Datasheet

Application Information
26
June 2006 Revised August 2011SLLS690C
5. When designing filters associated with power distribution, the power supply is a low-impedance source,
and the device power terminals are a low-impedance load. The best filter for this application is a T filter.
See Figure 5−2 for a T filter circuit. Some systems may require this type of filter design if the power
supplies or nearby components are exceptionally noisy. This type of filter design is recommended if a
significant amount of low frequency noise (frequencies less than 10 MHz) is present in a system.
6. For most applications a Pi filter is adequate. See Figure 5−2 for a Pi filter circuit. When implementing a
Pi filter, the two capacitors and the inductor must be located next to each other on the circuit board and
must be connected together with wide, low impedance traces. Capacitor ground connections must be
short and low-impedance.
7. If a significant amount of high frequency noise (frequencies greater than 300 MHz) is present in a system,
creating an internal circuit board capacitor helps reduce this noise. This capacitor is accomplished by
locating power and ground planes next to each other in the circuit board stack-up. A gap of 0.003 mils
between the power and ground planes significantly reduces this high frequency noise.
8. Another option for filtering high-frequency logic noise is to create an internal board capacitor using signal
layer copper plates. When a component requires a low-noise power supply, usually the Pi filter is located
near the component. A plate capacitor may be created directly under the Pi filter. In the circuit board
stack-up, select a signal layer that is physically located next to a ground plane. Then, generate an internal
0.25 inch by 0.25 inch plate on that signal layer. Assuming a 0.006 mil gap between the signal layer plate
and the internal ground plane, this arrangement generates a 12 pF capacitor. By connecting this plate
capacitor to the trace between the Pi filter and the component’s power terminals, an internal circuit board
high frequency bypass capacitor is created. This solution is extremely effective for switching frequencies
above 300 MHz.
Figure 5−2 illustrates two different filter designs that may be used with the XIO1100 to provide low-noise power
to critical power terminals.
COMPONENT
SIDE
POWER SUPPLY
SIDE
Pi FILTER DESIGN
T FILTER DESIGN
COMPONENT
SIDE
POWER SUPPLY
SIDE
Figure 5−2. Filter Designs