CS8415A-CSZ datasheet(42/46 Pages) CIRRUS

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DS470F4

CS8415A

15.2

External Filter Components

15.2.1

General

The PLL behavior is affected by the external filter component values. Figure 5 on page 11 shows the rec-

ommended configuration of the two capacitors and one resistor that comprise the PLL filter. In Table 6,

the component values shown have the highest corner frequency jitter attenuation curve, takes the short-

est time to lock, and offers the best output jitter performance. The component values shown in Table 5

allows the lowest input sample rate to be 8 kHz, and increases the lock time of the PLL. Lock times are

worst case for an Fsi transition of 96 kHz.

15.2.2

Capacitor Selection

The type of capacitors used for the PLL filter can have a significant effect on receiver performance. Large

or exotic film capacitors are not necessary as their leads and the required longer circuit board traces add

undesirable inductance to the circuit. Surface mount ceramic capacitors are a good choice because their

own inductance is low, and they can be mounted close to the FILT pin to minimize trace inductance. For

CRIP, a C0G or NPO dielectric is recommended, and for CFILT, an X7R dielectric is preferred. Avoid ca-

pacitors with large temperature coefficients, or capacitors with high dielectric constants, that are sensitive

to shock and vibration. These include the Z5U and Y5V dielectrics.

15.2.3

Circuit Board Layout

Board layout and capacitor choice affect each other and determine the performance of the PLL. Figure

19 contains a suggested layout for the PLL filter components and for bypassing the analog supply voltage.

The 0.1 µF bypass capacitor is in a 1206 form factor. RFILT and the other three capacitors are in an 0805

form factor. The traces are on the top surface of the board with the IC so that there is no via inductance.

The traces themselves are short to minimize the inductance in the filter path. The VA+ and AGND traces

extend back to their origin and are shown only in truncated form in the drawing.

FLT

1000

.1µF

RIP

Figure 19. Recommended Layout Example

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