TMP421-Q1 Datasheet, PDF (29/38 Pages) Texas Instruments – Remote and Local Temperature Sensor

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TMP421-Q1, TMP422-Q1, TMP423-Q1
SBOS821 – NOVEMBER 2016
10 Power Supply Recommendations
The TMP421-Q1, TMP422-Q1, and TMP423-Q1 operates with a power-supply range of 2.7 V to 5.5 V. The
device is optimized for operation at a 3.3 V supply but can measure temperature accurately in the full supply
range. TI recommends a power-supply bypass capacitor. Place this capacitor as close as possible to the supply
and ground pins of the device. A typical value for this supply bypass capacitor is 0.1 μF. Applications with noisy
or high-impedance power supplies may require additional decoupling capacitors to reject power-supply noise.
11 Layout
11.1 Layout Guidelines
Remote temperature sensing on the TMP421-Q1, TMP422-Q1, and TMP423-Q1 measures very small voltages
using very low currents; therefore, noise at the device inputs must be minimized. Most applications using the
TMP421-Q1, TMP422-Q1, and TMP423-Q1 have high digital content, with several clocks and logic level
transitions creating a noisy environment. Layout must adhere to the following guidelines:
1. Place the TMP421-Q1, TMP422-Q1, and TMP423-Q1 as close to the remote junction sensor as possible.
2. Route the DXP and DXN traces next to each other and shield them from adjacent signals through the use of
ground guard traces, as shown in Figure 23. If a multilayer PCB is used, bury these traces between ground
or V+ planes to shield them from extrinsic noise sources. 5 mil (0.127 mm) 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 DXP
and DXN connections to cancel any thermocouple effects.
4. Use a 0.1-μF local bypass capacitor directly between the V+ and GND of the TMP421-Q1, TMP422-Q1, and
TMP423-Q1; see Figure 24. Minimize filter capacitance between DXP and DXN to 1000 pF or less for
optimum measurement performance. This capacitance includes any cable capacitance between the remote
temperature sensor and the TMP421-Q1, TMP422-Q1, and TMP423-Q1.
5. If the connection between the remote temperature sensor and the TMP421-Q1, TMP422-Q1, and TMP423-
Q1 is less than 8 in (20.32 cm) long, use a twisted-wire pair connection. Beyond 8 in, use a twisted, shielded
pair with the shield grounded as close to the TMP421-Q1, TMP422-Q1, and TMP423-Q1 as possible. Leave
the remote sensor connection end of the shield wire open to avoid ground loops and 60-Hz pickup.
6. Thoroughly clean and remove all flux residue in and around the pins of the TMP421-Q1, TMP422-Q1, and
TMP423-Q1 to avoid temperature offset readings as a result of leakage paths between DXP or DXN and
GND, or between DXP or DXN and V+.
V+
DXP
DXN
Ground or V+ layer
on bottom and
top, if possible.
GND
NOTE: Use minimum 5 mil (0.127mm) traces with 5 mil spacing.
Figure 23. Suggested PCB Layer Cross-Section
Copyright © 2016, Texas Instruments Incorporated
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Product Folder Links: TMP421-Q1 TMP422-Q1 TMP423-Q1