Remote temperature sensing on the
TMP9R00-SP device measures very small voltages using very low currents; therefore,
noise at the device inputs must be minimized. Most applications using the TMP9R00-SP
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:
- Place the TMP9R00-SP device as
close to the remote junction sensor as possible.
- Route the D+ and D– traces next
to each other and shield them from adjacent signals through the use of ground
guard traces (see Figure 8-5). 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.
- 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.
- Use a 0.1-μF local bypass
capacitor directly between the V+ and GND of the TMP9R00-SP. 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 TMP9R00-SP.
- If the connection between the
remote temperature sensor and the TMP9R00-SP 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 TMP9R00-SP device 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 device to avoid temperature offset
readings as a result of leakage paths between D+ and GND, or between D+ and
V+.