Real-World Component Selection - Sharp Blue Treak LH75400 User Manual

Introduction
The power (VDDA_PLL) path must be a single wire that runs:
• From the IC package pin to the high-frequency capacitor
• Then to the low-frequency capacitor
• Then through the series element (e.g., resistor)
• Then to board power (VDDC).
The distance from the IC pin to the high-frequency capacitor should be as short as possible.
Similarly, the ground (VSSA_PLL) path should run from the IC pin to the high-frequency
capacitor, then to the low-frequency capacitor. The distance from IC pin to high-frequency
capacitor should be very short.
The PLL has the DC ground connection made on chip. Therefore, the external VSSA_PLL
pin must be connected to the power supply filter only, not to PCB ground.
To minimize noise, especially non-common-mode noise, power and ground traces should
be run as closely and as parallel as possible, with large spacings to adjacent traces, for all
applications. The leads of the high-frequency capacitor must be kept short; this includes:
• Board wires
• Vias
• Capacitor wires
• Wires within the capacitor package.
Therefore, select components carefully. The area and impedance of the power loop must
be minimized, where the loop includes the high-frequency capacitor and VDDA_PLL and
VSSA_PLL board traces to the IC. The board layout should have the smallest total analog
power circuit, with short and adjacent wire traces. Extra connections should not be made
to board power planes; the only connections should be those described above.

1.5.2 Real-World Component Selection

Throughout the attenuating frequency range, there should be no resonant nonabsorptions.
This means the series element will be either a resistor or a very poor (i.e., resistive) inductor.
Using the series element with the greatest impedance possible (e.g., 100 Ω), the
electrolytic used is usually the largest capacitance tantalum that fits nicely on the board
(e.g., 25 µF). Similarly, the other capacitor is the highest value HF capacitor that can be
found in a small package (e.g., 100 nF).
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LH75400/01/10/11 (Preliminary) User's Guide
7/15/03