cs5317 Cirrus Logic, Inc., cs5317 Datasheet - Page 15

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

CS5317 PERFORMANCE
The CS5317 features 100% tested dynamic per-
formance. The following section is included to
illustrate the test method used for the CS5317.
FFT Tests and Windowing
The CS5317 is tested using Fast Fourier Trans-
form (FFT) techniques to analyze the converter’s
dynamic performance. A pure sine wave is ap-
plied to the CS5317 and a "time record" of 1024
samples is captured and processed. The FFT algo-
rithm analyzes the spectral content of the digital
waveform and distributes its energy among 512
"frequency bins". Assuming an ideal sinewave,
distribution of energy in bins outside of the fun-
damental and dc can only be due to quantization
effects and errors in the CS5317.
If sampling is not synchronized to the input sine-
wave it is highly unlikely that the time record will
contain an exact integer number of periods of the
input signal. However, the FFT assumes that the
signal is periodic, and will calculate the spectrum
of a signal that appears to have large discontinui-
ties, thereby yielding a severely distorted
spectrum. To avoid this problem, the time record
is multiplied by a window function prior to per-
forming the FFT. The window function smoothly
forces the endpoints of the time record to zero,
removing the discontinuities. The effect of the
"window" in the frequency domain is to convo-
lute the spectrum of the window with that of the
actual input.
DS27F4
CLKIN
14400
19200
7200
9600
(Hz)
* The values for R and C are as calculated using the described method. Component tolerances have
not been allowed for. Notice that Ko and Kd can vary over a wide range, so using tight tolerances
for R and C is not justified. Use the nearest conveniently available value.
CLKG2
CLKG2
CLKG1
CLKG1
Mode
512
512
256
256
N
Table 2 Example PLL Loop Filter R and C values
CLKOUT
1.8432
2.4576
1.8432
2.4576
(MHz)
1.0
1.0
1.0
1.0
The quality of the window used for harmonic
analysis is typically judged by its highest side-
lobe level. The Blackman-Harris window used to
test the CS5317 has a maximum side-lobe level
of -92 dB.
Figure 7 shows an FFT plot of a typical CS5317
with a 1 kHz sinewave input generated by an "ul-
tra-pure" sine wave generator and the output
multiplied by a Blackman-Harris window. Arti-
facts of windowing are discarded from the
signal-to-noise calculation using the assumption
that quantization noise is white. All FFT plots in
this data sheet were derived by averaging the FFT
results from ten time records. This filters the
spectral variability that can arise from capturing
finite time records, without disturbing the total
energy outside the fundamental. All harmonics
and the -92 dB side-lobes from the Blackman-
Harris window are therefore clearly visible in the
plots.
Signal
Amplitude
Relative to
Full Scale
2262
3016
4524
6032
Figure 7. CS5317 Dynamic Performance
-100dB
-120dB
-20dB
-40dB
-60dB
-80dB
3dB
0dB
dc
1215
1822
2430
911
1 kHz
n
Input Frequency
R * (k )
15.5
15.5
11.6
11.6
Sampling Rate: 19.2 kHz
Full Scale:
S/(N+D): 81.39 dB
CS5317
C * (nF)
+ 2.75 V
_
9.6 kHz
187
106
94
52
15