CS6403 datasheet(39/54 Pages) CIRRUS

39 / 54 page

Another important note about the differential

amplifier is that it is biased to swing around

1VDC. This is fine when the CS6403 gain stage

is on, as full scale into the part is 100mVpp.

When the gain stage is off, however, the full

scale input is 2Vpp, which forces the op-amp to

swing from 2V to 0V. Since the op-amp won’t

drive this close to the negative rail, it is advis-

able to change the bias point to about 2VDC.

This can be easily acheived by connecting pin 7

of the op-amp to the right side of R104, cutting

the trace above and below the right-hand pad of

R104, and strapping VCM to pin 5 of the op-

amp.

Far-End Analog Interface

In Mode 1 the far-end signals are provided to the

CS6403 via the MC145480 PCM codec by de-

fault.

The analog signals provided to the

MC145480 are shown in Figure 3. The FE_IN

terminal block provides a differential input for

the far-end input. The fullscale voltage for this

input is 1.575Vp referenced to 2.4V in its cur-

rent unity gain configuration (see PCM Codec

below).

The FE_OUT terminal block provides the far-

end output either differentially or single-ended

depending on whether J8 or J10 is shorted. The

output level of each individual output is 1.575Vp

referenced about 2.4V. The FE_OUT is capable

of driving a 2k

Ω load.

Signal polarity is indicated by "+" and "-" sym-

bols silkscreened near the connectors.

These

connectors are not used in Mode 2.

CS6403

The CS6403 Echo Cancelling Codec, shown in

Figure 4, is the heart of the evaluation board.

See the CS6403 datasheet for full details on this

part.

The evaluation board is a good example of

proper layout and grounding of the CS6403.

Note that the part resides completely on the ana-

log ground plane and that all the power supplies

are decoupled with a 0.1

µF and 1µF capacitor

with the smaller capacitor closest to the chip.

VCM are also well bypassed and test points exist

to monitor these values.

A test point labeled MICIN allows the evaluator

to monitor the signal at the MICIN pin of the

CS6403. This signal should be a maximum of

2Vpp with the internal gain stage off (GPIN3

high), and 100mVpp with the internal gain stage

on (GPIN3 low).

SPKROUTP and SPKROUTN also have test

points on either side of the screw terminal. J14

and J15 are provided so that intermediate cir-

cuitry can intercept and process the SPKROUT

signals, if desired.

Note that SPKROUTP and

SPKROUTN each drive a maximum of 1.75Vpp

into an 8

Ω load. Since they are 180 degrees out

of phase, this can be applied differentially to

produce 3. 5Vpp.

Do no t gr ou nd either

SPKROUTP or SPKROUTN as this may damage

the speaker driver internally.

An 8.192MHz crystal is provided for Mode 1

applications. If the CS6403 is configured to op-

erate in Mode 2, the crystal should be removed

to prevent possible noise from interfering clocks.

The crystal is socketed to facilitate removal.

FE_OUT+

AGND

J10

FE_OUT-

FE_IN+

FE_IN-

Figure 3. Far-End Input and Output

Analog Connections

CDB6403

DS192DB3

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