CS4227 Datasheet, PDF (13/36 Pages) Cirrus Logic – Six Channel, 20-Bit Codec

CS4227
2.2.2 Adjustable Input Gain
The signals from the line inputs are routed to a pro-
grammable gain circuit which provides up to 9 dB
of gain in 3 dB steps, adjustable through the Input
Control Byte. Right and left channel gain settings
are controlled independently with the GNR1/0 and
GNL1/0 bits. To minimize audible artifacts, level
changes should be done with the channel muted, as
the changes occur immediately on register updates.
The ADC Status Report Byte provides feedback of
input level for each ADC channel. This register
continously monitors the ADC output and records
the peak output level since the last register read.
Reading this register causes it to reset to 0, where-
upon peak monitoring begins again.
2.2.3 High Pass Filter
The operational amplifiers in the input circuitry
driving the CS4227 may generate a small DC offset
into the A/D converter. The CS4227 includes a
high pass filter after the decimator to remove any
DC offset which could result in recording a DC lev-
el, possibly yielding "clicks" when switching be-
tween devices in a multichannel system.
The characteristics of this first-order high pass fil-
ter are outlined below for an output sample rate of
44.1 kHz. This filter response scales linearly with
sample rate.
2.3 Analog Outputs
2.3.1 Line Level Outputs
The CS4227 contains an on-chip buffer amplifier
producing single-ended outputs capable of driving
10 kΩ loads. Each output (AOUT 1-6) will produce
a nominal 2.83 V_pp (1 V_rms) output with a 2.3 volt
quiescent voltage for a full scale digital input. The
recommended off-chip analog filter is a 2nd order
Butterworth with a -3 dB corner at Fs (see
Figure 7). This filter provides out-of-band noise at-
tenuation along with a gain of 2, providing a 2 V_rms
output signal. A 3rd order Butterworth filter with a
-3 dB corner at 0.75 Fs can be used if greater out of
band noise filtering is desired. The CS4227 DAC
interpolation filter is a linear phase design which
has been pre-compensated for an external 2nd or-
der Butterworth filter to provide a flat frequency re-
sponse and linear phase response over the
passband. If this filter is not used, small frequency
response magnitude and phase errors will occur.
11 kΩ
22 kΩ
3.9 kΩ
CMOUT
1000pF
5 kΩ
150pF
_
+
Example
Op-Amps
are
MC33078
0.47 µF
Frequency Response
Phase Deviation
Passband Ripple
-3dB @ 3.4 Hz
-0.13 dB @ 20 Hz
10 degrees @ 20 Hz
None
Table 2. High Pass Filter Characteristics
2-Pole Butterworth Filter
560 pF
5.85 kΩ
1.1 kΩ 4.75 kΩ
AOUT
1.21 kΩ
_
5600 pF
5600 pF ⁺
CMOUT
5 kΩ
0.47 µF
3-Pole Butterworth Filter
Figure 7. Butterworth Filters
DS281PP2
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