AD8494/AD8495/AD8496/AD8497
Rev. C | Page 13 of 16
Keeping the AD849x at the Same Temperature
as the Reference Junction
RECOMMENDATIONS FOR BEST CIRCUIT
PERFORMANCE
The AD849x compensates for thermocouple reference junction
temperature by using an internal temperature sensor. It is
critical to keep the reference junction (thermocouple-to-PCB
connection) as close to the AD849x as possible. Any difference
in temperature between the AD849x and the reference junction
appears directly as temperature error. Temperature difference
between the device and the reference junction may occur if the
AD849x is not physically close to the reference junction or if the
AD849x is required to supply large amounts of output power.
Input Filter
A low-pass filter before the input of the AD849x is strongly
recommended (see Figure 29), especially when operating in an
electrically noisy environment. Long thermocouple leads can
function as an excellent antenna and pick up many unwanted
signals.
The filter should be set to a low corner frequency that still
allows the input signal to pass through undiminished. The
primary purpose of the filter is to remove RF signals, which,
if allowed to reach the AD849x, can be rectified and appear
as temperature fluctuations.
08529-010
AD849x
PCB
TRACES
KEEP
TRACES
SHORT
KEEP JUNCTION AND
AD849x AT SAME
TEMPERATURE
MEASUREMENT
JUNCTION
REFERENCE
JUNCTION
THERMOCOUPLE WIRES
08529-011
R
R
AD849x
C
D
C
C
C
C
1MΩ
CONNECT WHEN
THERMOCOUPLE TIP
TYPE IS UNKNOWN
FILTER FREQUENCY
DIFF
=
1
2πR(2C
D
+ C
C
)
FILTER FREQUENCY
CM
=
WHERE C
D
≥ 10C
C
1
2πRC
C
Figure 31. Compensating for Thermocouple Reference Junction Temperature
Driving the Reference Pin
The AD849x comes with a reference pin, which can be used
to offset the output voltage. This is particularly useful when
reading a negative temperature in a single-supply system.
INCORRECT
V
CORRECT
AD849x
AD8613
+
–
V
08529-006
REF
AD849x
REF
Figure 29. Filter for Any Thermocouple Style
To prevent input offset currents from affecting the measurement
accuracy, the filter resistor values should be less than 50 k.
Ground Connection
It is always recommended that the thermocouple be connected
to ground through a 100 k to 1 M resistor placed at the
negative (inverting) input of the amplifier on the PCB (see
Figure 30). This solution works well regardless of the thermo-
couple tip style.
08529-038
1MΩ
Figure 32. Driving the Reference Pin
For best performance, the reference pin should be driven with a
low output impedance source, not a resistor divider. The AD8613
and the OP777 are good choices for the buffer amplifier.
Figure 30. Ground the Thermocouple with a 1 MΩ Resistor
Debugging Tip
If there is no electrical connection at the measurement junction
(insulated tip), the resistor value is small enough that no mean-
ingful common-mode voltage is generated. If there is an electrical
connection through a grounded or exposed tip, the resistor value
is large enough that any current from the measurement tip to
ground is very small, preventing measurement errors.
If the AD849x is not providing the expected performance, a
useful debugging step is to implement the ambient temperature
configuration in Figure 34. If the ambient temperature sensor
does not work as expected, the problem is likely with the AD849x
or with the downstream circuitry. If the ambient temperature
sensor configuration is working correctly, the problem typically
lies with how the thermocouple is connected to the AD849x.
Common errors include an incorrect grounding configuration
or lack of filtering.
The AD849x inputs require only one ground connection or source
of common-mode voltage. Any additional ground connection is
detrimental to performance because ground loops can form
through the thermocouple, easily swamping the small
thermocouple signal. Grounding the thermocouple through a
resistor as recommended prevents such problems.
