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ADM1031
environment, a capacitor of value up to 1000 pF can be
placed between the D+ and D– inputs to filter the noise.
To measure D V BE , the sensor is switched between
operating currents of I and N ? I. The resulting waveform is
passed through a 65 kHz low-pass filter to remove noise,
then to a chopper-stabilized amplifier that performs the
functions of amplification and rectification of the waveform
to produce a dc voltage proportional to D V BE . This voltage
is measured by the ADC to give a temperature output in
11-bit twos complement format. To further reduce the
effects of noise, digital filtering is performed by averaging
the results of 16 measurement cycles. An external
temperature measurement nominally takes 9.6 ms.
Layout Considerations
Digital boards can be electrically noisy environments and
care must be taken to protect the analog inputs from noise,
particularly when measuring the very small voltages from a
remote diode sensor. The following precautions should be
taken:
1. Place the ADM1031 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.
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. Ten mil track minimum
width and spacing is recommended.
6. 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.
7. For extra long distances (up to 100 feet), use a
shielded twisted pair cable, such as the Belden #8451
microphone cable. Connect the twisted pair to D+
and D– and the shield to GND close to the
ADM1031. Leave the remote end of the shield
unconnected to avoid ground loops.
Because the measurement technique uses switched
current sources, excessive cable and/or filter capacitance
can affect the measurement. When using long cables, the
filter capacitor C1 can be reduced or removed. In any case
the total shunt capacitance should not exceed 1000 pF.
Cable resistance can also introduce errors. One ohm series
resistance introduces about 0.5 ? C error.
Addressing the Device
ADD (Pin 13) is a three-state input. It is sampled, on
powerup to set the lowest two bits of the serial bus address.
Up to three addresses are available to the systems designer
via this address pin. This reduces the likelihood of conflicts
with other devices attached to the system management bus.
The Interrupt System
The ADM1031 has two interrupt outputs, INT and
THERM. These have different functions. INT responds to
violations of software programmed temperature limits and
is maskable.
THERM is intended as a “fail-safe” interrupt output that
cannot be masked. If the temperature is below the low
GND
D+
D ?
GND
10 MIL
10 MIL
10 MIL
10 MIL
10 MIL
10 MIL
10 MIL
temperature limit, the INT pin is asserted low to indicate an
out-of-limit condition. If the temperature exceeds the high
temperature limit, the INT pin is also asserted low. A third
limit, THERM limit, can be programmed into the device to
set the temperature limit above which the overtemperature
THERM pin is asserted low. The behavior of the high limit
and THERM limit is as follows:
1. Whenever the temperature measured exceeds the
high temperature limit, the INT pin is asserted low.
Figure 19. 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.
Thermocouple effects should not be a major
problem as 1 ? C corresponds to about 200 m V, and
thermocouple voltages are about 3 m V/ ? C of
temperature difference. Unless there are two
thermocouples with a big temperature differential
between them, thermocouple voltages should be
much less than 200 m V.
5. Place a 0.1 m F bypass capacitor close to the
ADM1031.
2. If the temperature exceeds the THERM limit, the
THERM output asserts low. This can be used to
throttle the CPU clock. If the THERM-to-Fan
Enable bit (Bit 7 of THERM behavior/revision
register) is cleared to 0, then the fans do not run
full-speed. The THERM limit can be programmed
at a lower temperature than the high temperature
limit. This allows the system to run in silent mode,
where the CPU can be throttled while the cooling
fan is off. If the temperature continues to increase,
and exceeds the high temperature limit, an INT is
generated. Software can then decide whether the
fan should run to cool the CPU. This allows the
system to run in silent mode.
3. If the THERM-to-Fan Enable bit is set to 1, then
the fan runs full-speed whenever THERM is
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