|ADM1022ARQ-REEL7(2003)||Low-Cost PC Temperature Monitor and Fan Control ASIC|
|ADM1022ARQ-REEL7 Datasheet PDF : 21 Pages |
Figure 3 shows the input signal conditioning used to measure
the output of an external temperature sensor. This ﬁgure
shows the external sensor as a substrate transistor, provided
for temperature monitoring on some microprocessors, but it
could equally well be a discrete transistor.
N ؋ I IBIAS
fC = 65kHz
Digital boards can be electrically noisy environments, and care
must be taken to protect the analog inputs from noise, particu-
larly when measuring the very small voltages from a remote
diode sensor. The following precautions should be taken:
1. Place the ADM1022 as close as possible to the remote sens-
ing diode. Provided that the worst noise sources such as
clock generators, data/address buses and CRTs are avoided,
this distance can be four to eight 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
Figure 3. Signal Conditioning
If a discrete transistor is used, the collector will not be grounded,
and should be linked to the base. If a PNP transistor is used the
base is connected to the D– input and the emitter to the D+ input.
If an NPN transistor is used, the emitter is connected to the D–
input and the base to the D+ input.
Table II. Temperature Data Format
To prevent ground noise interfering with the measurement, the
more negative terminal of the sensor is not referenced to ground,
but is biased above ground by an internal diode at the D– input.
If the sensor is used in a very noisy environment, a capacitor of
value up to 1000 pF may be placed between the D+ and D–
inputs to ﬁlter the noise.
To measure DVBE, 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, thence to a chopper-
stabilized ampliﬁer that performs the functions of ampliﬁcation
and rectiﬁcation of the waveform to produce a dc voltage pro-
portional to DVBE. This voltage is measured by the ADC to give
a temperature output in 8-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 takes nominally 9.6 ms.
Figure 4. 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 mV, and thermocouple voltages are
about 3 mV/oC of temperature difference. Unless there are
two thermocouples with a big temperature differential between
them, thermocouple voltages should be much less than 200 mV.
5. Place 0.1 mF bypass and 1000 pF input ﬁlter capacitors close
to the ADM1022.
6. If the distance to the remote sensor is more than eight inches,
the use of twisted pair cable is recommended. This will work
up to about 6 to 12 feet.
7. For really long distances (up to 100 feet) use a shielded
twisted pair such as Belden #8451 microphone cable. Con-
nect the twisted pair to D+ and D– and the shield to GND
close to the ADM1022. 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
may be reduced or removed. In any case, the total shunt capaci-
tance should not exceed 1000 pF.
Cable resistance can also introduce errors. 1 W series resistance
introduces about 0.5∞C error.
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