CS42L56 datasheet(34/92 Pages) CIRRUS

34 / 92 page

DS851F2

CS42L56

4.2.3

Microphone Inputs

Any of the line inputs can be configured as a microphone input by using the MICBIAS pin to power the

external microphone circuit and by configuring the additional +10 or +20 dB gain in the PGA to properly

boost the low-level microphone signal.

4.2.3.1 External Passive Components

The analog inputs are internally biased to VQ. Input signals must be AC coupled using external capacitors

with values consistent with the desired high-pass filter design. The analog input resistance may be com-

bined with an external capacitor to achieve the desired cutoff frequency. The equation below gives an ex-

ample:

An electrolytic capacitor must be placed such that the positive terminal is positioned relative to the side

with the greater bias voltage. The MICBIAS voltage level is controlled by the BIAS_LVL[1:0] bits.

The MICBIAS series resistor must be selected based on the requirements of the particular microphone

used.

4.2.4

Optional VCM Buffer

Leaving an analog input pin floating when not being used might inject distortion in the analog input signal

path. To prevent this, the analog inputs may be internally biased to VCM by using a weak internal VCM

buffer when not being used. The VCM buffer outputs a weakly buffered version of the internal common-

mode voltage and biases the chip-side of the analog input AC-coupling capacitor to a constant DC level.

This prevents the analog signal from being distorted when that particular channel is not selected by either

the PGA or ADC input MUX. If an analog signal is routed to any place other than just the CS42L56, it is

recommended to set this bit to 0b. If all analog signals are only routed to the CS42L56, this bit may be left

set to 1b.

4.2.5

Automatic Level Control (ALC)

The function of the ALC is to maintain the level of the analog input signal between the maximum and min-

imum threshold settings programmed in the ALCMAX[2:0] and ALCMIN[2:0] registers. When enabled, the

ALC monitors the signal level after the digital volume control block in the input signal path and detects

Referenced Control

BIAS_LVL[1:0] ..................... “Microphone Bias Output Level” on page 77

Referenced Control

PDN_VBUF[1:0]................... “Power Down VCM Bias Buffer” on page 58

 4 k

 1 F



--------------------------------------------

39.79 Hz

MICx+

MICx-



MICBIAS

Figure 17. MIC Input Mix w/Common Mode Rejection

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