f75377 Feature Integration Technology Inc., f75377 Datasheet - Page 19

• Manufacturer Part Number: f75377
• Description: Monitor With Automatic Speed Control
• Manufacturer: Feature Integration Technology Inc.

6.6 PCB Layout Guide
PCB can be electrically noisy environments, and the F75377 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:
Place the F75377 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.
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.
Use wide tracks to minimize inductance and reduce noise pickup. 10 mil track minimum width and
spacing is recommended.
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
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.
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 F75377.
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.
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
0.5
MINIMUM
10MILS
10MILS
℃
er
ror.
GND
DXP
DXN
GND
19
10MILS
10MILS
℃
corre
sponds to about 200µV. It
F75377
March.
V0.12P
, 2008