AD8497 AD [Analog Devices], AD8497 Datasheet - Page 11

• Manufacturer Part Number: AD8497
• Description: Precision Thermocouple Amplifiers with Cold Junction Compensation
• Manufacturer: AD [Analog Devices]

THEORY OF OPERATION
THERMOCOUPLES
A thermocouple is a rugged, low cost temperature transducer
whose output is proportional to the temperature difference
between a measurement junction and a reference junction. It
has a very wide temperature range. Its low level output (typically
tens of microvolts per °C) requires amplification. Variation in
the reference junction temperature results in measurement
error unless the thermocouple signal is properly compensated.
A thermocouple consists of two dissimilar metals. These metals
are connected at one end to form the measurement junction,
also called the hot junction. The other end of the thermocouple
is connected to the metal lines that lead to the measurement
electronics. This connection forms a second junction: the
reference junction, also called the cold junction.
To derive the temperature at the measurement junction (T
the user must know the differential voltage created by the thermo-
couple. The user must also know the error voltage generated by
the temperature at the reference junction (T
for the reference junction error voltage is typically called cold
junction compensation. The electronics must compensate for
any changes in temperature at the reference (cold) junction so
that the output voltage is an accurate representation of the hot
junction measurement.
THERMOCOUPLE SIGNAL CONDITIONER
The AD8494/AD8495/AD8496/AD8497 thermocouple amplifiers
provide a simple, low cost solution for measuring thermocouple
temperatures. These amplifiers simplify many of the difficulties
of measuring thermocouples. An integrated temperature sensor
performs cold junction compensation. A fixed-gain instrumen-
tation amplifier amplifies the small thermocouple voltage to
provide a 5 mV/°C output. The high common-mode rejection
of the amplifier blocks common-mode noise that the long
thermocouple leads can pick up. For additional protection, the
high impedance inputs of the amplifier make it easy to add
extra filtering.
Table 6 shows an example of a J type thermocouple voltage for
various combinations of 0°C and 50°C on the reference and
measurement junctions. Table 6 also shows the performance
of the AD8494 amplifying the thermocouple voltage and
compensating for the reference junction temperature changes,
thus eliminating the error.
MEASUREMENT
JUNCTION
THERMOCOUPLE WIRES
Figure 26. Thermocouple Junctions
REFERENCE
JUNCTION
TRACES
PCB
RJ
AD849x
). Compensating
MJ
),
Rev. 0 | Page 11 of 16
Table 6. J Type Thermocouple Voltages and AD8494 Readings
Measurement
Junction
Temperature
(T
50°C
50°C
0°C
0°C
AD8494/AD8495/AD8496/AD8497 ARCHITECTURE
Figure 27 shows a block diagram of the AD849x circuitry. The
AD849x consists of a low offset, fixed-gain instrumentation
amplifier and a temperature sensor.
The AD849x output is a voltage that is proportional to the tem-
perature at the measurement junction of the thermocouple (T
To derive the measured temperature from the AD849x output
voltage, use the following transfer function:
An ideal AD849x achieves this output with an error of less than
±2°C, within the specified operating ranges listed in Table 7.
Instrumentation Amplifier
A thermocouple signal is so small that considerable gain is
required before it can be sampled properly by most ADCs. The
AD849x has an instrumentation amplifier with a fixed gain that
generates an output voltage of 5 mV/°C for J type and K type
thermocouples.
To accommodate the nonlinear behavior of the thermocouple,
each amplifier has a different gain so that the 5 mV/°C is accu-
rately maintained for a given temperature measurement range.
•
•
THERMO-
COUPLE
MJ
)
T
V
The AD8494 and AD8496 (J type) have an instrumentation
amplifier with a gain of 96.7 and 90.35, respectively.
The AD8495 and AD8497 (K type) have an instrumentation
amplifier with a gain of 122.4.
1MΩ
MJ
OUT
+IN
–IN
= (V
= (T
AD8494/AD8495/AD8496/AD8497
OUT
ESD AND
ESD AND
MJ
OVP
OVP
− V
× 5 mV/°C) + V
Reference
Junction
Temperature
(T
0°C
50°C
0°C
50°C
RJ
REF
)
Figure 27. Block Diagram
)/(5 mV/°C)
A2
A1
AD8494/AD8495/
AD8496/AD8497
REF
COLD JUNCTION
COMPENSATION
Thermocouple
Voltage
+2.585 mV
0 mV
0 mV
−2.585 mV
SENSE
REF
AD8494
Reading
250 mV
250 mV
0 mV
0 mV
A3
OUT
MJ
).