CDB4228 datasheet(13/30 Pages) CIRRUS

13 / 30 page

CS4228

DS307PP1

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.

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

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

controlled

the

HMUTE56/34/12 bits in the DAC Mute2 Control

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.

560

2SC2878

2.2 k

MUN2IIIT1

10 k

MUTEC

Line Out

MUTEDRV

10 k

22 µF

100 k

AOUT

Figure 7. Passive Output Filter with Mute

C=142

µF

3.16 k

AOUT

3.16 k

MC33078

1nf

GND

1nf

10 µf

MUTE DRV

Line

Out

MUTE

100 pf

3.16 k

1.78 k

+12

-12

3.16 k

AOUT

3.16 k

MC33078

1nf

GND

1nf

10 µf

MUTE DRV

Line

Out

MUTE

100 pf

3.16 k

1.78 k

+12

-12

Figure 8. Butterworth Output Filter with Mute

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