CS4297
CrystalClear™ SoundFusion™ Audio Codec ’97
The inputs to the output mixer are: the input mixer
output, the PC Beep mono input, and the Phone
mono input.
Microphone Level Inputs
The microphone level inputs, MIC1 and MIC2, in-
clude a selectable -34.5 dB to +12 dB gain stage for
interfacing to an external microphone. An addition-
al 20 dB gain block is also available. Figure 6 illus-
trates a single-ended microphone input buffer
circuit that will support lower gain mics. The cir-
cuit in Figure 6 supports dynamic mics and phan-
tom-powered mics that use the right channel (ring)
of the jack for power.
All analog inputs to the CS4297, including
CD_GND, should be capacitively coupled to the
input pins.
Since many analog levels can be as large as
2 V
, the circuit shown in Figure 4 can be used
RMS
to attenuate the analog input by 6 dB (to 1 V
RMS)
which is the maximum voltage allowed for all the
stereo line-level inputs: LINE_IN, AUX_IN, and
VIDEO_IN.
2 k
Ω
6.8 k
Ω
1.0
µ
F
Vrefout
0.1
F
µ
+
R
L
47 k
Ω
1 F
µ
MC33078 or
MC33178
1.0
µ
F
0.33
F
µ
47
Ω
0.33
F
µ
6.8 k
Ω
6.8 k
X7R
MIC1
or
MIC2
6.8 k
Ω
Ω
100 pF
NPO
47 k
Ω
Figure 4. Line Inputs
220 pF
NPO
6.8 K
Ω
The CD line-level inputs have an extra pin,
CD_GND, which provides a pseudo-differential
input for both CD_L and CD_R. This pin takes the
common-mode noise out of the CD inputs when
connected to the ground coming from the CD ana-
log source. Connecting the CD pins as shown in
Figure 5 provides extra attenuation of common
mode noise coming from the CDROM drive, there-
by producing a higher quality signal. One percent
resistors are recommended since the better the re-
sistors match, the better the common-mode attenu-
ation of unwanted signals. If CD is not used, the
inputs should be connected through AC capacitors
to analog ground or connected to Vrefout.
+
3.3
F
µ
Figure 6. Microphone Input
Mono Inputs
The mono input, PC_BEEP, is useful for mixing
the output of the “beeper” (timer chip), provided in
all PCs, with the rest of the audio signals. The at-
tenuation control allows 16 levels in -3 dB steps. In
addition, a mute control is provided. The attenuator
is a single channel block with the resulting signal
sent to the output mixer where it is mixed with the
left and right outputs. Figure 7 illustrates a typical
input circuit for the PC_BEEP input. If PC_BEEP
is driven from a CMOS gate, the 4.7 kΩ should be
tied to AGND instead of VA+. Although this input
is described for a low-quality beeper, the input is of
the same high-quality as all other analog inputs and
may be used for other purposes.
(All resistors 1%)
6.8 k
Ω
1.0 F
µ
CD_L
CD_GND
CD_R
3.4 k
Ω
2.0 F
µ
6.8 k
Ω
1.0 F
µ
6.8 k
6.8 k
Ω
Ω
3.4 k
Ω
The mono input, PHONE_IN, can be used to inter-
face to the output of a MODEM Analog Front End
Figure 5. Differential CDROM In
30
DS242F5
CIRRUS [ CIRRUS LOGIC ]
