CS6422 datasheet(11/48 Pages) CIRRUS

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CS6422

of higher amplitude will clip the ADC input and

will result in poor echo canceller performance. See

Section 4., “Design Considerations” for more de-

tails.

The outputs are delta-sigma digital to analog con-

verters (DACs) and have similar requirements to

the ADCs. The DACs are pre-compensated to ex-

pect a single-pole RC filter with a corner frequency

at 4 kHz. The full scale voltage output from a DAC

is 1.1 Vrms (3.1 Vpp) maximum, 1 Vrms typical, bi-

ased around 2.12 VDC.

3.1.1

Acoustic Interface

The pins API (pin 20), APO (pin 18), AO (pin 3),

and MB (pin 19) form the Acoustic Interface. A

block diagram of the Acoustic Interface is shown in

Figure 6.

API and APO are, respectively, the input and out-

put of the built-in microphone pre-amplifier. The

pre-amplifier is an inverting amplifier with a fixed

gain of 34 dB biased around an input offset voltage

of 2.12 V. APO is the output of the pre-amplifier

after a 1 k

Ω (typical) resistor. The circuitry con-

nected to the amplifier input must present low

source impedance (<100

Ω) to the API pin or the

gain will be reduced. When using the internal mic

preamp, a 0.022

µF capacitor should be placed be-

tween APO and ground to provide the anti-aliasing

filter required by the ADC, as shown in Figure 4.

The pre-amplifier may be bypassed by clearing the

‘Mic’ bit (Register 0, bit 15) using the Microcontrol-

ler Interface (see Section 3.2, “Microcontroller Inter-

face”). If the internal mic preamp is not used, a

0.022

µF capacitor should be tied between API and

ground, and APO should be driven directly. In this

case, the signal into APO must be low-pass filtered

by a single-pole RC filter with a corner frequency at

8 kHz (see Figure 5).

Following the pre-amplifier is a programmable an-

alog gain stage, called TGain, which is controlled

APO

(0,6,9.5,12 dB)

ADC

DAC

ADC

FAR-END

Transmit Path

Receive Path

API

Ω

Voltage

Reference

34 dB

(0,6,9.5,12 dB)

NEAR-END

TGain

RGain

CS6422

Mic

Figure 6. Analog Interface

CS6422

DS295F1

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