CS4228 Datasheet, PDF (13/30 Pages) Cirrus Logic – 24-Bit, 96 kHz Surround Sound Codec

CS4228
High frequency noise beyond the audio passband,
resulting from the delta-sigma conversion process
produces high frequency noise beyond the audio
passband, most of which is removed by the on-chip
analog filters. The remaining out-of-band noise can
be attenuated using an off-chip low pass filter. For
most applications, a simple passive filter as show in
Figure 7 can be used. Note that this circuit also
serves to block the DC present at the outputs. Fig-
ure 8 gives an example of a filter which can be used
in applications where greater out of band attenua-
tion is desired. The 2-pole Butterworth filter has a
-3 dB frequency of 50 kHz, a passband attenuation
of 0.1 dB at 20 kHz providing optimal out-of-band
filtering for sample rates from 44.1 kHz to 96 kHz.
The filter has and a gain of 1.56 providing a 2 Vrms
output signal.
MUTEC
10 k
10 k
AOUT
22 µF
560
+
100 k
C=142µF
F_s
2SC2878
MUN2IIIT1
MUTEDRV
Line Out
C
2.2 k
Figure 7. Passive Output Filter with Mute
1 nf
3.16 k
AOUT
3.16 k
+12
5
+
1 nf ⁶ _
7
MC33078
GND
-12
+
10 µf
MUTE
MUTE DRV
3.16 k
1.78 k
100 pf
Line
Out
Figure 8. Butterworth Output Filter with Mute
Digital Volume Control
Each DAC’s output level is controlled via the Dig-
ital Volume Control register operating over the
range of 0 to 90.5 dB attenuation with 0.5 dB reso-
lution. Volume control changes do not occur in-
stantaneously. Instead they ramp in increments of
0.125 dB at a variable rate controlled by the
RMP1:0 bits in the Digital Volume Control regis-
ter.
Each output can be independently muted via mute
control bits MUT6-1 in the DAC Mute1 Control
register. When asserted, MUT attenuates the corre-
sponding DAC to its maximum value (90.5 dB).
When MUT is deasserted, the corresponding DAC
returns to the attenuation level set in the Digital
Volume Control register. The attenuation is
ramped up and down at the rate specified by the
RMP1:0 bits.
To achieve complete digital attenutation of an in-
coming signal, Hard Mute controls are provided.
When asserted, Hard Mute will send zero data to a
corresponding pair of DACs. Hard Mute is not
ramped, so it should only be asserted after setting
the two corresponding MUT bits to prevent high
frequency noise from appearing on the DAC out-
puts. Hard Mute is controlled by the
HMUTE56/34/12 bits in the DAC Mute2 Control
register.
Mute Control
The Mute Control pin is typically connected to an
external mute control circuit as shown in Figure 7
and Figure 8. Mute Control is asserted during pow-
er up, power down, and when serial port clock er-
rors are present. The pin can also be controlled by
the user via the control port, or automatically as-
serted when zero data is present on all six DAC in-
puts. To prevent large transients on the output, it is
desirable to mute the DAC outputs before the Mute
Control pin is asserted. Please see the MUTEC pin
in the Pin Descriptions section for more informa-
tion.
DS307PP1
13