F75386
1. Place the F75386 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 Pin5 GND(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+ & Pin5 GND(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
D+
10MILS
MINIMUM
Pin5 GND(D-)
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 Pin5 GND(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+ & Pin5 GND(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 VDD pin. In very noisy environments, place an external
3300pF input filter capacitors across D+, Pin5 GND(D-) close to the F75386.
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 around 6 to 12 feet.
7. Because the measurement technique uses switched current sources, excessive cable and/or filter
capacitance will affect the measurement accuracy. When using long cables, the filter capacitor may be
reduced or removed. Cable resistance can also induce errors. 1 Ω series resistance introduces about
-18-
F75386
July, 2007
V0.27P
FINTEK [ FEATURE INTEGRATION TECHNOLOGY INC. ]
