AD524B AD [Analog Devices], AD524B Datasheet - Page 15

• Manufacturer Part Number: AD524B
• Description: Precision Instrumentation Amplifier
• Manufacturer: AD [Analog Devices]

REV. E
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.
The circuit is calibrated by adjusting R
with the measuring junction at a known reference temperature
TYPE
S, R
K
E
T
J
MEASURING
JUNCTION
NOMINAL
VALUE
52.3
41.2
61.4
40.2
5.76
R
A
Figure 47. Cold-Junction Compensation
V
IRON
T
E
CONSTANTAN
+15 C < T
REFERENCE
JUNCTION
O
V
= V
T
T
– V
A
V
A
< +35 C
A
+
52.3 I
T
1 +
A
52.3
A
+V
+ 2.5V
R
S
AD590
Figure 48. Microprocessor Controlled Data Acquisition System
I
A
CU
– 2.5V
T
AD7507
EN A1
LATCH
for proper output voltage
A0 A2
2.5V
NOMINAL VALUE
52.3
R
R
A
T
7.5V
9135
AD712
RG
RG
AD580
1/2
8.66k
E
1k
20k
O
2
1
G = 100
AMPLIFIER
OR METER
OUTPUT
+V
10k
–V
5k
AD524
S
S
ADDRESS BUS
AD524
AD712
1/2
ADDRESS BUS
–15–
20k
and the circuit near 25 C. If resistors with low tempcos are
used, compensation accuracy will be 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 the table. For other ranges of ambient temperature,
the equation in the figure may be solved for the optimum values
of R
The microprocessor controlled data acquisition system shown in
Figure 48 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
since its output has an inverted scale. The autogain cycle con-
verts the A/D reference and compares it with full scale. A multi-
plicative correction factor is then computed and applied to
subsequent readings.
For a comprehensive study of instrumentation amplifier design
and applications, refer to the Instrumentation Amplifier Applica-
tion Guide, available free from Analog Devices.
AD7524
–V
T
REF
and R
AD583
DECODE
AGND
V
A
I N
.
CONTROL
AD574A
V
REF
PROCESSOR
MICRO-
AD524