Cirrus Logic CS4297A - Manual (Page 35)

CS4297A
35
6. ANALOG HARDWARE
DESCRIPTION
The analog line-level input hardware consists of
four stereo inputs (LINE_IN_L/R, CD_L/GND/R,
VIDEO_L/R, and AUX_L/R), two selectable
mono microphone inputs (MIC1 and MIC2), and
two mono inputs (PC_BEEP and PHONE). The an-
alog line-level output hardware consists of a mono
output (MONO_OUT), and dual stereo line outputs
(LINE_OUT_L/R and ALT_LINE_OUT_L/R).
This section describes the analog hardware needed
to interface with these pins. The designs presented
in this section comply with specifications detailed
in Chapter 17 of the Microsoft

PC Design Guide-
lines [7] (referred to as PC 99). For EMI reduction
techniques refer to the application note N165:
CS4297A/CS4299 EMI Reduction Techniques [5].
6.1 Analog Inputs
All analog inputs to the CS4297A, including
CD_GND, should be capacitively coupled to the
input pins. Unused analog inputs should be tied to-
gether and connected through a capacitor to analog
ground or tied to the Vrefout pin directly. The max-
imum allowed voltage for analog inputs, except the
microphone input, is 1 V
RMS
. For the microphone
input the maximum allowed voltage depends on the
selected boost setting.
6.1.1 Line-Level Inputs
Figure 10 shows circuitry for a line-level stereo in-
put. Replicate this circuit for the Line, Video and
Aux inputs. This design attenuates the input by
6 dB, bringing the signal from the PC 99 specified
2 V
RMS
, to the CS4297A maximum allowed
1 V
RMS
.
6.1.2 CD Input
The CD line-level input has an extra pin,
CD_GND, providing 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 CD analog source ground. Follow-
ing the reference designs in Figure 11 and
Figure 12 provides extra attenuation of common
mode noise coming from the CD-ROM drive,
thereby producing a higher quality signal. One per-
cent resistors are recommended since closely
matched resistor values provide better com-
mon-mode attenuation of unwanted signals. The
circuit shown in Figure 11 can be used to attenuate
a 2 V
RMS
CD input signal by 6 dB. The circuit
shown in Figure 12 can be used for a 1 V
RMS
CD in-
put signal.
6.8 k
Ω
6.8 k
Ω
1.0
µF
1.0
µF
R
L
6.8 k
Ω
6.8 k
Ω
Figure 10. Line Input (Replicate for Video and Aux)
(All resistors 1%)
6.8 k Ω
CD_L
CD_COM
CD_R
1.0 µF
CD_L
CD_R
CD_GND
6.8 k Ω 1.0 µF
3.4 k Ω
6.8 k Ω
2.2 µF
3.4 k Ω 6.8 k Ω
AGND
Figure 11. Differential 2 V
RMS
CD Input
100 Ω
CD_L
CD_COM
CD_R
1.0 µF
CD_L
CD_R
CD_GND
100 Ω 1.0 µF
100 Ω
47 k Ω
2.2 µF
47 k Ω 47 k Ω
AGND
Figure 12. Differential 1 V
RMS
CD Input
DS318PP6 35
CS4297A