AD8494/AD8495/AD8496/AD8497
Rev. C | Page 12 of 16
The small thermocouple voltages mean that signals are quite
vulnerable to interference, especially when measured with
single-ended amplifiers. The AD849x addresses this issue in
several ways. Low input bias currents and high input impedance
allow for easy filtering at the inputs. The excellent common-mode
rejection of the AD849x prevents variations in ground potential
and other common-mode noise from affecting the measurement.
Temperature Sensor (Cold Junction Compensation)
The AD849x also includes a temperature sensor for cold junc-
tion compensation. This temperature sensor is used to measure
the reference junction temperature of the thermocouple and to
cancel its effect.
• The AD8494/AD8495 cold junction compensation is
optimized for operation in a lab environment, where the
ambient temperature is around 25°C. The AD8494/AD8495
are specified for an ambient range of 0°C to 50°C.
• The AD8496/AD8497 cold junction compensation is
optimized for operation in a less controlled environment,
where the temperature is around 60°C. The AD8496/AD8497
are specified for an ambient range of 25°C to 100°C.
Application examples for the AD8496/AD8497 include
automotive applications, autoclave, and ovens.
Thermocouple Break Detection
The AD849x offers open thermocouple detection. The inputs
of the AD849x are PNP type transistors, which means that the
bias current always flows out of the inputs. Therefore, the input
bias current drives any unconnected input high, which rails the
output. Connecting the negative input to ground through a
1 MΩ resistor causes the AD849x output to rail high in an open
thermocouple condition (see Figure 6, Figure 28, and the
Ground Connection section).
08529-008
1MΩ
Figure 28. Ground the Negative Input Through a 1 MΩ Resistor
for Open Thermocouple Detection
Input Voltage Protection
The AD849x has very robust inputs. Input voltages can be up
to 25 V from the opposite supply rail. For example, with a +5 V
positive supply and a −3 V negative supply, the part can safely
withstand voltages at the inputs from −20 V to +22 V. Voltages
at the reference and sense pins should not go beyond 0.3 V of
the supply rails.
MAXIMUM ERROR CALCULATION
As is normally the case, the AD849x outputs are subject to
calibration, gain, and temperature sensitivity errors. The user
can calculate the maximum error from the AD849x using the
following information.
The five primary sources of AD849x error are described in this
section.
AD849x Initial Calibration Accuracy
Error at the initial calibration point can be easily calibrated out
with a one-point temperature calibration. See Table 2 for the
specifications.
AD849x Ambient Temperature Rejection
The specified ambient temperature rejection represents the
ability of the AD849x to reject errors caused by changes in the
ambient temperature/reference junction. For example, with
0.025°C/°C ambient temperature rejection, a 20°C change in the
reference junction temperature adds less than 0.5°C error to the
measurement. See Table 2 for the specifications.
AD849x Gain Error
Gain error is the amount of additional error when measuring away
from the measurement junction calibration point. For example,
if the part is calibrated at 25°C and the measurement junction is
100°C with a gain error of 0.1%, the gain error contribution is
(100°C − 25°C) × (0.1%) = 0.075°C. This error can be calibrated
out with a two-point calibration if needed, but it is usually small
enough to ignore. See Tabl e 2 for the specifications.
Manufacturing Tolerances of the Thermocouple
Consult the data sheet for your thermocouple to find the
specified tolerance of the thermocouple.
Linearity Error of the Thermocouple
Each part in the AD849x family is precision trimmed to optimize
a linear operating range for a specific thermocouple type and
for the widest possible measurement and ambient temperature
ranges. The AD849x achieves a linearity error of less than ±2°C,
within the specified operating ranges listed in Table 7. This error
is due only to the nonlinearity of the thermocouple.
Table 7. AD849x ±2°C Accuracy Temperature Ranges
Part
Thermo-
couple
Type
Max
Error
Ambient
Temperature
Range
Measurement
Temperature
Range
AD8494 J ±2°C 0°C to 50°C −35°C to +95°C
AD8495 K ±2°C 0°C to 50°C −25°C to +400°C
AD8496 J ±2°C 25°C to 100°C +55°C to +565°C
AD8497 K ±2°C 25°C to 100°C −25°C to +295°C
For temperature ranges outside those listed in Table 7 or for
instructions on how to correct for thermocouple nonlinearity
error with software, see the AN-1087 Application Note for
additional details.
