Cirrus Logic CS4297A - Manual (Page 37)

CS4297A
37
6.1.5 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 16 shows a design for a modem
connection where the output is fed from the
CS4297A 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
.
6.2 Analog Outputs
The analog line-level output section provides two
stereo outputs and a mono output. The
LINE_OUT_L/R, ALT_LINE_OUT_L/R, and
MONO_OUT pins require 680 pF to 1000 pF NPO
capacitors between the corresponding pin and ana-
log ground. Each analog output is DC-biased up to
the Vrefout signal reference, nominally 2.3 V. This
requires the outputs be AC-coupled to external cir-
cuitry (AC load must be greater than 10 k
Ω) or DC
coupled to a buffer op-amp biased at Vrefout.
6.2.1 Stereo Outputs
See Figure 18 for a line-level stereo output refer-
ence design. See Figure 17 for a recommended
headphone stereo output reference design.
6.2.2 Mono Output
The mono output, MONO_OUT, can be either a
sum of the left and right output channels, attenuat-
ed by 6 dB to prevent clipping at full scale, or the
selected Mic signal. The mono out channel can
drive the PC internal mono speaker using an appro-
priate buffer circuit
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 16. Modem Connection
ALT_LINE_OUT_R
ALT_LINE_OUT_L
27 k Ω
1000 pF
NPO
AGND
3
2
1
220 µF
ELEC
220 µF
ELEC
+
+
1000 pF
NPO
27 k Ω
47 k Ω
AGND
22 pF
NPO
39 k Ω
5
6
7
22 pF
NPO
39 k Ω
+
-
+
-
TDA1308 Vrefout
10 Ω
1/4 Watt
10 Ω
1/4 Watt
47 k Ω
Headphone
Out
Figure 17. Alternate Line Output as Headphone Output
AGND
ALT_LINE_OUT_R
27 k Ω
1000 pF
NPO
1000 pF
NPO
1
2 3
4
AGND
0.1 µF Y5V
220 µF
+ 5
-
39 k Ω
1
2 3
4
+ 3
-
22 pF
NPO
22 pF
NPO
6
2
7
1
TDA1308
TDA1308
ALT_LINE_OUT_L
VREFOUT
+ ELEC 1/4 WATT
10 Ω
HP_OUT_R
220 µF
+ ELEC 1/4 WATT
10 Ω
HP_OUT_L
47 k Ω
1
3 4
AGND
2
1 µF
Figure 18. Stereo Output
DS318PP6 37
CS4297A