Cirrus Logic AN311 User Manual
AN311
CS5530/31/32/33/34:
Rejection of 50 Hz & 60 Hz Line Interference
1. Introduction
The small signals from sensors in an industrial environment can often be affected by interference from the AC power
mains. To improve measurement accuracy, it is desirable that this line interference be removed from the signal being measured. This can be accomplished by using an analog filter or an ADC that has an appropriate digital filter.
This application note presents information about the digital filter in the CS5530/1/2/3/4 series of ADCs. It describes
the filter attenuation characteristics of the filter at 60 Hz when the converter is set to output data at 50 Sps. And it
illustrates how a 3.2768 MHz clock can be used to enable the converter to simultaneously reject both 50 Hz and
60 Hz line interference with a very high level of attenuation.
2. The Sinc Filter & Its Attenuation
Delta-sigma A/D converters consist of a modulator and a digital filter. The digital filter defines the characteristics of
the gain and phase of the frequencies across the converter's passband.
One of the most common filters found in delta-sigma A/D converters that are used in low-frequency measurement
is from a family of sinc filters. The basic sinc filter function can be implemented by averaging a group of samples.
Figure 1 illustrates the attenuation of this type of filter.
20.00
0.00
-20.00
-40.00
-60.00
-80.00
0.10 1 10
Frequency (Hz)
Figure 1. First-order Sinc (Sinc1) Filter Response
In delta sigma converters it is common for the sinc filter to be convolved with itself multiple times. This results in a
multi-order sinc such as Sinc
2
, Sinc3, Sinc4 or Sinc5.
-6 dB per Octave
-20 dB per Decade
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AN311REV1
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Figure 2 illustrates each Sinc filter function from Sinc1 to Sinc5 with the frequency axis scaled logarithmically and
the output word rate normalized to 1.
Sinc1 -6 dB/Octave
2
Sinc
-12 dB/Octave
3
Sinc
-18 dB/Octave
4
Sinc
Attenuation (dB)
Frequency (Hz)
Figure 2. First-order Sinc Filter Response
-24 dB/Octave
5
Sinc
-30 dB/Octave
While a Sinc1 filter has a single zero at the output rate (normalized to 1 in Figure 1), a Sinc2 filter would have two
zeroes at the output word rate. A Sinc
each time the order of the Sinc filter increases, the magnitude response has 6 dB more of attenuation as the frequency doubles. Therefore a Sinc
5
filter will have five zeroes at the output word rate. Notice in Figure 2 that
1
filter has a slope of 6 dB per octave (20 dB per dacade); a Sinc5 filter has a
slope of 30 dB per octave (100 dB per decade).
3. The CS5530/31/32/33/34 Filter:
Attenuating 60 Hz at 50 Sps Output Word Rate
The CS5530/31/32/33/34 series ADCs use Sinc filters. There are two stages of filtering. The first filter is a Sinc
filter. This filter outputs words at the converter's highest rate. The second stage of the digital filter is a programmable
3
Sinc
filter. It accepts the output of the Sinc5 filter stage and provides the remainder of the availableword rates.
The actual output rate depends upon the master clock (nominally 4.9152 MHz) and the setting of the FRS (Filter
Rate Selection) bit in the CS5530/31/32/33/34 configuration register. Refer to Table 1.
.
Table 1. Crystal Frequency vs. Output Word Rate
Crystal Freq. FRS Available Word Rates
4.9152 MHz
If the FRS bit is set to logic 0, the converter can provide output word rates of 3840, 1920, 960, 480, 240, 120, 60,
30, 15, and 7.5 samples per second (Sps). When set to output word rates of 60, 30, 15, or 7.5 Sps, the Sinc
output will provide zeroes in the transfer function that will yield excellent rejection of 60 Hz line interference components.
0 3840, 1920, 960, 480, 240, 120, 60, 30, 15, 7.5
1 3200, 1600, 800, 400, 200, 100, 50, 25, 12.5, 6.25
3
filter
5
2 AN311REV1
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