Datasheet

ADT7316/ADT7317/ADT7318
Rev. B | Page 41 of 44
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. Take the following precautions:
• P
lace the ADT7316/ADT7317/ADT7318 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 inches
to 8 inches.
• R
oute 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.
• U
se wide tracks to minimize inductance and reduce noise
pickup. A 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
02661-045
Figure 61. Arrangement of Signal Tracks
• 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−
paths and at the same temperature. Thermocouple effects
should not be a major problem as 1°C corresponds to about
240 µV, and thermocouple voltages are about 3 µV/°C of
the temperature difference. Unless there are two thermo-
couples with a big temperature differential between them,
thermocouple voltages should be much less than 200 mV.
• P
lace 0.1 µF bypass and 2200 pF input filter capacitors
close to the ADT7316/ADT7317/ADT7318.
• I
f the distance to the remote sensor is more than 8 inches,
the use of the twisted pair cable is recommended. This
works for distances from 6 feet to 12 feet.
• F
or 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 ADT7316/ADT7317/ADT7318. Leave the remote end
of the shield unconnected to avoid ground loops.
• B
ecause the measurement technique uses switched current
sources, excessive cable and/or filter capacitance can affect
the measurement. When using long cables, the filter capacitor
may be reduced or removed.
Cable resistance can also introduce errors. Series resistance
o
f 1 Ω introduces about 0.5°C error.