Datasheet CN-0281 Datasheet (ANALOG DEVICES)

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

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

www.analog.com

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

Circuit Note CN-0281

0

56

CLOCK TICKS

SYNC

PULSE

STATUS

PULSE

D1 TO D6 CRC

PULSE

1 0 7 1 2 8 0

10889-002

0.30

0.25

0.20

0

0.05

0.10

0.15

–0.05

–210 –140 –70 0 70 140 210 280 350

ERROR (°C)

TEMPERATURE (°C)

10889-003

SENT Interface

The SENT interface is a single pin, unidirectional (sensor to host),
time modulated signal used mainly in automotive systems for
interfacing distributed sensors to host CPUs.

The main requirements for SENT include the following:

• It must have a 0 V to 5 V signal swing with EMC filtering.

• The clock used for the SENT signal must be accurate to ±20%.

• The SENT output circuit must be robust enough to tolerate

short circuit to ground and short circuit to supply voltages.

The associated source code uses the P0.4 digital pin of the

ADuC7061 as the SENT output pin. The packet format used is

the single sensor packet format as detailed in A.4 section of the
SAE J2716 standard (SENT Protocol) document. The source
code, particularly the SENT.h and Sent.c files can be modified to
support other packet formats. The overall temperature result is
returned in /°C format in Data Nibble 1 to Data Nibble 3. In
summary, the returned output packet is

• 56 clock ticks for sync pulse

• Status pulse (7 ticks to 15 ticks)

• Data Nibble 1 (Bit 11 to Bit 8 of the temperature result)

• Data Nibble 2 (Bit 7 to Bit 4 of the temperature result)

• Data Nibble 3 (Bit 3 to Bit 0 of the temperature result)

• Data Nibble 4 (Bit 7 to Bit 4 of the counter)

• Data Nibble 5 (Bit 3 to Bit 0 of the counter)

• Data Nibble 6 (inverse of Nibble 1)

An example packet is shown in Figure 2.

Code Description

Timer1 is used to control the SENT output pin, P0.4. When a
temperature result is been calculated from the ADC results for
the thermocouple and the cold junction temperature, the SENT
packet structure, SENT_PACKET, is updated, and Timer1 started.
The fields of the structure are clocked out on the P0.4 pin, as
shown in Figure 2. The first stage of the packet is the sync sequence.
The host determines the clock period from this pulse, and it
determines the subsequent nibble values from this.

To get a temperature reading, measure the temperature of the
thermocouple and the RTD. The RTD temperature is converted
to its equivalent thermocouple voltage via a lookup table. These
two voltages are added together to give the absolute value of the
voltage at the thermocouple.

First, the voltage between the two wires of the thermocouple (V1)
is measured. Then, the RTD voltage is measured and converted to
a temperature via a lookup table. This temperature is then
converted to its equivalent thermocouple voltage (V2). V1 and
V2 are then added to give the overall thermocouple voltage, and
this value is then converted to the final temperature measurement.

A piecewise linearization scheme is used to calculate the final
temperature value. The respective temperatures for a fixed number
of voltages are stored in an array, and values in between are
calculated using a linear interpolation between the adjacent points.
Figure 3 gives the algorithm error using the ideal thermocouple
voltages. Figure 4 shows the error obtained when using the
ADC0 pin on the ADuC7060/ADuC7061 to measure the
52 thermocouple voltages over the full thermocouple
operating range. The overall worst-case error is <1°C.

Figure 2. Example SENT Packet Measured at the P0.4 Pin

The circuit must be constructed on a multilayer printed circuit
board (PCB) with a large area ground plane. Proper layout,
grounding, and decoupling techniques must be used to achieve
optimum performance (see Tu tor i al M T-031, Grounding Data

Converters and Solving the Mystery of "AGND" and "DGND,"

Tutor ia l MT-101, Decoupling Techniques, and the ADuC7060/
ADuC7061 evaluation board layouts).

Figure 3. Error When Using Piecewise Linear Approximation Using

Rev. 0 | Page 3 of 6

52 Calibration Points and Ideal Measurements

CN-0281 Circuit Note

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

–210 –140 –70 0 70 140 210 280 350

ERROR (°C)

TEMPERATURE (°C)

10889-004

10889-005

Figure 4. Error When Using Piecewise Linear Approximation Using

52 Calibration Points Measured by the ADC0 Pin of the

The RTD temperature is calculated using lookup tables and is
implemented for the RTD the same way as for the thermocouple.
Note that the RTD has a different polynomial describing its
temperatures as a function of resistance.

