CS4297A
CS4297A
6.1.3 Microphone Inputs
6.1.4 PC Beep Input
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.
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 kΩ 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.
2 k
Ω
Vrefout
µ
0.1 F
+
47 k
Ω
µ
1 F
MC33078 or
MC33178
+5VA (Low Noise) or
0.33
µ
F
AGND if CMOS Source
47
Ω
0.33
X7R
µ
F
MIC1
or
MIC2
100 pF
NPO
4.7 k
Ω
47 k
Ω
47 k
Ω
PC_BEEP
PC-BEEP-BUS
220 pF
NPO
0.1 µF
2.7 nF
X7R
6.8 K
Ω
+
X7R
µ
3.3 F
AGND
Figure 13. Microphone Input
Figure 15. PC_BEEP Input
+5 VA
+5 VA
U1A
MC33078D
kΩ
47
8
+
3
2
1
-
+
4
10 µF
68 kΩ
47 k
Ω
AGND
AGND
AGND
100 k
Ω
47 kΩ
2.7 k
Ω
4
3
5
2
AGND
0.068 µF
X7R
+5 VA
8
U1B
MC33078D
1
5
6
+
1 µF
7
MIC1/MIC2
220 pF 220 pF
-
X7R
kΩ
6.8
4
+
10 µF
AGND
AGND
220 pF
CGND
AGND
47 kΩ
Figure 14. Microphone Pre-amplifier
36
36
DS318PP6
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