CS4297 datasheet(30/46 Pages) CIRRUS

30 / 46 page

DS242F5

CS4297

CrystalClear™ SoundFusion™ Audio Codec ’97

The inputs to the output mixer are: the input mixer

output, the PC Beep mono input, and the Phone

mono input.

All analog inputs to the CS4297, including

CD_GND, should be capacitively coupled to the

input pins.

Since many analog levels can be as large as

2VRMS, the circuit shown in Figure 4 can be used

to attenuate the analog input by 6 dB (to 1 VRMS)

which is the maximum voltage allowed for all the

stereo line-level inputs: LINE_IN, AUX_IN, and

VIDEO_IN.

The CD line-level inputs have an extra pin,

CD_GND, which provides 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 ground coming from the CD ana-

log source. Connecting the CD pins as shown in

Figure 5 provides extra attenuation of common

mode noise coming from the CDROM drive, there-

by producing a higher quality signal. One percent

resistors are recommended since the better the re-

sistors match, the better the common-mode attenu-

ation of unwanted signals. If CD is not used, the

inputs should be connected through AC capacitors

to analog ground or connected to Vrefout.

Microphone Level Inputs

The microphone level inputs, MIC1 and MIC2, in-

clude a selectable -34.5 dB to +12 dB gain stage for

interfacing to an external microphone. An addition-

al 20 dB gain block is also available. Figure 6 illus-

trates a single-ended microphone input buffer

circuit that will support lower gain mics. The cir-

cuit in Figure 6 supports dynamic mics and phan-

tom-powered mics that use the right channel (ring)

of the jack for power.

Mono Inputs

The mono input, PC_BEEP, is useful for mixing

the output of the “beeper” (timer chip), provided in

all PCs, with the rest of the audio signals. The at-

tenuation control allows 16 levels in -3 dB steps. In

addition, a mute control is provided. The attenuator

is a single channel block with the resulting signal

sent to the output mixer where it is mixed with the

left and right outputs. Figure 7 illustrates a typical

input circuit for the PC_BEEP input. If PC_BEEP

is driven from a CMOS gate, the 4.7 k

Ω should be

tied to AGND instead of VA+. Although this input

is described for a low-quality beeper, the input is of

the same high-quality as all other analog inputs and

may be used for other purposes.

The mono input, PHONE_IN, can be used to inter-

face to the output of a MODEM Analog Front End

6.8 k

Ω

6.8 k

Ω

1.0

µF

1.0

µF

6.8 k

Ω

6.8 k

Ω

Figure 4. Line Inputs

6.8 k

Ω

6.8 k

Ω

1.0

µF

1.0

µF

CD_L

CD_R

6.8 k

Ω

6.8 k

Ω

3.4 k

Ω

CD_GND

2.0

µF

3.4 k

Ω

(All resistors 1%)

Figure 5. Differential CDROM In

0.33

µF

220 pF

NPO

3.3

µF

6.8 K

Ω

47 k

Ω

MC33078 or

MC33178

Vrefout

MIC1

MIC2

µF

0.33

µF

X7R

2 k

Ω

47 k

Ω

0.1

µF

Ω

100 pF

NPO

Figure 6. Microphone Input

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