CS6420-CS datasheet(9/52 Pages) CIRRUS

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CS6420

DS205PP2

and they are post-compensated internally to pre-

vent any resultant passband droop. The ADCs also

expect a maximum of 1 Vrms (2.8 Vpp) at their in-

puts (which are biased around 2.12 VDC). A signal

of higher amplitude will clip the ADC input and

may result in poor echo canceller performance. See

the Design Considerations section for more details.

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 Vrms (2.8 Vpp) swinging around a DC bias of

2.12 V.

Acoustic Interface

The pins API (pin 20), APO (pin 18), MB (pin 19),

and AO (pin 3) make up the Acoustic Interface. A

block diagram of the Acoustic Interface is shown in

Figure 4.

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

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

amplifier is an inverting amplifier with a fixed gain

of 34 dB biased around an input offset voltage

(Voff) of 2.12 V. APO is the output of the pre-am-

plifier after a 1 k

Ω resistor. The circuitry connected

to the amplifier input must present low source im-

pedance (<100

Ω) to the API pin or the gain will be

reduced. When using the pre-amplifier, connecting

a 0.022

µF capacitor to ground off APO will pro-

vide the anti-aliasing filter required by the ADC, as

shown in Figure 2. The pre-amplifier may be by-

passed by clearing Mic (Register 0, bit 15) using the

Microcontroller Interface (see Microcontroller In-

terface section), grounding API through a capacitor,

and driving APO 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 3).

Following the pre-amplifier is a programmable an-

alog gain stage (PGA) which is controllable

through the Microcontroller Interface. This gain

19 MB

18 APO

0,6,9.5,12 dB

ADC

DAC

ADC

FAR-END

Transmit Path

Receive Path

API

3.5V

2.12V

Ω

BANDGAP

34 dB

0,6,9.5,12 dB

NEAR-END

PGA

Figure 4. Analog Interface

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