ANALOG DEVICES CN-0281 Service Manual

Circuit Note

CN-0281

Rev. 0

engineers. Standard engineering practices have been employed in the design and construction of
each circuit, and their function and performance have been tested and verified in a lab environment at

However, you are solely responsible for testing the circuit and determining its

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

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.

Devices Connected/Referenced

ADuC7060/

Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support visit www.analog.com/CN0281.

ADuC7061

ADT7311

AD8628 Precision, Low Power, Zero Drift Op Amp

ADP7102 Low Dropout Linear Regulator

Automotive SENT Interface-Based Thermocouple Temperature Sensor with Cold Junction

Compensation Using the ADuC7060/ADuC7061 Precision Analog Microcontroller

EVALUATION AND DESIGN SUPPORT

Design and Integration Files

Schematic, Layout Files, Source Code

CIRCUIT FUNCTION AND BENEFITS

This circuit uses the ADuC7060 or the ADuC7061 precision
analog microcontroller in an accurate thermocouple temperature
monitoring application. The ADuC7060/ADuC7061 integrate
dual 24-bit sigma-delta (Σ-Δ) analog-to-digital converters (ADCs),
dual programmable current sources, a 14-bit digital-to-analog
converter (DAC), and a 1.2 V internal reference, as well as an
ARM7 core, 32 kB flash, 4 kB SRAM, and various digital peripherals
such as UART, timers, serial peripheral interface (SPI), and I
interfaces.

In the circuit, the ADuC7060/ADuC7061 are connected to a
thermocouple and a 100 Ω platinum resistance temperature

detector (RTD). The RTD is used for cold junction compensation.

As an extra option, the ADT7311 digital temperature sensor
can be used to measure the cold junction temperature instead of
the RTD.

In the source code, an ADC sampling rate of 4 Hz was chosen.
When the ADC input programmable gain amplifier (PGA) is
configured for a gain of 32, the noise-free code resolution of the

ADuC7060/ADuC7061 is greater than 18 bits.

The single edge nibble transmission (SENT) interface to the host
is implemented by using a timer to control a digital output pin.
This digital output pin is then level shifted externally to 5 V using
an external NPN transistor. An EMC filter is provided on the
SENT output circuit as recommended in Section 6.3.1 of the SENT
protocol (SAE J2716 Standard). The data is measured as falling
edge to falling edge, and the duration of each pulse is related to the
number of system clock ticks. The system clock rate is determined
by measuring the SYNC pulse. The SYNC pulse is transmitted

2

C

at the start of every packet. More details are provided in the
SENT Interface section.

CIRCUIT DESCRIPTION

The following features of the ADuC7060/ADuC7061 are used
in this application:

• A primary, 24-bit Σ-Δ ADC with a PGA. The PGA is set

for a gain of 32 in the software for this application. The
primary ADC was switched continuously between
sampling the thermocouple and the RTD voltages.

• If an RTD is used for measuring the cold junction temperature,

programmable excitation current sources force a controlled
current through the RTD. The dual current sources are
configurable in 200 µA steps from 0 µA to 2 mA. For this
example, a 200 µA setting was used to minimize the error
introduced by the RTD self-heating.

• If the ADT7311 is used for measuring the cold junction

temperature, the SPI interface is used in master mode to
connect the ADT7311 slave device.

• An internal 1.2 V reference for the ADC in the ADuC7060/

ADuC7061. For measuring the thermocouple voltage, the

internal voltage reference was used due to its precision.

• An external voltage reference for the ADC in the ADuC7060/

ADuC7061. For measuring the RTD resistance, a ratiometric

setup was used where an external reference resistor (R
was connected across the external VREF+ and VREF− pins.

• The AD8628 si

high impedance reference voltage across R

• The OP193 is another option instead of the AD8628.

• A DAC that was used to set the thermocouple common-

mode voltage to 850 mV above ground.

