Cirrus Logic CS8420 - Manual (Page 88)

88 DS245F4
CS8420
16.2.2 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
C
RIP
, a C0G or NPO dielectric is recommended, and for C
FILT
, 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.
16.2.3 Circuit Board Layout
Board layout and capacitor choice affect each other and determine the performance of the PLL. Figure
42 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. R
FILT
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.
16.3 Component Value Selection
When transitioning from one revision of the part another, component values need to be changed. It is man-
datory for customers to change the external PLL component values when transitioning from revision D to
revision D1.
16.3.1 Identifying the Part Revision
The first line of the part marking on the package indicates the part number and package type
CS8420-xx.
Table 17 shows a list of part revisions and their corresponding second line part marking,
which indicates what revision the part is.
Table 17. Second Line Part Marking
Revision Pre-October 2002 (10-Digit) New (12-Digit)
D Zxxxxxxxxx ZFBADXxxxxxx
D1 Rxxxxxxxxx RFBAD1xxxxxx
VA+ AGND FILT
Cfilt
1000
pF
.1µF
R
fil
t
Crip
Figure 42. Recommended Layout Example