CS53L21 datasheet(23/66 Pages) CIRRUS

23 / 66 page

DS700PP1

CS53L21

4.3.2

High-Pass Filter and DC Offset Calibration

The high-pass filter continuously subtracts a measure of the DC offset from the output of the decimation

filter. If the high-pass filter is “frozen” during normal operation, the current value of the DC offset for the

corresponding channel is held. It is this DC offset that will continue to be subtracted from the conversion

result. This feature makes it possible to perform a system DC offset calibration by:

1. Running the A/D with the high-pass filter enabled and the DC offset not “frozen” until the filter settles.

See the Digital Filter Characteristics for filter settling time.

2. Freezing the DC offset.

The high-pass filters are controlled using the ADCx_HPFRZ and ADCx_HPFEN bits.

If a particular ADC channel is used to measure DC voltages, the high-pass filter may be disabled using

the ADCx_HPFEN bit.

4.3.3

Digital Routing

The digital output of the ADC may be internally routed to the Signal Processing Engine (SPE). ADC output

volume may be controlled using the ADCMIX [6:0] bits, and channel swaps can be done using the

ADCA[1:0] and ADCB[1:0] bits. This “processed” ADC data can be selected for output in place of the ADC

output data using the DIGMIX bit.

4.3.4

Differential Inputs

The stereo pair inputs act as a single differential input when the MICMIX bit is enabled. This provides com-

mon mode rejection of noise in digitally intense PCB’s where the microphone signal traverses long traces,

or across long microphone cables as illustrated in Figure 8.

Since the mixer provides a differential combination of the two signals, the potential input mix may exceed

the maximum full-scale input and result in clipping. The level out of the mixer, therefore, is automatically

attenuated 6 dB. Gain may be applied using either the analog PGA or MIC Pre-amp or the digital ADCMIX

volume control to re-adjust a small signal to desired levels.

The analog inputs may also be used as a differential input pair as illustrated in Figure 9. The two channels

are differentially combined when the MICMIX bit is enabled.

4.3.4.1 External Passive Components

The microphone input is internally biased to VQ. Input signals must be AC coupled using external capaci-

tors with values consistent with the desired high-pass filter design. The MICINx input resistance of 50 kW

may be combined with an external capacitor of 1 mF to achieve the cutoff frequency defined by the equa-

tion,

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 MICBIAS_LVL[1:0] bits.

Software

Controls:

“ADC Control (Address 06h)” on page 45.

Software

Controls:

“ADCx Mixer Volume Control: ADCA (Address 0Eh) & ADCB (Address 0Fh)” on page 51, “Inter-

face Control (Address 04h)” on page 43.

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