CS4294 datasheet(30/42 Pages) CIRRUS

30 / 42 page

CS4294

DS326PP4

7. ANALOG HARDWARE

DESCRIPTION

The analog hardware consist of three line-level ste-

reo inputs, one mono microphone input, and dual,

independent stereo line outputs. This section de-

scribes the analog hardware needed to interface

with these pins.

7.1

Line-Level Inputs

The analog inputs consist of three stereo analog in-

puts and one mono input. As shown in Figure 8, the

input to the ADCs comes from the Input Mux

which selects one of the following: Mic1 (Mono),

CD, Aux, Line In, Stereo Input Mix, or the Mono

Input Mix (Mono). Unused analog inputs should be

connected together and then connected through a

capacitor to analog ground or tied to the Vrefout

line directly.

The analog input mixer is designed to accommo-

date four stereo inputs and one mono input. These

inputs are: a stereo line-level input (LINE), a mono

microphone input (MIC), a stereo CD-ROM input

(CD), a stereo auxiliary line-level input (AUX),

and the PCM output from the DACs (if the POP bit

is cleared). Each of the stereo inputs has separate

volume controls for each channel and one mute

control for each left/right pair. The mono micro-

phone input has one mute and one volume control.

The inputs to the output mixer are: the input mixer

output, the stereo enhanced mix, and the DAC out-

put (if the POP bit is set).

All analog inputs to the CS4294, 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 10 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 and AUX_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 11 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.

7.2

Microphone Level Inputs

The microphone level inputs include a selectable -

34.5 dB to +12 dB gain stage for interfacing to an

external microphone. An additional 20 dB gain

block is also available. Figure 12 illustrates a sin-

gle-ended microphone input buffer circuit that will

support lower gain mics. The circuit in Figure 12

supports dynamic mics and phantom-powered mics

that use the right channel (ring) of the jack for pow-

er.

6.8 k

Ω

6.8 k

Ω

1.0

µF

1.0

µF

6.8 k

Ω

6.8 k

Ω

Figure 10. Line Inputs

Figure 11. Differential CDROM In

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%)

CDROM IN

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