Cirrus Logic CS4202 - Manual (Page 50)

CS4202
50 DS549PP2
10. ANALOG HARDWARE
DESCRIPTION
The analog input section consists of four stereo
line-level inputs (LINE_L/R, CD_L/C/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 output section consists of a mono output
(MONO_OUT), a stereo headphone output
(HP_OUT_L/R), and a stereo line-level output
(LINE_OUT_L/R). This section describes the ana-
log hardware needed to interface with these pins.
The designs presented in this section are compliant
with Chapter 17 of Microsoft’s
®
PC 99 System De-
sign Guide [7] (referred to as PC 99) and Chapter
11 of Microsoft’s
®
PC 2001 System Design Guide
[8] (referred to as PC 2001). For information on
EMI reduction techniques refer to the application
note AN165: CS4297A/CS4299 EMI Reduction
Techniques [5].
10.1 Analog Inputs
All analog inputs to the CS4202, including CD_C,
should be capacitively coupled to the input pins.
Unused analog inputs should be tied together and
connected through a capacitor to analog ground or
tied to the Vrefout pin directly. The maximum al-
lowed voltage for analog inputs, except the micro-
phone input, is 1 V
RMS
. The maximum allowed
voltage for the microphone input depends on the
selected boost setting.
10.1.1 Line Inputs
Figure 18 shows circuitry for a line-level stereo in-
put. Replicate this circuit for the Video and Aux in-
puts. This design attenuates the input by 6 dB,
bringing the signal from the PC 99 specified
2 V
RMS
, to the CS4202 maximum allowed 1 V
RMS
.
10.1.2 CD Input
The CD line-level input has an extra pin, CD_C,
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 design in Figure 19 provides extra
attenuation of common mode noise coming from
the CD-ROM drive, thereby producing a higher
quality signal. One percent resistors are recom-
mended since closely matched resistor values pro-
vide better common-mode attenuation of unwanted
signals. The circuit shown in Figure 19 can be used
for a 1 V
RMS
CD input signal.
10.1.3 Microphone Inputs
Figure 20 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
CS4202 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
LINE_IN_R
LINE_IN_L
6.8 k Ω
1.0 µ F
1.0 µ F
AGND
AGND
6.8 k Ω
6.8 k Ω 6.8 k Ω
Figure 18. Line Input (Replicate for Video and AUX)
100 Ω
CD_L
CD_COM
CD_R
1.0 µF
CD_L
CD_R
CD_C
100 Ω 1.0 µF
100 Ω
47 k Ω
2.2 µF
47 k Ω 47 k Ω
AGND
Figure 19. Differential 1 V
RMS
CD Input