ADT7461
http://onsemi.com
17
Transistors, such as the 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 internal temperature sensor being
at the same temperature as the environment being measured;
many 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
sensor’s mass causes a lag in the response of the sensor to a
temperature change. With a remote sensor, this should not be
a problem since it will be either a substrate transistor in the
processor or a small package device, such as the SOT-23,
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. The thermal time constant
of the SOIC-8 package in still air is about 140 seconds, and
if the ambient air temperature quickly changed by
100 degrees, it would take about 12 minutes (5 time
constants) for the junction temperature of the ADT7461 to
settle within 1 degree of this. In practice, the ADT7461
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 ADT7461 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. With the ADT7461, 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 SOIC-8 package is about 121°C/W.
Layout Considerations
Digital boards can be electrically noisy environments, and
the ADT7461 is measuring very small voltages from the
remote sensor, so care must be taken to minimize noise
induced at the sensor inputs. The following precautions
should be taken:
1. Place the ADT7461 as close as possible to the
remote sensing diode. Provided the worst noise
sources, such as 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 pick-up, 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°C corresponds to about 200 mV, and
thermocouple voltages are about 3 mV/°C 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 may be placed across D+ and D− close to
the ADT7461. This capacitance can effect the
temperature measurement, so care must be taken to
ensure 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.
5. If the distance to the remote sensor is more than
8 inches, the use of twisted pair cable is
recommended. This works up to about 6 to 12 feet.
For extremely long distances (up to 100 feet), use
a shielded twisted pair, such as the Belden No.
8451 microphone cable. Connect the twisted pair
to D+ and D− and the shield to GND close to the
ADT7461. 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 may be reduced or removed.
Application Circuit
Figure 24 shows a typical application circuit for the
ADT7461 using a discrete sensor transistor connected via a
shielded, twisted pair cable. The pull-ups on SCLK, SDATA,
and ALERT
are required only if they are not already provided
elsewhere in the system.
The SCLK and SDATA pins of the ADT7461 can be
interfaced directly to the SMBus of an I/O controller, such
as the Intel
®
820 chipset.
