ANALOG DEVICES CN-0271 Service Manual

Circuit Note

tested for quick and easy system integration to help solve today’s

16-Bit, Single-Channel, Ultralow Power, Sigma­Delta ADC

Ultralow Noise, 2.5 V, LDO, XFET Voltage

Rev. A

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whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page)

Fax: [phone] ©2012 Analog Devices, Inc. All rights reserved.

AD8495

OUT

SENSE

REF

–V

S

+V

S

+V

S

–V

S

INP

INN

0.1µF 10µF

+5V

+2.5V

COLD

JUNCTION

COMPENSATION

THERMO­COUPLE

1MΩ

100Ω

49.9kΩ

0.01µF

0.01µF

1.0µF

100Ω

0.1µF

0.1µF

10µF

+5V +2.5V

IN-AMP

+OUT

–OUT

AD8476

10kΩ

10kΩ

10kΩ

10kΩ

100Ω

0.01µF

0.01µF

1.0µF

100Ω

SERIAL

INTERFACE

INTERNAL

CLOCK

16-BIT

ADC

GND

REFIN

AD7790

DIGITAL

PGA

BUF

V

DD

V

DD

GND

+5V

ADR441

+5V

+2.5V

VIN VOUT

GND

10598-001

Circuits from the Lab™ reference circuits are engineered and

analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0271.

K-Type Thermocouple Measurement System with Integrated Cold Junction

Compensation

EVALUATION AND DESIGN SUPPORT

Circuit Evaluation Boards

CN-0271 Circuit Evaluation Board (EVAL-CN0271-SDPZ)
System Demonstration Platform, SDP-B (EVAL-SDP-CB1Z)

Design and Integration Files

Schematics, Layout Files, Bill of Materials

CIRCUIT FUNCTION AND BENEFITS

The circuit shown in Figure 1 is a complete thermocouple signal
conditioning circuit with cold junction compensation followed
by a 16-bit sigma-delta (Σ-Δ) analog-to-digital converter (ADC).
The AD8495 thermocouple amplifier provides a simple, low cost
solution for measuring K type thermocouple temperatures,
including cold junction compensation.

CN-0271

Devices Connected/Referenced

AD8495

AD8476

AD7790

ADR441

A fixed gain instrumentation amplifier in the AD8495 amplifies
the small thermocouple voltage to provide a 5 mV/°C output. The
high common-mode rejection of the amplifier blocks common­mode 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.

The AD8476 differential amplifier provides the correct signal levels
and common-mode voltage to drive the AD7790 16-bit, Σ-Δ ADC.

The circuit provides a compact low cost solution for thermocouple
signal conditioning and high resolution analog-to-digital
conversion.

Full K-Type Range 0°C to 50°C Thermocouple
Amplifier with Cold Junction Compensation

Low Power, Unity-Gain Fully Differential
Amplifier and ADC Driver

Reference with Current Sink and Source

Figure 1. K-Type Thermocouple Measurement System with Integrated Cold Junction Compensation (Simplified Schematic: All Connections Not Shown)

each circuit, and

suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone]

www.analog.com

)2log(

57.16.6

log















××××

=

BandwidthGainDensityNoiseVoltage

V

BitsFreeNoise

MAX

OUT

bits12.4

log(2)

Hz8001.57122.4)HznV/(326.6

V4.9

log

=















××××

=

–2.0

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

2.0

–50

0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

850

900

950

ERROR (°C)

JUNCTION T E M P E R ATURE (° C)

AD8495
CN-0271
CN-0271 WITH

NONLINEARITY CORRECTION

10598-002

CN-0271 Circuit Note

CIRCUIT DESCRIPTION

The thermocouple is a simple, widely used component for
measuring temperature. It consists of a junction 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

MJ), the user must know the differential voltage created by

the thermocouple. The user must also know the error voltage
generated by the temperature at the reference junction (T

RJ).

The AD8476 is a very low power, fully differential precision
amplifier with integrated thin film, laser trimmed 10 kΩ gain
resistors for unity gain. It is an ideal choice for this application
because it presents a relatively high impedance load to the AD8495.

The AD7790 is a low power, complete analog front end for low
frequency measurement applications. It contains a low noise,
16-bit, Σ-Δ ADC with one differential input that can be buffered or
unbuffered.

Compensating for the reference junction error voltage is 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.

The circuit uses the AD8495 thermocouple amplifier on a single
5 V supply. The output voltage of the AD8495 is calibrated for
5 mV/°C. On a single 5 V supply, the output is linear between
approximately 75 mV and 4.75 V, corresponding to a temperature
range of 15°C to 950°C. The output of the AD8495 drives the
noninverting input of the AD8476 unity-gain differential amplifier,
which converts the single-ended input to differential outputs for
driving the AD7790 16-bit, Σ-Δ ADC.

A low-pass differential and common-mode filter before the
input of the AD8495 prevents RF signals, which, if allowed to
reach the AD8495, can be rectified and appear as temperature

Test Results

An important measure of the performance of the circuit is the
amount of linearity error. The AD8495 output is accurate to
within 2°C from −25°C to +400°C. To achieve even greater
accuracy when operating at or outside of this range, a linearity
correction algorithm must be implemented in software. The

CN-0271 evaluation software uses NIST thermoelectric voltage

lookup tables to achieve an output error within 1°C from 15°C
to 950°C.

Figure 2 compares the performance of the AD8495 with the

CN-0271 system, and the result of applying the linearization

correction to the ADC output. For details on how the algorithm
was implemented in the software, see the AN-1087 Application

Note, Thermocouple Linearization When Using the AD8494/

AD8495/AD8496/AD8497.

fluctuations. The two 100 Ω resistors and the 1 µF capacitor
form a differential filter with a cutoff frequency of 800 Hz. The
two 0.01 µF capacitors form common-mode filters with a cutoff
frequency of 160 kHz. A similar filter is used at the output of
the AD8476 differential amplifier before the signal is applied
to the AD7790 ADC.

The AD8495 inputs are protected from input voltage excursions
up to 25 V from the opposite supply rail. For example, in this
circuit, with a 5 V positive supply rail and the negative supply
rail tied to GND, the part can safely withstand voltages at the
inputs from −20 V to +25 V. Voltages at the reference and sense
pins should not go beyond 0.3 V of the supply rails. This feature
is of particular importance in applications with power supply
sequencing issues that can cause the signal source to be active
before the supplies to the amplifier are applied.

The theoretical resolution of the system can be calculated from
the bandwidth, voltage noise density, and gain of the AD8495.
The peak-to-peak (noise free code) resolution in bits is

Rev. A | Page 2 of 5

Figure 2. Output Error of AD8495, Total CN-0271 Circuit Error, and

Total CN-0271 Circuit Error with Thermocouple Nonlinearity Correction