Datasheet AD524 (Analog Devices) - 23

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Data Sheet AD524
Figure 53 shows a simple application in which the variation of ambient temperature, the equation in Figure 53 may be of the cold junction voltage of a Type J thermocouple-iron ± solved for the optimum values of RT and RA. constantan is compensated for by a voltage developed in series The microprocessor controlled data acquisition system shown by the temperature sensitive output current of an AD590 in Figure 54 includes both auto-zero and autogain capability. By semiconductor temperature sensor. dedicating two of the differential inputs, one to ground and one
RA NOMINAL REFERENCE
to the A/D reference, the proper program calibration cycles can
JUNCTION TYPE VALUE +VS 7.5V +15°C < TA < +35°C
eliminate both initial accuracy errors and accuracy errors over
J 52.3Ω IA T 2.5V K 41.2Ω A
temperature. The auto-zero cycle, in this application, converts a
AD580 E 61.4Ω G = 100
number that appears to be ground and then writes that same
T 40.2Ω VA AD590 +V S, R 5.76Ω S AD524
number (8-bit) to the AD7524, which eliminates the zero error.
+ IRON RA
Because its output has an inverted scale, the autogain cycle
EO VT CONSTANTAN CU 52.3Ω
converts the A/D reference and compares it with full-scale. A
– MEASURING
multiplicative correction factor is then computed and applied to
8.66kΩ JUNCTION 52.3ΩIA + 2.5V –V EO = VT – VA + – 2.5V S 52.3Ω RT
subsequent readings.
1 + OUTPUT R 1kΩ AMPLIFIER = V ~ OR METER T REFERENCES NOMINAL VALUE
053
9135Ω
0500- 0 The following reference materials provide additional Figure 53. Cold Junction Compensation information that supplements material found in the data sheet. The circuit is calibrated by adjusting RT for proper output A Designer’s Guide to Instrumentation Amplifiers, 3RD Edition, voltage with the measuring junction at a known reference 2006. Analog Devices. temperature and the circuit near 25°C. If resistors with low Application Note, AN-306, Synchronous System Measures μΩs, temperature coefficients are used, compensation accuracy is Analog Devices. to within ±0.5°C, for temperatures between +15°C and +35°C. Other thermocouple types may be accommodated with the standard resistance values shown in Table 5. For other ranges
2 + AD583 VREF RG2 16 13 10 AD7507 12 AD524 9 VIN AD574A 11 6 AGND RG1 3 1 – –VREF A0, A2, 20kΩ EN, A1 20kΩ 10kΩ – – AD7524 + + 1/2 1/2 5kΩ AD712 AD712 DECODE LATCH CONTROL MICRO- PROCESSOR ADDRESS BUS
054 0- 050 0 Figure 54. Microprocessor Controlled Data Acquisition System Rev. G | Page 23 of 25 Document Outline FEATURES FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION PRODUCT HIGHLIGHTS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS CONNECTION DIAGRAMS ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS TEST CIRCUITS THEORY OF OPERATION INPUT PROTECTION INPUT OFFSET AND OUTPUT OFFSET GAIN INPUT BIAS CURRENTS COMMON-MODE REJECTION GROUNDING SENSE TERMINAL REFERENCE TERMINAL PROGRAMMABLE GAIN AUTO-ZERO CIRCUITS ERROR BUDGET ANALYSIS REFERENCES OUTLINE DIMENSIONS ORDERING GUIDE