ADM1032
Layout Considerations
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, 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.
3. Use wide tracks to minimize inductance and
reduce noise pickup. 10 mil track minimum width
and spacing is recommended.
GND
D+
D−
GND
10 MIL
10 MIL
10 MIL
10 MIL
10 MIL
10 MIL
10 MIL
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 6 feet to
12 feet.
7. For really long distances (up to 100 feet), use
shielded twisted pair, such as Belden #8451
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.
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 1C error.
Power Sequencing Considerations
Power Supply Slew Rate
When powering up the ADM1032 you must ensure that
the slew rate of V
DD
is less than 18 mV/ms. A slew rate larger
than this may cause power-on-reset issues and yield
unpredictable results.
THERM Pin Pullup
Figure 18. Typical 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 since 1C 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.
5. Place a 0.1
mF
bypass capacitor close to the V
DD
pin. In very noisy environments, place a 1000 pF
input filter capacitor across D+ and D− close to the
ADM1032.
As mentioned above, the THERM signal is open drain and
requires a pullup to V
DD
. The THERM signal must always
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
than V
DD
) is if the other supply is powered up simultaneous
with, or after the ADM1032 main V
DD
. This is to protect the
internal circuitry of the ADM1032. If the THERM pullup
supply rail were to rise before V
DD
, the POR circuitry may
not operate correctly.
Application Circuit
Figure 19 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.
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