CS5317 datasheet(10/32 Pages) CIRRUS

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Decimation

Aliasing effects due to decimation are identical in

the CLKOR and CLKG1 modes. Aliasing is dif-

ferent in the CLKG2 mode due to the difference

in output sample rates (10 kHz vs. 20 kHz) and

thus will be discussed separately.

Aliasing in the CLKOR and CLKG1 Modes

The delta-sigma modulator output is fed into the

digital low-pass filter at the input sampling rate,

in. The filter’s frequency response is shown in

Figure 3. In the process of filtering the digitized

signal the filter decimates the sampling rate by

128 (that is, fs

out = fsin/128). In its most elemen-

tary form, decimation simply involves ignoring -

or selectively reading - a fraction of the available

samples.

In the process of decimation the output of the

digital filter is effectively resampled at fs

out, the

output word rate, which has aliasing implications.

Residual signals after filtering at multiples of fs

out

will alias into the baseband. For example, an in-

put tone at 28 kHz will be attenuated by 39.9 dB.

If fs

out = 20 kHz, the residual tone will alias into

the baseband and appear at 8 kHz in the output

spectrum.

If the input signal contains a large amount of out-

of-band energy, additional analog and/or digital

antialias filtering may be required. If digital post-

filtering is used to augment the CS5317’s

rejection above fs

out/4 (that is, above 5 kHz), the

filtering will also reject residual quantization

noise from the modulator. This will typically in-

crease the converter’s dynamic range to 88 dB.

Further bandlimiting the digital output to fs

out/8

(2.5 kHz at full speed) will typically increase dy-

namic range to 90 dB.

Aliasing in the CLKG2 Mode

Aliasing effects in the CLKG2 mode can be mod-

eled exactly as those in the CLKG1 mode with

the output decimated by two (from 20 kHz to 10

kHz). This is most easily achieved by ignoring

every other sample. In the CLKG2 mode the ratio

of the output sampling rate to the filter’s -3 dB

point is two, with no oversampling beyond the

demands of the Nyquist criterion. Without the

ability to roll-off substantially before fs

out/2, the

on-chip digital filter’s antialiasing value is dimin-

ished.

The CLKG2 mode should therefore be used only

when the output data rate must be minimized due

to communication and/or storage reasons. In ad-

dition, adequate analog filtering must be provided

prior to the A/D converter.

Digital Design Considerations

The CS5317 presents its 16-bit serial output

MSB-first in 2’s complement format. The con-

verter’s serial interface was designed to easily

interface to a wide variety of micro’s and DSP’s.

Appendix A offers several hardware interfaces to

industry-standard processors.

CLKOUT

DATA

12 11

987

543

MSB

(sign bit)

LSB

15 14

f out

DOUT

Figure 4. Data Output

CS5317

DS27F4

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