Datasheet

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SBOS383C − DECEMBER 2006 − REVISED MAY 2008
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23
LAYOUT CONSIDERATIONS
Remote temperature sensing on the TMP411 measures
very small voltages using very low currents; therefore,
noise at the IC inputs must be minimized. Most
applications using the TMP411 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 TMP411 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 19. 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 TMP411, as shown in
Figure 20. 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 TMP411.
5. If the connection between the remote temperature
sensor and the TMP411 is less than 8 inches, use a
twisted-wire pair connection. Beyond 8 inches, use a
twisted, shielded pair with the shield grounded as
close to the TMP411 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 19. Example Signal Traces
1
2
3
4
8
7
6
5
TMP411
0.1
µ
F Capacitor
PCB Via
PCB Via
V+
GND
Figure 20. Suggested Bypass Capacitor
Placement