AD524 Analog Devices, Inc., AD524 Datasheet - Page 23

• Manufacturer Part Number: AD524
• Description: Precision Instrumentation Amplifier
• Manufacturer: Analog Devices, Inc.

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.
The circuit is calibrated by adjusting R
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.
TYPE
S, R
K
E
J
T
MEASURING
JUNCTION
NOMINAL
52.3Ω
41.2Ω
61.4Ω
40.2Ω
5.76Ω
VALUE
R
A
V
IRON
T
E
CONSTANTAN
~
= V
REFERENCE
JUNCTION
+15°C < T
O
Figure 53. Cold-Junction Compensation
= V
T
T
– V
V
A
A
A
< +35°C
+
52.3ΩI
T
1 +
A
52.3Ω
A
+V
+ 2.5V
R
S
AD590
I
A
CU
– 2.5V
AD7507
A0, A2,
EN, A1
LATCH
T
2.5V
for proper output
NOMINAL VALUE
52.3Ω
R
R
A
T
7.5V
Figure 54. Microprocessor Controlled Data Acquisition System
9135Ω
AD712
AD580
RG
RG
8.66kΩ
E
1kΩ
1/2
O
20kΩ
2
1
G = 100
–
+
AMPLIFIER
OR METER
13
12
16
11
+
–
2
3
1
OUTPUT
+V
–V
+
–
10kΩ
5kΩ
S
S
AD524
AD524
ADDRESS BUS
Rev. F | Page 23 of 28
AD712
1/2
10
6
–
+
20kΩ
9
AD7524
–V
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 R
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 (3
Analog Devices, Inc.
REF
DECODE
AD583
AGND
V
IN
CONTROL
AD574A
V
REF
PROCESSOR
MICRO-
rd
Edition), available free from
T
and R
A
.
AD524