interface CODE FOR SETTING UP THE AD7705/AD7706 C Code for Interfacing AD7705 to 68HC11 APPLICATIONS PRESSURE MEASUREMENT TEMPERATURE MEASUREMENT SMART TRANSMITTERS BATTERY MONITORING OUTLINE DIMENSIONS ORDERING GUIDE