ANALOG DEVICES CN-0206 Service Manual

Circuit Note

Circuits from the Lab™ circuits from Analog Devices have been designed and built by Analog Devices
engineers. Standard engineering practices have been employed in the design and construction of

d their function and performance have been tested and verified in a lab environment at

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

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

6VLITHIUM ION
BATTERY
(2 × 3V CELLS)

LCD

DRIVER

LCD

AIN1(+)

DV

DD

AV

DD

AIN1(–)

AD7793

SCLK

DIN

DOUT/RDY

CS

IOUT1
AIN2(+)

AIN2(–)
REFIN(+)

REFIN(–)

0.1µF

0.01µF

+

–

0.01µF

1kΩ

1kΩ

THERMISTOR

KTY81-110

1kΩ AT 25°C

THERMOCOUPLE

COLD JUNCTION

THERMOCOUPLE

CONNECTOR

2kΩ

0.1%

10ppm

0.1µF

1.5Ω

10µF

+

+

ADuC832

ANALOG

MICRO-

CONTROLLER

CLK GND

IOUT2

09776-001

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/CN0206.

Complete Thermocouple Measurement System Using the

AD7793 24-Bit Sigma-Delta ADC

EVALUATION AND DESIGN SUPPORT

Design and Integration Files
Schematics, Layout Files, Bill of Materials

CIRCUIT FUNCTION AND BENEFITS

The circuit, shown in Figure 1, is a complete thermocouple system
based on the AD7793 24-bit sigma-delta ADC. The AD7793 is
a low power, low noise, complete analog front end for high
precision measurement applications. The device includes a
PGA, internal reference, internal clock, and excitation currents,
thereby greatly simplifying the thermocouple system design.
The system noise is approximately 0.02°C peak-to-peak.

CN-0206

Devices Connected/Referenced

3-Channel, Low Noise, Low Power,

AD7793

ADuC832 Precision Analog Microcontroller

The AD7793 consumes only 500 µA maximum, making
it suitable for any low power application, such as smart
transmitters where the complete transmitter must consume less
than 4 mA. The AD7793 also has a power down option. In this
mode, the complete ADC along with its auxiliary functions are
powered down so that the part consumes 1 µA maximum.

Since the AD7793 provides an integrated solution for
thermocouple design, it interfaces directly to the thermocouple.
For the cold junction compensation, a thermistor along with a
precision resistor is used. These are the only external
components required for the cold junction measurement
other than some simple R-C filters for EMC considerations.

24-Bit Σ-Δ ADC with On-Chip In-Amp
and Reference

Figure 1. Thermocouple Measurement System with Cold Junction Compensation (Simplified Schematic: All Connections and Decoupling Not Shown)

Rev.0

each circuit, an

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)

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CN-0206 Circuit Note

20

15

10

5

0

–5

–10

–300 –200 –100 0 100 200 300 400

09776-002

THERMOCOUPLE EMF (mV)

TEMPERATURE (°C)

APPROXIMATELY
LINEAR REGION

TYPE “T” THERMOCOUPLE

CIRCUIT DESCRIPTION

A type "T" thermocouple is used in the circuit. This
thermocouple (made from copper and constantan) measures
temperature from −200°C to +400°C. It generates a typical
temperature dependent voltage of 40 µV/°C.

A thermocouple does not have a linear transfer function. For a
temperature range of 0°C to +60°C , the response is quite linear.
However, for wider temperature ranges, a linearization routine
is required.

The circuit tested does not include linearization. Therefore, the
useful measurement range of the circuit is from 0°C to +60°C.
For this temperature range, the thermocouple generates a
voltage from 0 mV to 2.4 mV. The internal 1.17 V reference is
used for the thermocouple conversions. So, the AD7793 is
configured for a gain of 128.

Since the AD7793 operates from a single power supply, the
signal generated by the thermocouple must be biased above
ground so that it is within the acceptable range of the ADC. For
a gain of 128, the absolute voltage on the analog inputs must be
between GND + 300 mV and AVDD – 1.1 V.

The bias voltage generator onboard the AD7793 biases the
thermocouple signal so that it has a common-mode voltage of
AVDD/2. This ensures that the input voltage limits are met with
significant margin.

The thermistor has a value of 1 kΩ at +25°C. The typical
resistance at 0°C is 815 Ω and 1040 Ω at +30°C. Assuming a
linear transfer function between 0°C and 30°C , the relationship
between cold junction temperature and thermistor resistance R is

Cold Junction Temperature = 30 × (R – 815)/(1040 – 815)

The 1 mA excitation current on the AD7793 is used to supply
the thermistor and the 2 kΩ precision resistor. The reference
voltage is generated using this external precision 2 kΩ resistor.
This architecture gives a ratiometric configuration—the
excitation current is used to supply the thermistor and to
generate the reference voltage. Therefore, any deviation in the
value of the excitation current does not alter the accuracy of the
system.

The AD7793 operates at a gain of 1 when sampling the
thermistor channel. For a maximum cold junction of +30°C,
the maximum voltage generated across the thermistor is 1 mA ×
1040 Ω = 1.04 V.

The precision resistor is chosen so that the maximum voltage
generated across the thermistor multiplied by the PGA gain is
less than or equal to the voltage generated across the precision
resistor.

For a conversion value of ADC_CODE, the corresponding
thermistor resistance R equals

One other consideration is the output compliance of the IOUT1
pin of the AD7793. When the 1 mA excitation current is used,
the output compliance equals AVDD – 1.1 V. From the previous
calculations, this specification is met since the maximum
voltage at IOUT1 equals the voltage across the precision resistor
plus the voltage across the thermistor, which equals 2 V + 1.04 V
= 3.04 V.

The AD7793 is configured to operate with an output data rate of

16.7 Hz. For every ten conversions read from the thermocouple,
one conversion is read from the thermistor. The resultant
temperature equals

The conversions from the AD7793 are processed by the

ADuC832 analog microcontroller, and the resultant

temperature is displayed on the LCD display.

The thermocouple design is operated from 6 V (2 × 3 V
Lithium Ion) batteries. A diode reduces the 6 V to a level
suitable for the AD7793 and the ADuC832 analog
microcontroller. An RC filter is placed between the ADuC832
power supply and the AD7793 power supply so that the power
supply digital noise to the AD7793 is minimized.

Figure 2 shows the relationship between voltage generated across
the thermocouple and temperature for a T-type thermocouple. The
circled area is the region from 0°C to +60°C where the transfer
function is approximately linear.

Rev. 0 | Page 2 of 4

R = (ADC_CODE – 0x800000) × 2000/2

23

Temperature = Thermocouple Temperature + Cold
Junction Temperature

Figure 2. Thermocouple EMF vs Temperature