AD8494/AD8495/AD8496/AD8497

Rev. 0 | 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.

AD849x

PCB

TRACES

KEEP

TRACES

SHORT

KEEP JUNCTION AND

AD849x AT SAME

TEMPERATURE

MEASUREMENT

JUNCTION

REFERENCE

JUNCTION

THERMOCOUPLE WIRES

R

R

AD849x

1M

Ω

CONNECT WHEN

THERMOCOUPLE TIP

TYPE IS UNKNOWN

1

2

1

2

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

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 kΩ.

Ground Connection

It is always recommended that the thermocouple be connected

to ground through a 100 kΩ to 1 MΩ 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.

1M

Ω

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.