ADM1032
Layout Considerations
microphone cable. Connect the twisted pair to D+
and D− and the shield to GND close to the
ADM1032. Leave the remote end of the shield
unconnected to avoid ground loops.
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.
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 can be reduced or removed.
Cable resistance can also introduce errors. 1 W series
resistance introduces about 1°C error.
1. Place the ADM1032 as close as possible to the
remote sensing diode. Provided that the worst
noise sources, that is, 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.
Power Sequencing Considerations
Power Supply Slew Rate
When powering up the ADM1032 you must ensure that
the slew rate of V is less than 18 mV/ms. A slew rate larger
DD
3. Use wide tracks to minimize inductance and
reduce noise pickup. 10 mil track minimum width
and spacing is recommended.
than this may cause power−on−reset issues and yield
unpredictable results.
THERM Pin Pullup
As mentioned above, the THERM signal is open drain and
10MIL
10MIL
GND
D+
requires a pullup to V . The THERM signal must always
DD
be pulled up to the same power supply as the ADM1032,
unlike the SMBus signals (SDA, SCL and ALERT) that can
be pulled to a different power rail. The only time the
THERM pin can be pulled to a different supply rail (other
10MIL
10MIL
10MIL
10MIL
10MIL
than V ) is if the other supply is powered up simultaneous
DD
D–
with, or after the ADM1032 main V . This is to protect the
DD
internal circuitry of the ADM1032. If the THERM pullup
supply rail were to rise before V , the POR circuitry may
DD
GND
not operate correctly.
Application Circuit
Figure 18. Typical Arrangement of Signal Tracks
Figure 20 shows a typical application circuit for the
ADM1032, using a discrete sensor transistor connected via
a shielded, twisted pair cable. The pullups 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 ADM1032 can be
interfaced directly to the SMBus of an I/O controller, such
as the Intel 820 chipset.
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 since 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.
0.1m F
V
3V TO 3.6V
DD
ADM1032
TYP 10kW
D+
SCLK
SDATA
ALERT
D–
SMBUS
CONTROLLER
5. Place a 0.1 mF bypass capacitor close to the V
2N3906 SHIELD
OR
CPU THERMAL
DIODE
DD
pin. In very noisy environments, place a 1000 pF
input filter capacitor across D+ and D− close to the
ADM1032.
V
DD
5V OR 12V
THERM
TYP 10kW
GND
FAN
6. If the distance to the remote sensor is more than
eight inches, the use of twisted pair cable is
recommended. This works up to about six feet to
12 feet.
CONTROL
CIRCUIT
FAN
ENABLE
Figure 19. Typical Application Circuit
7. For really long distances (up to 100 feet), use
shielded twisted pair, such as Belden #8451
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