Datasheet AD7792, AD7793 (Analog Devices) - 29

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AD7792/AD7793 TEMPERATURE MEASUREMENT USING AN RTD
material and of equal length), and IOUT1 and IOUT2 match, To optimize a 3-wire RTD configuration, two identically the error voltage across RL2 equals the error voltage across RL1, matched current sources are required. The AD7792/AD7793, and no error voltage is developed between AIN1(+) and which contain two well-matched current sources, are ideally AIN1(–). Twice the voltage is developed across RL3 but, suited to these applications. One possible 3-wire configuration because this is a common-mode voltage, it does not introduce is shown in Figure 21. In this 3-wire configuration, the lead errors. The reference voltage for the AD7792/AD7793 is also resistances result in errors if only one current is used, as the generated using one of these matched current sources. It is excitation current flows through RL1, developing a voltage error developed using a precision resistor and applied to the between AIN1(+) and AIN1(–). In the scheme outlined, the differential reference pins of the ADC. This scheme ensures that second RTD current source is used to compensate for the error the analog input voltage span remains ratiometric to the introduced by the excitation current flowing through RL1. The reference voltage. Any errors in the analog input voltage due to second RTD current flows through RL2. Assuming RL1 and the temperature drift of the excitation current are compensated RL2 are equal (the leads would normally be of the same by the variation of the reference voltage.
GND AVDD REFIN(+) REFIN(–) IOUT1 BAND GAP REFERENCE GND AVDD RL1 AIN1(+) RTD DOUT/RDY AIN1(–) SERIAL INTERFACE DIN RL2 BUF IN-AMP Σ-Δ IOUT2 AND ADC SCLK CONTROL LOGIC CS RL3 REFIN(+) GND DV R DD REF INTERNAL AD7792/AD7793 REFIN(–) CLOCK
13 0 5- 85
CLK
04 Figure 21. RTD Application Using the AD7792/AD7793 Rev. B | Page 29 of 32 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS TIMING CHARACTERISTICS TIMING DIAGRAMS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS OUTPUT NOISE AND RESOLUTION SPECIFICATIONS EXTERNAL REFERENCE INTERNAL REFERENCE TYPICAL PERFORMANCE CHARACTERISTICS ON-CHIP REGISTERS COMMUNICATIONS REGISTER RS2, RS1, RS0 = 0, 0, 0 STATUS REGISTER RS2, RS1, RS0 = 0, 0, 0; Power-On/Reset = 0x80 (AD7792)/0x88 (AD7793) MODE REGISTER RS2, RS1, RS0 = 0, 0, 1; Power-On/Reset = 0x000A CONFIGURATION REGISTER RS2, RS1, RS0 = 0, 1, 0; Power-On/Reset = 0x0710 DATA REGISTER RS2, RS1, RS0 = 0, 1, 1; Power-On/Reset = 0x0000(00) ID REGISTER RS2, RS1, RS0 = 1, 0, 0; Power-On/Reset = 0xXA (AD7792)/0xXB (AD7793) IO REGISTER RS2, RS1, RS0 = 1, 0, 1; Power-On/Reset = 0x00 OFFSET REGISTER RS2, RS1, RS0 = 1, 1, 0; Power-On/Reset = 0x8000 (AD7792)/0x800000 (AD7793) FULL-SCALE REGISTER RS2, RS1, RS0 = 1, 1, 1; Power-On/Reset = 0x5XXX (AD7792)/0x5XXX00 (AD7793) ADC CIRCUIT INFORMATION OVERVIEW DIGITAL INTERFACE Single Conversion Mode Continuous Conversion Mode Continuous Read CIRCUIT DESCRIPTION ANALOG INPUT CHANNEL INSTRUMENTATION AMPLIFIER BIPOLAR/UNIPOLAR CONFIGURATION DATA OUTPUT CODING BURNOUT CURRENTS EXCITATION CURRENTS BIAS VOLTAGE GENERATOR REFERENCE RESET AVDD MONITOR CALIBRATION GROUNDING AND LAYOUT APPLICATIONS INFORMATION TEMPERATURE MEASUREMENT USING A THERMOCOUPLE TEMPERATURE MEASUREMENT USING AN RTD OUTLINE DIMENSIONS ORDERING GUIDE