cs5317 Cirrus Logic, Inc., cs5317 Datasheet - Page 10

• Manufacturer Part Number: cs5317
• Description: 16-bit, 20 Khz Oversampling A/d Converter
• Manufacturer: Cirrus Logic, Inc.

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,
f
Figure 3. In the process of filtering the digitized
signal the filter decimates the sampling rate by
128 (that is, f
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 f
output word rate, which has aliasing implications.
Residual signals after filtering at multiples of f
will alias into the baseband. For example, an in-
put tone at 28 kHz will be attenuated by 39.9 dB.
If f
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 f
filtering will also reject residual quantization
CLKOUT
10
s
in
. The filter’s frequency response is shown in
DOUT
DATA
s
out
= 20 kHz, the residual tone will alias into
s
out
s
(sign bit)
= f
out
MSB
/4 (that is, above 5 kHz), the
s
15 14
in
/128). In its most elemen-
13 12 11 10
s
Figure 4. Data Output
out
, the
s
9
out
8
f out
noise from the modulator. This will typically in-
crease the converter’s dynamic range to 88 dB.
Further bandlimiting the digital output to f
(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 f
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.
7
6
5
4
3
2
1
LSB
0
CS5317
15 14
s
out
DS27F4
/2, the
s
out
/8