AD8494/AD8495/AD8496/AD8497

Rev. 0 | 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).

1M

Ω

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 Table 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 product page for the AD8494,

AD8495, AD8496, or AD8497, or contact an Analog Devices

representative.