CS4222_02 Datasheet, PDF (16/29 Pages) Cirrus Logic – 20-Bit Stereo Audio Codec with Volume Control

CS4222
4.2.2 Input Level Monitoring
The CS4222 includes independent Peak Input Level Monitoring for each channel. The analog-to-digital con-
verter (ADC) continually monitors the peak digital signal for both channels, prior to the digital limiter, and
records these values in the LVL2-0 (left channel) and LVR2-0 (right channel) bits in the Converter Status
Report Byte (#6). These bits indicate whether the input level is clipping, -1 to -6 dB from full scale in 1 dB
resolution, or below -6 dB from full scale. The LVL/LVR bits are "sticky" bits and are reset to zero when the
DSP Port Mode Byte (#5) is read.
4.2.3 High Pass Filter
The operational amplifiers in the input circuitry driving the CS4222 may generate a small DC offset into the
A/D converter. The CS4222 includes a high pass filter after the decimator to remove any DC offset which
could result in recording a DC level, possibly yielding "clicks" when switching between devices in a multi-
channel system. The characteristics of this first-order high pass filter are outlined below for Fs equal to
48 kHz. The filter response scales linearly with sample rate. The high pass filter in the CS4222 may be de-
feated independently for the left and right channels by writing HPDR and HPDL in the ADC control byte (#1).
Table 2. High Pass Filter Characteristics
Frequency Response
Phase Deviation
Passband Ripple
-3 dB @ 3.7 Hz
-0.1 dB @ 20 Hz
10 degrees @ 20 Hz
None
4.3 Analog Outputs
4.3.1 Line Level Outputs
The CS4222 contains an on-chip buffer amplifier producing differential outputs capable of driving 10 kΩ
loads. Each output (AOUTL+, AOUTL-, AOUTR+, AOUTR-) will produce a nominal 2.83 Vpp (1 Vrms) out-
put with a 2.3 volt common mode for a full scale digital input. This is equivalent to a 5.66 Vpp (2 Vrms) dif-
ferential signal as shown in Figure 6. The recommended off-chip analog filter is either a 2nd order
Butterworth or a 3rd order Butterworth, if greater out-of-band noise filtering is desired. The CS4222 DAC
interpolation filter has been pre-compensated for an external 2nd order Butterworth filter with a 3 dB corner
at Fs, or a 3rd order Butterworth filter with a 3 dB corner at 0.75 Fs to provide a flat frequency response and
linear phase over the passband (see Figure 7 for Fs = 48 kHz). If the recommended filter is not used, small
frequency response magnitude and phase errors will occur. In addition to providing out-of-band noise atten-
uation, the output filters shown in Figure 7 provide differential to single-ended conversion.
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