CS4298 datasheet(38/52 Pages) CIRRUS

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CS4298

DS315PP2

7.2

Microphone Level Inputs

The microphone level inputs, MIC1 and MIC2, include a selectable -34.5 dB to +12 dB gain stage

for interfacing to an external microphone. An additional 20 dB gain block is also available. Figure 12

illustrates a single-ended microphone input buffer circuit that will support lower gain mics. The cir-

cuit in Figure 12 supports dynamic mics and phantom-powered mics that use the right channel (ring)

of the jack for power..

7.3

Line Level Outputs

The analog output section provides a stereo line-level output and an alternate stereo line-level output.

LINE_OUT_L, LINE_OUT_R, ALT_LINE_OUT_L, and ALT_LINE_OUT_R outputs should be

capacitively coupled to external circuitry.

The mono output, MONO_OUT, can be used as either a sum of the left and right output channels

attenuated by 6 dB to prevent clipping at full scale or the selected MIC_IN signal. The mono out

channel can be used to drive the PC internal mono speaker using an appropriate drive circuit. This

approach allows the traditional PC sounds to be integrated with the rest of the audio system. The mute

control is independent of the line outputs allowing the mono channel to mute the speaker without

muting the line outputs.

Each of the 5 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

external circuitry (AC load must be greater than 10 k

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

the Vrefout voltage (see Figure 13 for the recommended headphone op-amp circuit).

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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