11.1 Layout Guidelines

Remote temperature sensing on the TMP461 device measures very small voltages using very low currents; therefore, noise at the device inputs must be minimized. Most applications using the TMP461 have high digital content, with several clocks and logic-level transitions that create a noisy environment. Layout must adhere to the following guidelines:

1. Place the TMP461 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 21. If a multilayer PCB is used, bury these traces between the 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 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 TMP461 device. 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 TMP461 device.
5. If the connection between the remote temperature sensor and the TMP461 device is less than 8-in (20.32 cm) long, use a twisted-wire pair connection. For lengths greater than 8 in, use a twisted, shielded pair with the shield grounded as close to the TMP461 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 TMP461 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 21. Suggested PCB Layer Cross-Section

11.2 Layout Example

Figure 22. TMP461 Layout Example