Datasheet

Maxim Integrated │ 166
MAX98090 Ultra-Low Power Stereo Audio Codec
www.maximintegrated.com
EMI Considerations and Optional
Ferrite Bead Filter
Reducing trace length minimizes radiated EMI. On the
PCB, route SPKLP/SPKLN and SPKRP/SPKRN as differ-
ential pairs with the shortest trace lengths possible. This
minimizes trace loop area, and thereby, the inductance of
the circuit. If filter components are used on the speaker
outputs, minimize the trace length from any ground tied
passive components to SPK_GND to further minimize
radiated EMI.
In applications where speaker leads/wires are long
(exceeding approximately 12in), additional EMI suppres-
sion can be achieved by using a filter constructed from a
ferrite bead and a capacitor to ground (Figure 55). Use
a ferrite bead with low DC resistance, high frequency (>
600MHz) impedance between 100Ω and 600Ω, and rated
for at least 1A. The capacitor value varies based on the
ferrite bead chosen and the actual speaker lead length.
Select a capacitor less than 1nF with the value based
upon optimizing EMI performance.
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 and its har-
monics that is easily demodulated by audio amplifiers.
The device is designed specifically to reject RF signals;
however, PCB layout has a large impact on the suscepti-
bility of the end product.
In RF applications, improvements to both layout and
component selection decreases the susceptibility to RF
noise and prevent RF signals from being demodulated
into audible noise. Trace lengths should be kept below
1/4 of the wavelength of the RF frequency of interest.
Minimizing the trace lengths prevents them from function-
ing as antennas and coupling RF signals into the device.
The wavelength (λ) in meters is given by: λ = c/f, where c
= 3 x 10
8
m/s, and f = the RF frequency of interest.
Route audio signals on inner layers of the PCB to allow
ground planes above and below to shield them from RF
interference. Ideally the top and bottom layers of the PCB
should primarily be ground planes to create effective
shielding.
Additional RF immunity can also be obtained by rely-
ing on the self-resonant frequency of capacitors, as it
exhibits a frequency response similar to a notch filter.
Depending on the manufacturer, 10pF to 20pF capacitors
typically exhibit self-resonance at RF (high) frequencies.
These capacitors, when placed at the input pins, can
effectively shunt the RF noise at the inputs of the device.
For these capacitors to be effective, they must have a
low-impedance, low-inductance path to the ground plane.
Avoid using micro vias to connect to the ground plane as
these vias do not conduct well at RF frequencies. At the
Headphone outputs, additional RFI can be achieved by
using series ferrite beads with the parallel capacitors to
ground (Figure 56).
Figure 55. Optional Class D Ferrite Bead EMI Filter
Figure 56. Optional Class H Output RFI Filter
SPK_P
SPK_N
MAX98090
HPL
HPR
HPSNS
JACKSNS
MAX98090