F75383/F75384
electronically noisy environment. High frequency EMI is best filtered at D+ and D- with an external 2200pF capacitor. Too high
capacitance may introduce errors due to the rise time of the switched current source. Nearly all noise sources tested cause the ADC
measurement to be higher than the actual temperature, depending on the frequency and amplitude.
6.10 PCB Layout Guide
PCB can be electrically noisy environments, and the F75383/F75384 is measuring very small voltage from the remote sensor, so
care must be taken to minimize noise which is occurred at the sensor inputs. The following guideline should be taken to reduce the
measurement error of the temperature sensors:
1. Place the F75383/F75384 as close as practical to the remote sensing diode. In noisy environments, such as a computer
main-board, the distance can be 4 to 8 inches. (typ). This length can be increased if the worst noise sources are avoided. Noise
sources generally include clock generators, CRTs, memory buses and PCI/ISA bus etc.
2. Route the D+ and D- tracks close together, in parallel, with grounded guard tracks on each side. Provide a ground plane under
the tracks if possible. Do not route D+ & D- lines next to the deflection coil of the CRT. And also don’t route the trace across fast
digital signals which can easily induce bigger error.
GND
10MILS
DXP
10MILS
MINIMUM
DXN
10MILS
10MILS
GND
3. Use wide tracks to minimize inductance and reduce noise pickup. 10 mil track minimum width and spacing is recommended.
4. Try to minimize the number of copper/solder joints, which can cause thermocouple effects. Where copper/solder joints are used,
make sure that they are in both the D+ and D- path and at the same temperature. Thermocouple effects should not be a major
problem as 1℃corresponds to about 200µV. It means that a copper-solder thermocouple exhibits 3µV/℃, and takes about
200µV of the voltage error at D+ & D- to cause a 1℃ measurement error. Adding a few thermocouples causes a negligible
error.
5. Place a 0.1µF bypass capacitor close to the VCC pin. In very noisy environments, place an external 2200pF input filter
capacitors across D+, D- close to the F75383/F75384.
6. If the distance to the remote sensor is more than 8 inches, the use of twisted pair cable is recommended. It will work up to
F75383/F75384
-8-
July, 2007
V0.32P
FINTEK [ FEATURE INTEGRATION TECHNOLOGY INC. ]
