AD524CDZ 데이터 시트보기 (PDF) - Analog Devices
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AD524CDZ Datasheet PDF : 28 Pages
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Figure 53 shows a simple application in which the variation
of the cold-junction voltage of a Type J thermocouple-iron ±
constantan is compensated for by a voltage developed in series
by the temperature-sensitive output current of an AD590
semiconductor temperature sensor.
RA
NOMINAL
TYPE VALUE
REFERENCE
JUNCTION
+15°C < TA < +35°C
+VS
7.5V
J
K
E
T
S, R
52.3Ω
41.2Ω
61.4Ω
40.2Ω
5.76Ω
TA
VA
IRON
VT CONSTANTAN
IA
AD590
CU
2.5V
RA
52.3Ω
AD580
G = 100
+VS
AD524
+
EO
–
MEASURING
JUNCTION
EO = VT – VA +
52.3ΩIA + 2.5V – 2.5V
1 + 52.3Ω
RT
~= VT
R
8.66kΩ –VS
OUTPUT
1kΩ AMPLIFIER
OR METER
NOMINAL VALUE
9135Ω
Figure 53. Cold-Junction Compensation
The circuit is calibrated by adjusting RT for proper output
voltage with the measuring junction at a known reference
temperature and the circuit near 25°C. If resistors with low
temperature coefficients are used, compensation accuracy is
to within ±0.5°C, for temperatures between +15°C and +35°C.
AD524
Other thermocouple types may be accommodated with the
standard resistance values shown in Table 5. For other ranges
of ambient temperature, the equation in Figure 53 may be
solved for the optimum values of RT and RA.
The microprocessor controlled data acquisition system shown
in Figure 54 includes both autozero and autogain capability. By
dedicating two of the differential inputs, one to ground and one
to the A/D reference, the proper program calibration cycles can
eliminate both initial accuracy errors and accuracy errors over
temperature. The autozero cycle, in this application, converts a
number that appears to be ground and then writes that same
number (8-bit) to the AD7524, which eliminates the zero error.
Because its output has an inverted scale, the autogain cycle
converts the A/D reference and compares it with full scale. A
multiplicative correction factor is then computed and applied
to subsequent readings.
For a comprehensive study of instrumentation amplifier
design and applications, refer to the Designer’s Guide to
Instrumentation Amplifiers (3rd Edition), available free from
Analog Devices, Inc.
AD7507
A0, A2,
EN, A1
2+
RG2 16
13
10
12 AD524
9
AD583
VREF
VIN AD574A
RG1
11
3
1–
6
AGND
–VREF
20kΩ
20kΩ
LATCH
–
+
1/2
AD712
10kΩ
–
+
1/2
5kΩ AD712
AD7524
DECODE
CONTROL
MICRO-
PROCESSOR
ADDRESS BUS
Figure 54. Microprocessor Controlled Data Acquisition System
Rev. F | Page 23 of 28
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