CS4294 Datasheet, PDF (30/42 Pages) Cirrus Logic – SoundFusion Audio/Docking Codec 97 (AMC 97)

CS4294
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).
6.8 kΩ
6.8 kΩ
6.8 kΩ
1.0 µF
R
1.0 µF
L
6.8 kΩ
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
2 V_RMS, the circuit shown in Figure 10 can be used
to attenuate the analog input by 6 dB (to 1 V_RMS)
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.
CDROM IN (All resistors 1%)
6.8 kΩ
1.0 µF
3.4 kΩ 2.0 µF
CD_L
CD_GND
CD_R
6.8 kΩ
1.0 µF
6.8 kΩ
3.4 kΩ ^{6.8 kΩ}
Figure 10. Line Inputs
Figure 11. Differential CDROM In
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DS326PP4