ADT7320/ADT7420: Digital Temperature Sensors FAQ PDF

ADT7320/ADT7420 Digital Temperature Sensors
Frequently Asked Questions
Why should I use a digital temperature sensor over other
Digital sensors are fully integrated and calibrated temperature
measurement solutions and offer many advantages over analog type
sensor technologies such as thermistors and resistance temperature
detectors (RTDs).
• High performance
• High accuracy. Accuracy to 0.2°C (maximum) with all errors included
in the sensor specifications.
• High stability, repeatability, and reliability. Drift and repeatability are
Note that the digital temperature sensors are not typically used as
thermocouple replacements because thermocouples have a much wider
temperature range. However, digital temperature sensors are very popular
for providing the reference temperature for thermocouples, which is called
cold junction compensation and is described in more detail in the following
technical article: “Two Ways to Measure Temperature Using Thermocouples
Feature Simplicity, Accuracy, and Flexibility.” Analog Dialogue. Volume 44,
October 2010 at
In summary, digital temperature sensors provide easy, reliable, and cost
effective high performance temperature measurement.
How is the high accuracy of the temperature sensors achieved?
included within the sensor specifications.
• Fast thermal response. Thermal response is dependent on how the
sensor is used. For example, mounting the sensor on a large PCB
limits the thermal response because the PCB acts as a heat sink.
• Traceability. Sensors are traceable to national standards such as
those from the National Institute of Standards and Technology (NIST).
• Low cost
• No additional signal processing or additional components.
• No user calibration. Because the devices are precalibrated, all errors
involved in measuring and digitizing the temperature value are
included in the sensor’s accuracy specifications. In contrast, an
analog-based sensing element’s specified error, including temperature drift and noise, must be added to that of any ADC, amplifier,
reference, wiring, or other component that is used in conjunction
with the sensor. Calibration costs can be expensive and, in many
cases, exceed the cost of the sensor element itself.
• Easy implementation
• No complex calibration.
• Standard SPI (ADT7320) and I2C (ADT7420) digital interfaces.
• No self-heating or lead wire resistance concerns. The sensors are
very robust and do not suffer from moisture ingress issues, unlike
other sensors.
• Low power
• Low software overhead; no linearization.
• No additional components required.
• Multiple low power modes, including a 2 μA shutdown mode.
The latest sensor cores are based on a silicon band gap principle, which
is the same principle used by all temperature sensing ICs in the industry
today. In fact, ADI is one of the pioneers of this technique and released the
industry classic, AD590, in 1978. Based on knowledge gained since the
development of the AD590, ADI has optimized the temperature core and,
coupled with leading precision sigma-delta ADC technology, has achieved
high levels of accuracy. In addition to optimizing the design, ADI has developed a test capability that enables reliable testing of the sensors to high
accuracy specifications.
The architecture diagram in Figure 1 highlights the band gap-based temperature sensor core and the high precision sigma-delta modulator that are
used to convert the voltage from the band gap to a digital value.
What is the response time of these sensors?
Thermal response is a function of the thermal mass of the sensor but is
heavily influenced by the mass of the object on which the IC is mounted.
For example, a large PCB acts as a large heat sink and slows the thermal
response. For best thermal response, it is recommended to mount the
sensor on as small a PCB as possible, such as a flex PCB that provides the
best thermal response. ADI has achieved thermal responses of <2 seconds
(including all signal processing) using the ADT7320/ADT7420 and a flex
PCB. Look for additional details in a forthcoming application note.
Do these sensors need to be calibrated?
No user calibration is required with these sensors because ADI precalibrates
the devices using our high accuracy test solution. Because the devices are
precalibrated, all errors involved in measuring and digitizing the temperature value are included in the sensor’s accuracy specifications.
Table 1 compares digital ICs against RTD- and thermistor-based solutions.
What are the repeatability and drift performance of the sensors?
What are the best practices for layout?
