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CDB6420

Full-Duplex Speakerphone Chip

Cirrus Logic 

CS6420

Noise and Grounding

Since the CS6420 is a mixed-signal integrated cir-

cuit, the system designer must pay special attention

to layout and decoupling to minimize noise con-

cerns. The three best methods to reduce noise when

using the CS6420 are to have good decoupling of

power supplies, separation of analog and digital

power and ground, and careful board layout.

Figure 6 shows the suggested placement of decou-

pling capacitors for the power supplies. Note that

the trace length from the power pin to the capaci-

tors is minimized. Also note that the smaller valued

capacitor is placed closer to the pin than the larger

valued capacitor. The smaller capacitor decouples

high frequency noise and the larger capacitor atten-

uates lower frequencies.

The separation of analog and digital power and

ground is done in two ways. The power is separated

by deriving the digital power for the CS6420 from

the analog through a ferrite bead to isolate analog

from digital, as shown in Figure 7. The ferrite bead

serves as a low-pass filter to remove CS6420 digi-

tal switching noise from the analog power supply.

The ground is separated by isolating all the digital

components of the system board on one ground

plane and all the analog and linear components on

a different ground plane. The CS6420 should be

placed over the analog ground plane. This prevents

digital switching noise from the digital components

of the board from coupling into the converters and

aliasing into the passband.

+5V

(Analog)

Ferrite Bead

1µF 0.1µF

AVDD

AGND

DVDD

DGND

0.1µF 1µF

Analog Ground Plane

Digital Ground Plane

Microcontroller

Figure 7. Ground Planes

DS205PP2

21

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