The source code in the design support package was generated using
KEIL µVision V3.90.

For details on linearization and maximizing the performance of
the RTD, refer to Application Note AN-0970, RTD Interfacing

and Linearization Using an ADuC706x Microcontroller.

COMMON VARIATIONS

Instead of using an external RTD, the ADT7311 temperature
sensor can be used to measure the cold junction temperature.

Source code using the ADT7311 is provided with the software
package in the design support package:

www.analog.com/CN0281-DesignSupport.

An extra external regulator is required to power the ADT7311.
The ADP7102 was used in this circuit, but an alternative is the

ADP120.

If more GPIO pins are required on the microcontroller, the

ADuC7060, which comes in a 48-lead LFCSP or a 48-lead L Q FP,

is another option. Note that the ADuC7060/ADuC7061 can be
programmed or debugged via a standard JTAG interface.

When using the external reference resistor as the reference source
for the RTD measurement, it is recommended to buffer the input
to the VREF+ pin using an op amp in unity-gain mode. This is to
ensure that the input leakage current to the VREF+ pin does
not distort the measurement accuracy. In Figure 9, the OP193
was used for this purpose. Figure 1 shows the AD8628 which is
characterized to work in the −40°C to +150°C operating range.

ADuC7060/ADuC7061

For a wider temperature range, a different thermocouple can be
used, such as a Type J. To minimize the cold junction compensation
error, a thermistor can be placed in contact with the actual cold
junction instead of on the PCB.

Instead of using the RTD and external reference resistor for
measuring the cold junction temperature, an external digital
temperature sensor can be used. For example, the ADT7311
can connect to the ADuC7060/ADuC7061 via the I

2

C interface.

For additional information on cold junction compensation, see
the Sensor Signal Conditioning, Analog Devices, Chapter 7,

"Temperature Sensors."

CIRCUIT EVALUATION AND TEST

Rev. 0 | Page 4 of 6

SENT Interface Tests

The SENT interface output was evaluated using a digital oscillo­scope and a logic analyzer. The SENT interface was only tested
with a SENT clock period of 100 µs, which is the maximum
allowed by the SENT standard. The limiting factors in this
implementation include the following:

• Variation in FIQ interrupt latency for the timer. The interrupt

latency of an ARM7 can be up to 45 CPU clocks. With a

10.24 MHz CPU clock, this is up to 4.4 µs. The minimum
is 5× CPU clocks (0.5 µs). The worst-case latency occurs
when the ARM7 core is executing a LDMIA or STMIA (load
or store multiple values to/from memory). Select the Split
Load and Store Multiple compiler option to minimize this,
and see

Figure 5 to see

how to do this.

• The 10.24 MHz oscillator on the ADuC7060/ADuC7061

has a worst-case accuracy of ±3% over the −40°C to +125°C
operating temperature range.

Figure 5. Selecting Split Load and Store Multiple Option

Despite these limitations, this implementation is well within the
required ±20% timing accuracy of the SENT specification when
a SENT clock period of 100 µs is selected.

Circuit Note CN-0281

4.5

4.0

3.5

3.0

2.

5

2.0

AMPLITUDE (V)

TIME (µs)

1.5

1.0

0.5

5 10 15 20 25 30 35 400

0

10889-006

EVAL-ADuC7061MKZ

WAVTEK 4808

MULTIFUNCTION

CALIBRATOR

PC

J2-9

J2-8

J2-5

THERMOCOUPLE

JUNCTION

SEE TEXT

USB

CABLE

10889-007

0

–0.01

–25 –5

ERROR (°C)

TEMPERATURE (°C)

–0.02

–0.03

–0.04

–0.05

–0.06

–0.07

–0.08

–0.09

–0.10

15 35 55

75 95 115

10889-009

An EMC filter has been catered for on the SENT output in Figure 1.
Chose values for this filter (R1, C1, C2) to meet the targeted
transmitter output waveform, as shown in Figure 6, to reduce
EMC emissions due to the SENT output.

Only limited testing of this filter has been done; therefore, the
resistor and capacitor values for the output filter were omitted,
as is shown in Figure 1 (R1, C1, and C2).

Figure 6. Example SENT Shaped Waveform Transmitter Output

To further test and evaluate the circuit, the thermocouple
measurements and the RTD measurements were evaluated
separately.

