Cirrus Logic AN330 REV2 - Manual (Page 3)

AN330REV2 3
AN330
Figure 3.
In this case, the effective noise bandwidth of the digital filter would be that of the Sinc
3
filter. This is 0.275 times the
sample rate. In the figure above, with a sample rate of 120 Sps, the effective noise bandwidth would be
(0.275)(120) = 33 Hz.
The right half of Figure 3 is the noise characteristic of the CS5513 operating at 107 Sps. The noise in the CS5513
converter behaves differently than that of the ADC with the Sinc
3
filter. The CS5513 is stated to exhibit 12 microvolts
rms noise in its output codes with a VREF voltage of 5 V. The CS5513 runs on an internal oscillator and its conver-
sion rate is about 107 Sps. As shown in the spectral plot, the noise is not attenuated across the frequency spectrum.
This is due to the particular design of the CS5512/CS5513 device in which some noise is aliased back into the pass-
band. Because of this behavior, the noise is nearly flat across the frequency band from 0 to 53.5 Hz even though
the filter has significant attenuation with increasing frequency as shown in the right portion of Figure 3.
Therefore, based upon the spectral plot for the noise which indicates that the noise is flat across frequency in the
CS5513 ADC, an estimate of the noise per
√
Hz would be 12 μV / √
(53.5) = 1640 nV
√
Hz.
0 60 120
180
160
140
120
100
80
60
40
20
0
Frequency (Hz)
Magnitude (-dB)
0 53.5 107
180
160
140
120
100
80
60
40
20
0
Frequency (Hz)
Magnitude (-dB)