Cirrus Logic CS4205 - Manual (Page 65)

CS4205
DS489PP4 65
12.1.3 Microphone Inputs
Figure 28 illustrates an input circuit suitable for dy-
namic and electret microphones. Electret, also
known as phantom-powered, microphones use the
right channel (ring) of the jack for power. The de-
sign also supports the recommended advanced fre-
quency response for voice recognition as specified
in PC 99 and PC 2001. The microphone input of the
CS4205 has an integrated pre-amplifier. Using
combinations of the 10dB bit in the Misc. Crystal
Control Register (Index 60) and the 20dB bit in the
Mic Volume Register (Index 0Eh) the pre-amplifier
gain can be set to 0 dB, 10 dB, 20 dB, or 30 dB.
12.1.4 PC Beep Input
The PC_BEEP input is useful for mixing the output
of the “beeper” (timer chip), provided in most PCs,
with the other audio signals. When the CS4205 is
held in reset, PC_BEEP is passed directly to the
line output. This allows the system sounds or
“beeps” to be available before the AC ’97 interface
has been activated. Figure 29 illustrates a typical
input circuit for the PC_BEEP input. If PC_BEEP
is driven from a CMOS gate, the 4.7 k
Ω resistor
should be tied to analog ground instead of +5VA.
Although this input is described for a low-quality
“beeper”, it is of the same high-quality as all other
analog inputs and may be used for other purposes.
12.1.5 Phone Input
One application of the PHONE input is to interface
to the output of a modem analog front end (AFE)
device so that modem dialing signals and protocol
negotiations may be monitored through the audio
system. Figure 30 shows a design for a modem
connection where the output is fed from the
CS4205 MONO_OUT pin through a divider. The
divider ratio shown does not attenuate the signal,
providing an output voltage of 1 V
RMS
. If a lower
output voltage is desired, the resistors can be re-
placed with appropriate values, as long as the total
load on the output is kept greater than 10 k
Ω. The
PHONE input is divided by 6 dB to accommodate
a line-level source of 2 V
RMS
.
12.2 Analog Outputs
The analog output section provides a stereo and a
mono output. The MONO_OUT, LINE_OUT_L,
and LINE_OUT_R pins require 680 pF to 1000 pF
NPO dielectric capacitors between the correspond-
ing pin and analog ground. Each analog output is
DC-biased up to the Vrefout voltage signal refer-
0.1 µ F
X7R
100 Ω
0.1 µ F
X7R
MIC1/MIC2
10 µF
ELEC
AGND
+5VA
2.2 k Ω
1.5 k Ω
AGND
AGND
+
Figure 28. Microphone Input
4.7 k Ω
PC_BEEP
+5VA (Low Noise) or
AGND if CMOS Source
PC-BEEP-BUS
47 k Ω
2.7 nF
X7R
0.1 µF
X7R
AGND
Figure 29. PC_BEEP Input
PHONE
MONO_OUT
PHONE
MONO_OUT
6.8 k Ω 1.0 µF
0 Ω
6.8 k Ω
1.0 µF
47 k Ω
AGND AGND
1000 pF
Figure 30. Modem Connection