Cirrus Logic CS4297A - Manual (Page 36)

CS4297A
36
6.1.3 Microphone Inputs
Figure 13 illustrates an input circuit suitable for dy-
namic and electret microphones. Electret, or phan-
tom-powered, microphones use the right channel
(ring) of the jack for power. The design also sup-
ports the recommended advanced frequency re-
sponse for voice recognition as specified in PC 99.
Note the microphone input to the CS4297A has an
integrated pre-amplifier. Using the 20dB bit in the
Microphone Volume Register (Index 0Eh) the
pre-amplifier gain can be set to 0 dB or 20 dB.
Figure 14 shows an external pre-amplifier circuit
for an additional 18 dB gain.
6.1.4 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 CS4297A 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 15 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.
0.33 µF
220 pF
NPO
3.3 µF
+
6.8 K Ω
47 k Ω
MC33078 or
MC33178
Vrefout
MIC1
or
MIC2
1 µF +
0.33 µF
X7R
2 k Ω
47 k Ω
0.1 µF
47 Ω
100 pF
NPO
Figure 13. Microphone Input
AGND
4
8
+5 VA
100
1
2
5
3
4
2.7
CGND
220 pF 220 pF
68
AGND
0.068 µF
X7R
AGND
+
-
AGND
4
1
8
+5 VA
U1A
MC33078D
3
2
10 µF
AGND
+
47
AGND
+5 VA
47
+
-
6.8
10 µF
AGND
+
6
5
220 pF
47 k Ω
U1B
MC33078D
1 µF
X7R
7
MIC1/MIC2
k Ω
k Ω
k Ω
k Ω
k Ω
k Ω
47 k Ω
Figure 14. Microphone Pre-amplifier
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 15. PC_BEEP Input
36 DS318PP6
CS4297A