AD524
REV. C
–12–
PRINTED
IN
U.S.A.
C722c–20–8/86
Figure 47 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 semi-
conductor temperature sensor.
+Vs
–Vs
AD524
VA
TA
IA
IRON
MEASURING
JUNCTION
CONSTANTAN
AD590
VT
CU
RA
52.3
RT
8.66k
1k
EO
2.5V
AD580
7.5V
+Vs
G = 100
OUTPUT
AMPLIFIER
OR METER
NOMINAL VALUE
9135
EO = VT – VA +
– 2.5V
52.3
I
A + 2.5V
1 +
52.3
R
V
T
TYPE
RA
NOMINAL
VALUE
52.3
41.2
61.4
40.2
5.76
J
K
E
T
S, R
REFERENCE
JUNCTION
15oC <TA <35
oC
Figure 47. Cold-Junction Compensation
The circuit is calibrated by adjusting RT for proper output volt-
age with the measuring junction at a known reference tempera-
ture and the circuit near 25
°C. If resistors with low tempcos are
used, compensation accuracy will be to within
±0.5°C, for tem-
peratures between +15
°C and +35°C. Other thermocouple
types may be accommodated with the standard resistance values
shown in the table. For other ranges of ambient temperature,
the equation in the figure may be solved for the optimum values
of RT and RA.
The microprocessor controlled data acquisition system shown
in Figure 48 includes both auto-zero and auto-gain 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 auto-zero cycle, in this application, con-
verts a number that appears to be ground and then writes that
same number (8 bit) to the AD7524 which eliminates the zero
error since its output has an inverted scale. The auto-gain cycle
converts the A/D reference and compares it with full scale. A
multiplicative correction factor is then computed and applied to
subsequent readings.
2
16
13
12
11
3
1
VREF
9
AD524
6
10
AD7524
AD574A
AD583
AGND
–VREF
VIN
MICRO-
PROCESSOR
ADDRESS BUS
20k
10k
5k
1/2
AD712
AD7507
EN
A1
A0 A2
1/2
AD712
RG2
RG1
CONTROL
DECODE
LATCH
Figure 48. Microprocessor Controlled Data Acquisition
System
For a comprehensive study of instrumentation amplifier design
and applications, refer to the Instrumentation Amplifier Applica-
tion Guide, available free from Analog Devices.
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
20-Terminal Leadless Chip Carrier (E) Package
16-Pin Ceramic (D) Package