MAX9879 datasheet(29/32 Pages) AD

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MAX9879

Stereo Class D Audio Subsystem

with DirectDrive Headphone Amplifier

Maxim Integrated

RF Susceptibility

GSM radios transmit using time-division multiple access

(TDMA) with 217Hz intervals. The result is an RF signal

with strong amplitude modulation at 217Hz that is easily

demodulated by audio amplifiers. Figure 14 shows the

susceptibility of the MAX9879 to a transmitting GSM

radio placed in close proximity. Although there is mea-

surable noise at 217Hz and its harmonics, the noise is

well below the threshold of hearing using typical head-

phones.

In RF applications, improvements to both layout and

component selection decreases the MAX9879’s sus-

ceptibility to RF noise and prevent RF signals from

being demodulated into audible noise. Trace lengths

should be kept below 1/4 the wavelength of the RF fre-

quency of interest. Minimizing the trace lengths pre-

vents them from functioning as antennas and coupling

RF signals into the MAX9879. The wavelength

λ in

meters is given by:

λ = c/f

where c = 3 x 108 m/s, and f = the RF frequency of

interest.

Route audio signals on middle layers of the PCB to

allow ground planes above and below shield them from

RF interference. Ideally the top and bottom layers of the

PCB should primarily be ground planes to create effec-

tive shielding.

Additional RF immunity can also be obtained from rely-

ing on the self-resonant frequency of capacitors as it

exhibits the frequency response similar to a notch filter.

Depending on the manufacturer, 10pF to 20pF capaci-

tors typically exhibit self resonance at RF frequencies.

These capacitors, when placed at the input pins, can

effectively shunt the RF noise at the inputs of the

MAX9879. For these capacitors to be effective, they

must have a low-impedance, low-inductance path to

the ground plane. Do not use microvias to connect to

the ground plane as these vias do not conduct well at

RF frequencies.

UCSP Applications Information

For the latest application details on UCSP construction,

dimensions, tape carrier information, PCB techniques,

bump-pad layout, and recommended reflow tempera-

ture profile, as well as the latest information on reliability

testing results, refer to the Application Note 1891:

Understanding the Basics of the Wafer-Level Chip-

Scale Package (WL-CSP) on Maxim’s website at

www.maxim-ic.com/ucsp. See Figure 15 for the rec-

ommended PCB footprint for the MAX9879.

250

μm

±5μm

Figure 15. PCB Footprint Recommendation Diagram

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