CS4294 datasheet(31/42 Pages) CIRRUS

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CS4294

DS326PP4

7.3

Line Level Outputs

The analog output section provides a stereo line-

level output and an alternate stereo line-level out-

put.

LINE_OUT_L,

LINE_OUT_R,

ALT_LINE_OUT_L, and ALT_LINE_OUT_R

outputs should be capacitively coupled to external

circuitry.

Each of the 4 analog outputs, if used in the design,

require 680 pF or larger NPO dielectric capacitors

between the corresponding pin and AGND. Each

analog output is DC biased up to the Vrefout volt-

age signal reference which is nominally 2.2 V. This

requires that the output either be AC coupled to ex-

ternal circuitry (AC load must be greater than 10

Ω) or DC coupled to a buffer op-amp biased at the

Vrefout voltage (see Figure 13 for the recommend-

ed headphone op-amp circuit).

7.4

Miscellaneous Analog Signals

The AFILT1 and AFILT2 pins must have a

1000 pF NPO capacitor (must not be smaller than

390 pF) to analog ground. These capacitors, along

with an internal resistor, provide a single-pole low-

pass filter at the inputs to the ADCs. By placing

these filters at the input to the ADCs, low-pass fil-

ters at each analog input pin are not necessary.

The REFFLT pin lowers the noise of the internal

voltage reference. A 1

µF (must not be greater than

µF) and 0.1 µF capacitor to analog ground should

be connected with a short, wide trace to this pin. No

other connection should be made, as any coupling

onto this pin will degrade the analog performance

of the Codec. Likewise, digital signals should be

kept away from REFFLT for similar reasons.

The Vrefout pin is typically 2.2 V and provides a

common mode signal for single-supply external

circuits. Vrefout only supports light DC loads and

should be buffered if AC loading is needed. For

typical use, a 0.1

µF in parallel with a 1 µF capaci-

tor should be connected to Vrefout.

AGND

+5 VA

100 k

Ω

2.7 k

Ω

CGND

220 pF

68 k

Ω

AGND

0.068 µF

X7R

AGND

+5 VA

U1A

MC33078D

47 k

Ω

10 µF

AGND

47 k

Ω

AGND

+5 VA

47 k

Ω

6.8 k

Ω

10 µF

AGND

220 pF

47 k

Ω

U1B

MC33078D

1 µF

X7R

MIC1

Figure 12. PC ‘99 Microphone Pre-amplifier

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