CS8427-CSZ datasheet(56/60 Pages) CIRRUS

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CS8427

DS477F5

20.2 External Filter Components

20.2.1 General

The PLL behavior is affected by the external filter

component values. Figure 5 on page 11 shows the

recommended configuration of the two capacitors

and one resistor that comprise the PLL filter. In

Table 7 and Table 8, the component values shown

for the 32 to 96 kHz range have the highest corner

frequency jitter attenuation curve, takes the short-

est time to lock, and offers the best output jitter per-

formance. The component values shown in

Table 6 and Table 8 for the 8 to 96 kHz range al-

lows 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.

20.2.2 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 capacitors with

large temperature coefficients, or capacitors with

high dielectric constants, that are sensitive to

shock and vibration. These include the Z5U and

Y5V dielectrics.

20.2.3 Circuit Board Layout

Board layout and capacitor choice affect each

other and determine the performance of the PLL.

Figure 31 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.

Cfilt

1000

.1µF

Crip

Figure 31. Recommended Layout Example

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