ADT7482
http://onsemi.com
18
This offset can 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 ADT7482,
the best accuracy is obtained by choosing devices according
to the following criteria:
?SPAN class="pst ADT7482ARMZ-REEL_2296055_3"> Base-emitter voltage greater than 0.25 V at 6 mA, at the
highest operating temperature.
?SPAN class="pst ADT7482ARMZ-REEL_2296055_3"> Base-emitter voltage less than 0.95 V at 100 mA, at the
lowest operating temperature.
?SPAN class="pst ADT7482ARMZ-REEL_2296055_3"> Base resistance less than 100 W.
?SPAN class="pst ADT7482ARMZ-REEL_2296055_3"> Small variation in h
FE
(such as 50 to 150) that indicates
tight control of V
BE
characteristics.
Transistors, such as 2N3904, 2N3906, or equivalents in
SOT23 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.
If it is not, the thermal inertia caused by the sensors 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 is either a substrate transistor in the
processor or a small package device, such as SOT23,
placed in close proximity to it.
The on-chip sensor, however, is often remote from the
processor and only monitors the general ambient
temperature around the package. In practice, the ADT7482
package is 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 also affects the accuracy.
Self-heating due to the power dissipated in the ADT7482 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. In the case of the ADT7482, the worst-case
condition occurs when the device 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 MSOP10
package is about 142癈/W.
Layout Considerations
Digital boards can be electrically noisy environments, and
the ADT7482 is measuring very small voltages from the
remote sensor, so care must be taken to minimize noise
induced at the sensor inputs. Take the following precautions:
1. Place the ADT7482 as close as possible to the
remote sensing diode. Provided that the worst
noise sources, that is, clock generators,
data/address buses, and CRTs, are avoided, this
distance can be 4 inches to 8 inches.
2. Route the D+ and D tracks close together, in
parallel, with grounded guard tracks on each side.
To minimize inductance and reduce noise pickup,
a 5 mil track width and spacing is recommended.
Provide a ground plane under the tracks, if
possible.
Figure 23. Typical Arrangement of Signal Tracks
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
GND
D
D+
GND
3. 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癈 corresponds to about 200 mV, and
thermocouple voltages are about 3 mV/癈 of
temperature difference. Unless there are two
thermocouples with a big temperature differential
between them, thermocouple voltages should be
much less than 200 mV.
4. Place a 0.1 mF bypass capacitor close to the V
DD
pin. In extremely noisy environments, an input
filter capacitor can be placed across D+ and D,
close to the ADT7482. This capacitance can effect
the temperature measurement, so care must be
taken to ensure that any capacitance seen at D+
and D is a maximum of 1000 pF. This maximum
value includes the filter capacitance, plus any
cable or stray capacitance between the pins and the
sensor diode.
5. 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 is
needed.
For 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 ADT7482. 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.
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