Cirrus Logic CS5571 - Manual (Page 24)

CS5571
24 DS768PP1
3/25/08
10:56
Figure 20 illustrates a noise histogram of 4096 samples with the input signal adjusted to almost the exact
center of a code with DITHER on. Figure 21 illustrates a noise histogram of 4096 samples with the input
signal at the center of a code with DITHER off.
Notice that with a signal at the center of a code that the converter outputs the same code over 96% of the
time. Figures 22 and 23 illustrate the noise histogram, DITHER on and then DITHER off with the input
signal at a code boundary. Notice that in the DITHER off case the converter only exhibits two codes of
noise.
3.9 DITHER
From the performance plots, one should conclude that the best AC performance for small signals occurs
with DITHER on. For capturing multiple samples and performing averaging, DITHER should also be on
because the dither will randomize the quantization noise of the converter and provide improved accuracy.
However, if only one conversion is to be taken on a DC input, DITHER should be set to off. With DITHER
off, the converter exhibits only two codes of noise if the signal is at any point other than the exact center
of a code. This means that with DITHER off the converter will nominally yield over 32,000 noise-free
counts.
Output (Codes)
3
2624
9
0
500
1000
1500
2000
2500
3000
3500
4000
4500
-2 -1 0
Output (Codes)
1 2
751 709
0
500
1000
1500
2000
2500
3000
3500
4000
4500
75
3940
81 0 0
-2 -1 0
Output (Codes)
1 2
Figure 20. Noise Histogram, 4096 Samples
Dither On, Code Center
Figure 21. Noise Histogram, 4096 Samples
Dither Off, Code Center
58 0
1953
66
2019
0
500
1000
1500
2000
2500
3000
3500
4000
4500
-2 -1 0
Output (Codes)
1 2
Output (Codes)
2050 2046
-2 -1 0 1 2
0
500
1000
1500
2000
2500
3000
3500
4000
4500
0 0 0
Figure 22. Noise Histogram, 4096 Samples
Dither On, Input at Code Boundary
Figure 23. Noise Histogram, 4096 Samples
Dither Off, Input at Code Boundary