Repeatability is the ability to obtain consistent results when measuring
a part with the same measuring instrument. For the ADT7320/ADT7420, the
repeatability performance is typically ±0.015°C.
For PCB temperature measurement, use a common GND plane between the
ADT7320/ADT7420 and the heat source. Ensure that both the GND pin and
paddle are connected to the heat source GND plane. Keep the ADT7320/
ADT7420 and heat source as close as possible to each other on the PCB.
Drift indicates how often a measurement needs recalibration. Drift includes
solder heat resistance and lifetime test performed per Jedec Standard
JESD22-A108. The ADT7320/ADT7420 typically drifts 0.0073°C over its
operating lifetime.
How do I use a sensor for point temperature measurement?
For applications where the sensor must measure the surface temperature
of an object (such as a metal plate), it is recommended to mount the sensor
on a thin flex PCB and thermally glue the sensor to the object being measured. It is also recommended to thermally insulate all parts of the sensor
not in contact with the object being measured. For more information on the
best solution for your application, please contact ADI.
What does NIST traceability mean?
NIST is the abbreviation of the U.S. Commerce Department’s National
Institute of Standards and Technology (see the NIST web site at Traceability is the establishment of an
unbroken chain of comparisons to stated references, which basically
indicates that the absolute temperature reported by the sensor is
traceable to a standard reference.
Are evaluation board and samples available?
An evaluation board is available for the ADT7320 and ADT7420. The board
(model EVAL-ADT7X20EBZ) is available for purchase from the ADT7420
product page at
Samples are also available at
What other resources are available?
ADI has a number of additional resources available to assist with the
incorporation of these sensors into an application. These include
• Circuits from the Lab™, which are tested circuit designs that address
common design challenges and have been engineered for quick and
easy system integration. Look for information about thermocouple cold
junction in a forthcoming Circuits from the Lab.
• Cold junction compensation technical article available at
• Accuracy demonstration video available at
• Temperature sensing selection guide available at
For the latest list of available resources, see the ADT7320 and
ADT7420 product pages at and
| ADT7320/ADT7420 Digital Temperature Sensors
For ambient temperature measurement, use a hash GND plane. Reduce
the GND plane area to increase thermal resistance. Keep the ADT7320/
ADT7420 as far away from the heat source as possible. Use a separate
GND plane for the ADT7320/ADT7420, and keep connections to the main
GND plane as low as possible. Use narrow GND connections to increase
thermal resistance. Use a solid GND plane under the main heat source and
expose the green solder mask. This provides the minimum thermal resistance for the main heat source to dissipate heat.
What is the MTTF?
MTTF refers to mean time to failure, which is the mean time it takes for a
first failure to occur under specified operating conditions. It is calculated
by dividing the total number of device operating hours by the number of
failures. Details of the ADT7320/ADT7420 MTTF can be obtained at
Does ADI offer other packaging options? Does ADI offer through-hole
ADI is currently evaluating a number of alternative packaging options as
part of its temperature sensing roadmap. Please contact ADI for more
Other Questions/Feedback
ADI values feedback on our temperature sensing solutions. Please send
feedback to ADI via [email protected]
Table 1.
Digital IC
(Thin Film)
NTC Thermistor
Accuracy 1
±0.1°C from 0°C to 70°C, ±0.3°C from 0°C
to 100°C; excludes data conversion, signal
conditioning, self-heating, noise, drift, and so on
Thermal Response
Medium to fast
Long Term Stability/Reliability
System Cost
High for low tolerance (±0.1/0.2°C)
Calibration Required
Extra Components Required
±0.27°C from 0°C to 100°C (DIN Class 1/3 B);
±0.2°C from −10°C to +85°C, ±0.25°C from
excludes data conversion, signal conditioning, self- −20°C to +105°C
heating, lead wire resistance, noise, and so on
Medium to high
Medium to fast
Medium to high
For thermistors and RTDs, actual tolerances degrade in the assembled system.
CS 4
Figure 1. ADT7320 Architecture Diagram | 3
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