Cirrus Logic AN311 - Manual

Copyright © Cirrus Logic, Inc. 2007
(All Rights Reserved)
http://www.cirrus.com
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 be-
ing 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.
Figure 1. First-order Sinc (Sinc
1
) 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
, Sinc
3
, Sinc
4
or Sinc
5
.
Frequency (Hz)
-80.00
-60.00
-40.00
-20.00
0.00
20.00
0.10 1 10
-6 dB per Octave
-20 dB per Decade
AN311
JUN
‘
07
AN311REV1