ADM1023
Rev. 8 | Page 16 of 18 | www.onsemi.com
APPLICATIONS
FACTORS AFFECTING ACCURACY
Remote Sensing Diode
The ADM1023 is designed to work with substrate transistors
built into processors or with discrete transistors. Substrate
transistors are generally PNP types with the collector connected
to the substrate. Discrete types can be either PNP or NPN,
connected as a diode (base-shorted to collector). If an NPN
transistor is used, the collector and base are connected to D+
and the emitter to D−. If a PNP transistor is used, the collector
and base are connected to D− and the emitter to D+.
The user has no choice with substrate transistors, but if a
discrete transistor is used, the best accuracy is achieved
by choosing devices according to the following criteria:
• Base emitter voltage greater than 0.25 V at 6 μA, at the
highest operating temperature.
• Base emitter voltage less than 0.95 V at 100 μA, at the
lowest operating temperature.
• Base resistance less than 100 Ω.
• Small variation in h
fe
(approximately 50 to 150),
which 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 internal temperature sensor being at the same
temperature as that being measured, and a number of factors
can affect this. Ideally, the sensor should be in good thermal
contact with the part of the system being measured, such as the
processor, for example. If it is not in good thermal contact, the
thermal inertia caused by the mass of the sensor causes a lag in
the response of the sensor to a temperature change. With the
remote sensor, this should not be a problem, as it will be either
a substrate transistor in the processor or a small package device,
such as SOT-23, placed in close proximity to it.
The on-chip sensor, however, is often remote from the proces-
sor and monitors only the general ambient temperature around
the package. The thermal time constant of the QSOP-16
package is about 10 seconds.
In practice, the package has electrical, and hence thermal,
connection to the printed circuit board. Therefore, the
temperature rise due to self-heating is negligible.
LAYOUT CONSIDERATIONS
Digital boards can be electrically noisy environments, and the
ADM1023 is measuring very small voltages from the remote
sensor; therefore, care must be taken to minimize noise induced
at the sensor inputs. The following precautions are needed:
• Place the ADM1023 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 be 4 to 8 inches.
• Route the D+ and D− tracks close together, in parallel, with
grounded guard tracks on each side. Provide a ground plane
under the tracks if possible (see Figure 20).
• Use wide tracks to minimize inductance and reduce noise
pickup. 10 mil track minimum width and spacing is
recommended.
10MIL
10MIL
10MIL
10MIL
10MIL
10MIL
10MIL
GND
D+
D–
GND
00058-020
Figure 20. Arrangement of Signal Tracks
• Try to minimize the number of copper/solder joints, which
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 240 μV, and thermocouple voltages are
about 3 μV/°C of temperature difference. Unless there are
two thermocouples with a big temperature differential
between them, thermocouple voltages should be much less
than 240 μV.
• Place a 0.1 μF bypass capacitor close to the V
DD
pin and
1000 pF input filter capacitors across D+, D− close to the
ADM1023.
• If the distance to the remote sensor is more than 8 inches, the
use of twisted pair cable is recommended. This is effective
up to approximately 6 to 12 feet.
