CS4218 datasheet(12/44 Pages) CIRRUS

12 / 44 page

OVERVIEW

The CS4218 contains two analog-to-digital con-

verters, two digital-to-analog converters,

adjustable input gain, and adjustable output level

control. Since the converters contain all the re-

quired filters in digital or sampled analog form,

the filters’ frequency responses track the sample

rate of the CS4218. Only a single-pole RC filter

is required for the analog inputs and outputs.

Communication with the CS4218 is via a serial

port, with separate pins for data input and out-

put. The filters and converters operate over a

sample rate range of 4 kHz to 50 kHz.

FUNCTIONAL DESCRIPTION

Analog Inputs and Outputs

Figure 8 illustrates the suggested connection dia-

gram for the CS4218. The line level inputs,

LIN1 or LIN2 and RIN1 or RIN2, are selected

by an internal input multiplexer. This multiplexer

is a source selector and is not designed for real-

time switching between inputs at the sample rate.

When using the CS4218 as a drop-in replace-

ment for the CS4216, existing recommended

circuits (shown in the CS4216 data sheet) may

be used as is without any noticeable degradation

in performance. Performance may vary with

user-specific input circuits and should be

checked when contemplating the use of CS4218

in existing CS4216 designs.

Unused analog inputs that are not selected have

a very high input impedance, so they may be

tied to AGND directly. Unused analog inputs

that are selected should be tied to AGND

through a 0.1

µF capacitor. This prevents any

DC current flow.

The analog inputs are single-ended and inter-

nally biased to the REFBUF voltage (nominally

2.1 V). The REFBUF output should be buffered

if it is to be used for microphone phantom

power.

The use of a single-pole RC filter is recom-

mended for use as an external anti-alias filter for

the CS4218. The maximum cutoff frequency

(lowpass) should not exceed 200 kHz. A lower

value for cuttoff frequency may be used, and is

dependent upon the application’s input band-

width.

The CS4218 inputs will accept a 1Vrms signal,

so a divide by two resistor network will serve as

a front-end interface to 2 Vrms line level sys-

tems. Figure 9 shows a simple input circuit that

includes a gain of 0.5 and the required RC filter.

The gain of 0.5 yields a full scale input sensitiv-

ity of 2 Vrms with the CS4218 programmable

gain set to 0.

The analog outputs are also single-ended and

centered around the REFBUF voltage. AC cou-

pling capacitors of >1

µF are recommended.

Refer to Figure 8 for the recommended analog

output circuit.

Line In

Right

Line In

Left

300 pF

NPO

5.6K

0.33 uF

LIN1

LIN2

5.6K

0.33 uF

300 pF

RIN1

RIN2

Figure 9. Line Inputs.

CS4218

DS135F1

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