ADM1032
A
PPLICATIONS INFORMATION
FACTORS AFFECTING ACCURACY
Remote Sensing Diode
The ADM1032 is designed to work with substrate transistors
built into processors’ CPUs or with discrete transistors. Sub-
strate transistors will generally be PNP types with the collector
connected to the substrate. Discrete types can be either PNP or
NPN transistor 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+.
Substrate transistors are found in a number of CPUs. To reduce
the error due to variations in these substrate and discrete
transistors, a number of factors should be taken into consideration:
1. The ideality factor,
n
f
, of the transistor. The ideality factor is
a measure of the deviation of the thermal diode from ideal
behavior. The ADM1032 is trimmed for an
n
f
value of 1.008.
The following equation may be used to calculate the error
introduced at a temperature T°C when using a transistor
whose
n
f
does not equal 1.008. Consult the processor
datasheet for
n
f
values.
temperature change. In the case of the remote sensor this should
not be a problem, as it will either be a substrate transistor in the
processor, or can be a small package device such as SOT-23
placed in close proximity to it.
The on-chip sensor, however, will often be remote from the
processor, and will only be monitoring the general ambient
temperature around the package. The thermal time constant of
the SO-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
ADM1032 to settle within 1 degree of this. In practice, the ADM1032
package will be in electrical, and hence thermal, contact with a printed
circuit board, and may also be in a forced airflow. How accurately
the temperature of the board and/or the forced airflow reflect the
temperature to be measured will also affect the accuracy.
Self-heating due to the power dissipated in the ADM1032 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 ADM1032, the worst-case condition occurs when
the device is converting at 16 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 11 mW. The thermal resistance,
θ
JA
, of the SO-8 package
is about 121°C/W.
In practice, the package will have electrical and hence thermal
connection to the printed circuit board, so the temperature rise
due to self-heating will be negligible.
LAYOUT CONSIDERATIONS
∆T =
(
n
natural
– 1.008
×
(
273.15
Kelvin
+
T
)
1.008
)
This value can be written to the offset register and is automati-
cally added to or subtracted from the temperature measurement.
2. Some CPU manufacturers specify the high and low current
levels of the substrate transistors. The high current level of
the ADM1032, I
HIGH
, is 230 A and the low level current,
I
LOW
, is 13 A. If the ADM1032 current levels do not match
the levels of the CPU manufacturers, then it may become
necessary to remove an offset. The CPU’s datasheet will
advise whether this offset needs to be removed and how to
calculate it. This offset may be programmed to the offset
register. It is important to note that if accounting for two or
more offsets is needed, then the algebraic sum of these offsets
must be programmed to the Offset Register.
If a discrete transistor is being used with the ADM1032 the best
accuracy will be 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
Ω.
•
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
Digital boards can be electrically noisy environments, and the
ADM1032 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 ADM1032 as close as possible to the remote sensing
diode. Provided that the worst noise sources, i.e., clock gen-
erators, data/address buses, and CRTs, are avoided, this distance
can be 4 to 8 inches.
2. 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.
3. Use wide tracks to minimize inductance and reduce noise
pickup. 10 mil track minimum width and spacing is
recommended.
GND
10MIL
10MIL
D+
10MIL
10MIL
D–
10MIL
10MIL
GND
10MIL
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, for example
the processor. If it is not, the thermal inertia caused by the mass
of the sensor will cause a lag in the response of the sensor to a
REV. 0
Figure 6. Arrangement of Signal Tracks
4. 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.
–11–
AD [ ANALOG DEVICES ]
