AD524
Rev. F | Page 23 of 28
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.
AD524
IRON
CONSTANTAN
AD590
CU
52.3Ω
8.66kΩ
1kΩ
E
O
2.5V
AD580
7.5V
G = 100
– 2.5V
1 +
52.3Ω
R
TYPE
J
K
E
T
S, R
R
A
NOMINAL
VALUE
REFERENCE
JUNCTION
+15°C < T
A
< +35°C
52.3Ω
41.2Ω
61.4Ω
40.2Ω
5.76Ω
+V
S
I
A
T
A
V
A
+V
S
+
–
–V
S
OUTPUT
AMPLIFIER
OR METER
R
T
NOMINAL VALUE
9135Ω
R
A
E
O
= V
T
– V
A
+
52.3ΩI
A
+ 2.5V
MEASURING
JUNCTION
= V
T
~
V
T
0500-053
Figure 53. Cold-Junction Compensation
The circuit is calibrated by adjusting R
T
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.
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
T
and R
A
.
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
rd
Edition), available free from
Analog Devices, Inc.
AD524
AD7524
AD574A
AD583
AGND
20kΩ
10kΩ
5kΩ
AD7507
CONTROL
DECODE
LATCH
ADDRESS BUS
A0, A2,
EN, A1
V
IN
V
REF
MICRO-
PROCESSOR
+
–
RG
2
RG
1
–V
REF
20kΩ
1/2
AD712
1/2
AD712
–
+
–
+
2
10
6
9
16
13
12
11
3
1
0500-054
Figure 54. Microprocessor Controlled Data Acquisition System
