CDB6422 Просмотр технического описания (PDF) - Cirrus Logic

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CDB6422

Enhanced Full-Duplex Speakerphone IC

Cirrus Logic 

CS6422

Receive Path

NI 17 RGain

ADC

(0,6,9.5,12 dB)

FAR-END

NO 4

DAC

DAC

3 AO

D

S

P

ADC

TGain

Mic 1kΩ

NEAR-END

34 dB 20 API

(0,6,9.5,12 dB)

CS6422

Voltage

Reference

18

19

APO

MB

Transmit Path

Figure 6. Analog Interface

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 kΩ (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

DS295F1

11

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