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参数资料
| 型号： | ADT7408CCPZ2-REEL7 |
| 厂商： | ANALOG DEVICES INC |
| 元件分类： | 温度/湿度传感器 |
| 英文描述： | DIGITAL TEMP SENSOR-SERIAL, 12BIT(s), 4Cel, SQUARE, SURFACE MOUNT |
| 封装： | 3 X 3 MM, MO-229VEED-4, LFCSP-8 |
| 文件页数： | 13/22页 |
| 文件大小： | 570K |
| 代理商： | ADT7408CCPZ2-REEL7 |
ADT7408
Preliminary Technical Data
Rev. PrC | Page 20 of 22
Application Hints
THERMAL RESPONSE TIME
The time required for a temperature sensor to settle to a
specified accuracy is a function of the thermal mass of the
sensor and the thermal conductivity between the sensor and the
object being sensed. Thermal mass is often considered
equivalent to capacitance. Thermal conductivity is commonly
specified using the symbol Q, and can be thought of as thermal
resistance. It is commonly specified in units of degrees per watt
of power transferred across the thermal joint. Thus, the time
required for the ADT7408 to settle to the desired accuracy is
dependent on the package selected, the thermal contact
established in that particular application, and the equivalent
power of the heat source. In most applications, the settling time
is probably best determined empirically.
SELF-HEATING EFFECTS
The temperature measurement accuracy of the ADT7408 might
be degraded in some applications due to self-heating. Errors can
be introduced from the quiescent dissipation and power
dissipated when converting. The magnitude of these
temperature errors is dependent on the thermal conductivity of
the ADT7408 package, the mounting technique, and the effects
of airflow. At 25°C, static dissipation in the ADT7408 is
typically 778 W operating at 3.3 V. In the 8-lead LFCSP
package mounted in free air, this accounts for a temperature
increase due to self-heating of
ΔT = PDISS × θJA = 778 W × ???°C/W = °C
It is recommended that current dissipated through the device be
kept to a minimum, because it has a proportional effect on the
temperature error.
Using the shutdown mode can reduce the current dissipated
through the ADT7408 subsequently reducing the self-heating
effect. When the ADT7408 is in shutdown mode and operating
at 25°C, static dissipation in the ADT7408 is typically 33W
with VDD = 3.3 V. In the 8-lead LFCSP package mounted in free
air, this accounts for a temperature increase due to self-heating
of
ΔT = PDISS × θJA = 33 W × ???°C/W = ???°C
SUPPLY DECOUPLING
The ADT7408 should be decoupled with a 0.1 F ceramic
capacitor between VDD and GND. This is particularly important
when the ADT7408 is mounted remotely from the power
supply. Precision analog products such as the ADT7408 require
a well-filtered power source. Because the ADT7408 operates
from a single supply, it might seem convenient to simply tap
into the digital logic power supply.
Unfortunately, the logic supply is often a switch-mode design,
which generates noise in the 20 kHz to 1 MHz range. In
addition, fast logic gates can generate glitches hundreds of mV
in amplitude due to wiring resistance and inductance.
If possible, the ADT7408 should be powered directly from the
system power supply. This arrangement, shown in Figure 4,
isolates the analog section from the logic switching transients.
Even if a separate power supply trace is not available, however,
generous supply bypassing reduces supply-line-induced errors.
Local supply bypassing consisting of a 0.1 F ceramic capacitor
is critical for the temperature accuracy specifications to be
achieved. This decoupling capacitor must be placed as close as
possible to the ADT7408 VDD pin.
Figure 4. Use Separate Traces to Reduce Power Supply Noise
TEMPERATURE MONITORING
The ADT7408 is ideal for monitoring the thermal environment
within electronic equipment. For example, the surface-mounted
package accurately reflects the exact thermal conditions that
affect nearby integrated circuits.
The ADT7408 measures and converts the temperature at the
surface of its own semiconductor chip. When the ADT7408 is
used to measure the temperature of a nearby heat source, the
thermal impedance between the heat source and the ADT7408
must be considered. Often, a thermocouple or other
temperature sensor is used to measure the temperature of the
source, while the temperature is monitored by reading back
from the ADT7408’s temperature value register.
Once the thermal impedance is determined, the temperature of
the heat source can be inferred from the ADT7408 output. As
much as 60% of the heat transferred from the heat source to the
thermal sensor on the ADT7408 die is discharged via the
copper tracks, the package pins and the bond pads. Of the pins
on the ADT7408, the GND pin transfers most of the heat.
Therefore, to measure the temperature of a heat source it is
recommended that the thermal resistance between the
ADT7408 GND pin and the GND of the heat source is reduced
as much as possible.
03340-0-013
0.1F
ADT7408
TTL/CMOS
LOGIC
CIRCUITS
POWER
SUPPLY
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相关代理商/技术参数 |
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| ADT7410 | 制造商:AD 制造商全称:Analog Devices 功能描述:±0.5°C Accurate, 16-Bit Digital I2C Temperature Sensor |
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