10.1 Layout Guidelines

Remote temperature sensing on the TMP468 device measures very small voltages using very low currents; therefore, noise at the device inputs must be minimized. Most applications using the TMP468 device have high digital content, with several clocks and a multitude of logic-level transitions that create a noisy environment. Layout must adhere to the following guidelines:

1. Place the TMP468 device 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 26. If a multilayer PCB is used, bury these traces between the ground or V+ planes to shield them from extrinsic noise sources. TI recommends 5-mil (0.127 mm) PCB traces.
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 0.1-μF local bypass capacitor directly between the V+ and GND of the TMP468. For optimum measurement performance, minimize filter capacitance between D+ and D– to 1000 pF or less. This capacitance includes any cable capacitance between the remote temperature sensor and the TMP468.
5. If the connection between the remote temperature sensor and the TMP468 is wired and is less than eight inches (20.32 cm) long, use a twisted-wire pair connection. For lengths greater than eight inches, use a twisted, shielded pair with the shield grounded as close to the TMP468 device 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 TMP468 device to avoid temperature offset readings as a result of leakage paths between D+ and GND, or between D+ and V+.

NOTE: Use a minimum of 5-mil (0.127 mm) traces with 5-mil spacing.

Figure 26. Suggested PCB Layer Cross-Section

10.2 Layout Example

Figure 27. TMP468 YFF Package Layout Example

Figure 28. TMP468 RGT Package Layout Example