11.1 Layout Guidelines
Remote temperature sensing on the TMP43x measures very small voltages using very low currents; therefore, noise at the IC inputs must be minimized. Most applications using the TMP43x have high digital content, with several clocks and logic level transitions creating a noisy environment. Layout must conform to the following guidelines:
- Place the TMP43x as close to the remote junction sensor as possible.
- Route the DXP and DXN traces next to each other and shield them from adjacent signals through the use of ground guard traces; see Figure 25. If a multilayer PCB is used, bury these traces between ground or VDD planes to shield them from extrinsic noise sources. TI recommends 5 mil (0.127 mm) PCB traces.
- 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.
- Use a 0.1-μF local bypass capacitor directly between the V+ and GND of the TMP43x. Figure 26 illustrates the suggested bypass capacitor placement for the TMP43x. This capacitance includes any cable capacitance between the remote temperature sensor and TMP43x.
- If the connection between the remote temperature sensor and the TMP43x is less than 8 inches
(20.32 cm), use a twisted-wire pair connection. Beyond 8 inches, use a twisted, shielded pair with the shield grounded as close to the TMP43x as possible. Leave the remote sensor connection end of the shield wire open to avoid ground loops and 60-Hz pickup.
- Thoroughly clean and remove all flux residue in and around the pins of the TMP43x to avoid temperature offset readings as a result of leakage paths between DXP or DXN and GND, or between DXP or DXN and V+.