ADT7481
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18
calculate it. This offset may be programmed to the
offset register. It is important to note that if more than
one offset must be considered, the algebraic sum of
these offsets must be programmed to the offset register.
If a discrete transistor is being used with the ADT7481, the
best accuracy is obtained by choosing devices according to
the following criteria:
• Base−emitter voltage greater than 0.25 V at 6 mA, at the
highest operating temperature.
• Base−emitter voltage less than 0.95 V at 100 mA, at the
lowest operating temperature.
• Base resistance less than 100 W.
• Small variation in h
FE
(say 50 to 150) that indicates
tight control of V
BE
characteristics.
Transistors, such as 2N3904, 2N3906, or equivalents in
SOT−23 packages, are suitable devices to use.
Thermal Inertia and Self−Heating
Accuracy depends on the temperature of the remote
sensing diode and/or the local temperature sensor being at
the same temperature as that being measured. A number of
factors can affect this. Ideally, the sensor should be in good
thermal contact with the part of the system being measured;
otherwise, the thermal inertia caused by the sensor’s mass
causes a lag in the response of the sensor to a temperature
change.
In the case of the remote sensor, this should not be a
problem, since it will either be a substrate transistor in the
processor or a small package device, such as an SOT−23,
placed in close proximity to it.
The on−chip sensor, however, will often be remote from
the processor and only monitors the general ambient
temperature around the package. In practice, the ADT7481
package will be in electrical, and hence, thermal contact with
a PCB and may also be in a forced airflow. How accurately
the temperature of the board and/or the forced airflow
reflects the temperature to be measured will also affect the
accuracy of the measurement. Self−heating, due to the
power dissipated in the ADT7481 or the remote sensor,
causes the chip temperature of the device (or remote sensor)
to rise above ambient. However, the current forced through
the remote sensor is so small that self−heating is negligible.
The worst−case condition occurs when the ADT7481 is
converting at 64 conversions per second while sinking the
maximum current of 1 mA at the ALERT
and THERM
output. In this case, the total power dissipation in the device
is about 4.5 mW. The thermal resistance, q
JA
, of the
MSOP−10 package is about 142°C/W.
Layout Considerations
Digital boards can be electrically noisy environments, and
the ADT7481 measures very small voltages from the remote
sensor, so care must be taken to minimize noise induced at
the sensor inputs. Take the following precautions:
• Place the ADT7481 as close as possible to the remote
sensing diode. Provided that the worst noise sources
such as clock generators, data/address buses, and CRTs
are avoided, this distance can range from 4 to 8 inches.
• Route the D+ and D− tracks close together, in parallel,
with grounded guard tracks on each side. To minimize
inductance and reduce noise pick up, a 5 mil track
width and spacing is recommended. Provide a ground
plane under the tracks if possible.
Figure 21. Typical Arrangement of Signal Tracks
GND
D+
D–
GND
5MIL
5MIL
5MIL
5MIL
5MIL
5MIL
5MIL
• Try to minimize the number of copper/solder joints that
can cause thermocouple effects. Where copper/solder
joints are used, make sure that they are in both the D+
and D− path and at the same temperature.
• Thermocouple effects should not be a major problem as
1°C corresponds to about 200 mV, and thermocouple
voltages are about 3 mV/°C of temperature difference.
• Unless there are two thermocouples with a large
temperature differential between them, thermocouple
voltages should be much less than 200 mV.
• Place a 0.1 mF bypass capacitor close to the V
DD
pin. In
extremely noisy environments, an input filter capacitor
may be placed across D+ and D− close to the
ADT7481. This capacitance can affect the temperature
measurement, so care must be taken to ensure that any
capacitance seen at D+ and D− is a maximum of 1,000
pF. This maximum value includes the filter capacitance,
plus any cable or stray capacitance between the pins
and the sensor diode.
• If the distance to the remote sensor is more than 8
inches, the use of twisted pair cable is recommended. A
total of 6 feet to 12 feet of cable is needed.
• For really long distances (up to 100 feet), use shielded
twisted pair, such as Belden No. 8451 microphone
cable. Connect the twisted pair to D+ and D− and the
shield to GND close to the ADT7481. Leave the remote
end of the shield unconnected to avoid ground loops.
Because the measurement technique uses switched
current sources, excessive cable or filter capacitance can
affect the measurement. When using long cables, the filter
capacitance can be reduced or removed.
