Datasheet

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SBOS371A − AUGUST 2006 − REVISED OCTOBER 2007
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21
LAYOUT CONSIDERATIONS
Remote temperature sensing on the TMP401 measures
very small voltages using very small currents; therefore,
noise at the IC inputs must be minimized. Most
applications using the TMP401 will have high digital
content, with several clocks and logic level transitions
creating a noisy environment. Layout should adhere to the
following guidelines:
1. Place the TMP401 as close to the remote junction
sensor as possible.
2. Route the D+ and D− traces next to each other and
shield them from adjacent signals through the use
of ground guard traces, as shown in Figure 18. If a
multilayer PCB is used, bury these traces between
ground or V
DD
planes to shield them from extrinsic
noise sources. 5 mil PCB traces are recommended.
3. Minimize additional thermocouple junctions caused
by copper-to-solder connections. If these junctions
are used, make the same number and approximate
locations of copper-to-solder connections in both
the D+ and D− connections to cancel any
thermocouple effects.
4. Use a 0.1µF local bypass capacitor directly
between the V+ and GND of the TMP401, as shown
in Figure 19. Minimize filter capacitance between
D+ and D− to 1000pF or less for optimum
measurement performance. This capacitance
includes any cable capacitance between the
remote temperature sensor and TMP401.
5. If the connection between the remote temperature
sensor and the TMP401 is between 8 inches and 12
feet, use a twisted-wire pair connection. Beyond
this distance (up to 100ft), use a twisted, shielded
pair with the shield grounded as close to the
TMP401 as possible. Leave the remote sensor
connection end of the shield wire open to avoid
ground loops and 60Hz pickup.
GND
(1)
D+
(1)
D
−
(1)
GND
(1)
NOTE: (1) 5 mil traces with 5 mil spacing.
Ground or V+ layer
on bottom and/or
top, if possible.
Figure 18. Example Signal Traces
1
2
3
4
8
7
6
5
TMP401
0.1
µ
F Capacitor
PCB Via
PCB Via
V+
GND
Figure 19. Suggested Bypass Capacitor
Placement