Cirrus Logic CS35L00 - Manual (Page 17)

CS35L00
DS906PP1 17
Z
IN
= Input impedance of the device being used (See “Electrical Characteristics - SD Mode” on page 10 ,
“Electrical Characteristics - FSD Mode” on page 11 , “Electrical Characteristics - HD Mode” on page 12 , or
“Electrical Characteristics - FHD Mode” on page 13 for this value.)
Z
EXT
= Value of the resistor added in series with the inputs
A
V
= Original gain of the device being used (See “Electrical Characteristics - All Operational Modes” on
page 9 for this value.)
5.3 Output Filtering with the CS35L00
The CS35L00 is specifically designed to minimize radiated electromagnetic interference (EMI) signals. All
of the devices are capable of meeting all stated data sheet performance numbers with no special filtering
required. Additionally, the device has shown to be below the compliance limits of both FCC and CISPR test-
ing with no external filtering required.
Ultimately, compliance with any radiated emissions requirements depends significantly on the entire system
under test. In applications where system-level trade-offs, such as compromised component layout or
lengthy speaker wires, have increased emissions levels, a passive output filter can be added to the outputs
of the device in order to decrease EMI levels.
5.3.1 Reduced Filter Order with the CS35L00
In applications which require an output filter, the unique design of the CS35L00 allows a much smaller,
less expensive output filter to be used than what is normally found in Class D amplifiers. In contrast to a
second order filter implemented with a series inductive element (traditional inductor or ferrite beads) and
a shunt capacitive element, basic filtering for the CS35L00 is accomplished by a single-order capacitive
element attached to the OUTx terminals. This is highlighted in
Figure 5 below. Of course, if the system
requires more aggressive filtering, a ferrite bead can be added in series with the outputs to further atten-
uate system level noise.
5.3.2 Filter Component Selection
Usually, the need for output filtering is determined after the system under test has failed EMI testing. Dur-
ing this testing, problem frequencies are easily identified by the peaks which appear in the spectral plots
gathered in the EMI testing.
Selection of the filter components should ensure that shunt elements (i.e. C
FILT
in Figure 5 on page 17 )
present a very low impedance at the frequency corresponding to the tallest peak in the spectral plot. If
needed, series components such as ferrite beads (i.e. L
FILT
in Figure 5 on page 17 ) should be chosen to
present a very high impedance at the frequency corresponding to the tallest peak in the spectral plot.
Careful attention should be paid to the current carrying capabilities of any included ferrite beads and the
impedance of the ferrite beads in the audio band. A proper trade-off in ferrite bead selection is one that
C
FILT
C
FILT
Traditional 2 nd Order Optional Filtering CS35L00’s Minimized Optional Filtering
OUT+
OUT-
C
FILT
C
FILT
x
x
L
FILT
L
FILT
OUT+
OUT- x
x
Figure 5. Optional Output Filter Components