ARM7-Based Microcontroller with Dual
24-Bit Σ-Δ ADCs

±0.5°C Accurate ,16-Bit Digital SPI
Temperature Sensor

ngle supply op amp was used to buffer the

to the ADC.

REF

REF

)

Circuits from the Lab™ circuits from Analog Devices have been designed and built by Analog Devices

room temperature.

be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause

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CN-0281 Circuit Note

5V

PWRIN

5V

PWRIN

10Ω 1.6Ω

10Ω

10Ω

10Ω

10Ω

10kΩ

C1R1C2

100Ω

Pt RTD

R

REF

5.6kΩ

(0.1%)

THERMOCOUPLE

JUNCTION

2.5V

0.1µF 4.7µF

0.1µF

0.1µF

0.1µF

0.1µF

0.1µF

0.1µF

PWRIN

AD8628

ADuC7016

5V

AVDD

AVDD

DVDD

BEAD

VIN

ADP7102

(ADJ)

VOUT

GND

ONLY REQUIRED FOR ADT7311

TEMPERATURE SENSOR OPTION

AVDD

AVDD

PWRIN

10Ω

0.1µF

P0.0 SCLK

0.1µF

2.7V

VIN

ADP7102

(ADJ)

ADT7311

VOUT

GND

IEXC0

ADC0

ADC1

VREF+

VREF–

P0.0

P0.4

PESD1LIN

P0.1

SPI

P0.2

P0.3

S2

S1

ADC2

ADC3

RESET

NTRST

DAC0

P1.0/SIN

PROGRAMMABLE

INTERFACE

P1.1/SOUT GND

V

DD

2.7V

P0.1

SPI

DOUT

P0.2 DIN

P0.3 CS

0.1µF

DVDD

DVDD

2N3904

SENT

10889-001

• An AR M7 TDMI® co re. The powerful 16-/32-bit A RM7 c ore

with integrated 32 kB flash and SRAM memory runs the
us er code that configures and controls the ADC, processes
the ADC conversions from the RTD, and controls the
com mu nicatio ns o ver t he S PI inte rfa ce.

• Timer1 and a digital output pin are use d to generate the

SENT output signal.

• An optional PESDLIN protection diode for suppression of

ESD, electrical fast transients (EFT), and surge transients
up to 23 k V.

• An EMC f ilte r on the SENT output as recommended in

Section 6.3.1 of the SAE J2716 standard (SENT Protocol).

• Two external switches are used to force the part into its

flash boot mode. By holding S1 low and toggling S2, the

ADuC7060/ADuC7061 e nter boot m ode instead of normal

user m ode. In boot mode, the internal flash can be
reprogrammed through the UART interface.

Bo th t he thermocouple and th e RTD generate ver y small s ignals;
therefore, a PGA is required to amplify those signals. As the
au xiliar y AD C on the ADuC7060/ADuC7061 does not have a
PGA, both were connected to the primary ADC, and switching
between the two was done in software.

The th erm oco uple u sed in this a pplicatio n is a Type T (copper­co nst antan ), w hich has a t emperatu re range of −200°C t o + 350°C.
Its sens itivity is approximately 40 µV/°C; the refore, in bipo lar
mode, the ADC , with a PGA g ain o f 32, can cover the entire
temperature range of the thermocouple.

Th e RTD was us ed for cold ju nction compensat ion. In this circuit,
a platinum, 100 Ω RTD, Enercorp PC S 1.1503.1 was used. It is
available in a 0805 surface-mount package. This RTD has a
temperature variation of 0.385 Ω/°C.

Note that the reference resistor, R

, s hou ld be a p recis ion

REF

5.6 kΩ (±0.1%).

Fi gure 1. ADuC7060/ADuC7061 as a Temperature Monitor Controller with a Thermocouple Interface (Simplified Schematic, All Connections Not Shown)

Rev. 0 | Pag e 2 of 6