Thermocouple Measurement Test

The basic test setup is shown in Figure 7. Two methods were
used to evaluate the performance of the circuit. Initially, the
circuit was tested with the thermocouple attached to the board
and used to measure the temperature of an ice bucket, and then
the temperature of boiling water.

A Wavetek 4808 multifunction calibrator was used to fully
evaluate the error, as shown in Figure 3 and Figure 4. In this
mode, the thermocouple was replaced with the calibrator as the
voltage source, as shown in Figure 7. To evaluate the entire range
of a Type T thermocouple, the calibrator was used to set the
equivalent thermocouple voltage at 52 points between −200°C
to +350°C for the negative and positive ranges of the Type T
thermocouple (see the ITS-90 Table for Type T thermocouple).

To evaluate the accuracy of the lookup algorithm, 551 voltage
readings, equivalent to temperatures in the range of −200°C to
+350°C spaced at +1°C, were passed onto the temperature
calculation functions. Errors were calculated for the linear method
and the piecewise linear approximation method as can be seen
in Figure 3

and Figure 4.

RTD Measurement Test

To evaluate the RTD circuit and linearization source code, the
RTD on the board was replaced with an accurate, adjustable
resistance source. The instrument used was the 1433-Z decade
resistor. RTD values from 90 Ω to 140 Ω, which represent an
RTD temperature range of −25°C to +114°C, was the range of
the RTD values tested.

The error results for the RTD tests are shown in Figure 8, and
the setup circuit is shown in Figure 9.

Figure 7. Test Setup Used to Calibrate and Test the Circuit Over Full

Thermocouple Output Voltage Range

Figure 8. Error in °C of the RTD Measurement Using Piecewise Linearization

Code and ADC0 Measurements

Rev. 0 | Page 5 of 6

CN-0281 Circuit Note

10Ω

10Ω

0.1µF

0.01µF

1433-Z DECADE

RESISTOR

0.01µF

AVDD

OP193

AVDD

ADuC7061

AVDD

ADC0

ADC1

VREF+

VREF–

P1.0/SIN P1.1/SOUT

RxD TxD

RxD

IN OUT

TxD

0.1µF

DVDD

DVDD

0.1µF

R

REF

5.6kΩ

(0.1%)

FT232QN

ADP3333-2.5

10Ω

4.7µF

4.7µF

AVDD

DVDD

BEAD

BEAD

USB

HEADER

BEAD

5V
D–
D+

GND

SHIELD

IEXC0

10889-008

+
–

(Continued from first page) Cir cuits from the Lab circuits are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you

reserves the right to change any Circuits from the Lab circuits at any time without notic e but is under no obligation to do so.

Figure 9. Test Setup Used to Calibrate and Test the RTD Output Voltage Range from −25°C to +114°C

LEARN MORE

CN0281 Design Support Package:

http://www.analog.com/CN0281-DesignSupport.

ADIsimPower Design Tool.

Kester, Walt. 1999. Sensor Signal Conditioning. Analog Devices.

Chapter 7, "Temperature Sensors."

Kester, Walt. 1999. Sensor Signal Conditioning. Analog Devices.

Chapter 8, "ADCs for Signal Conditioning."

Looney, Mike. RTD Interfacing and Linearization Using an

ADuC706x Microcontroller. AN-0970 Application Note.
Analog Devices.

MT-022 Tutorial, ADC Architectures III: Sigma-Delta ADC

Basics. Analog Devices.

MT-023 Tutorial, ADC Architectures IV: Sigma-Delta ADC

Advanced Concepts and Applications. Analog Devices.

MT-031 Tutorial, Grounding Data Converters and Solving the

Mystery of "AGND" and "DGND." Analog Devices.

MT-101 Tutorial, Decoupling Techniques. Analog Devices.

ITS-90 Table for Type T Thermocouple.

SAE J2716 Standard (SENT Protocol).

may use the Circuits from the Lab circuits in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by
application or use of the Circuits from the Lab circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab circuits are supplied
"as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular
purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices

©2012 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
C

N10889-0-7/12(0)

Rev. 0 | Page 6 of 6

Data Sheets and Evaluation Boards

ADuC7060/ADuC7061 Data Sheet

ADuC7060/ADuC7061 Evaluation Kit

ADM3202 Data Sheet

ADT7311 Data Sheet

AD8628 Data Sheet

ADP7102 Data Sheet

REVISION HISTORY

7/12—Revision 0: Initial Version