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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

) 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

, Sinc

or Sinc

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

1 2 3 4 5 6

Sommario

Pagina 1 - CS5530/31/32/33/34:

Pagina 2 - Attenuation (dB)

2 AN311REV1AN311Figure 2 illustrates each Sinc filter function from Sinc1 to Sinc5 with the frequency axis scaled logarithmically andthe output word r

Pagina 3 - 4. The CS5530/31/32/33/34:

AN311REV1 3AN311If the FRS bit is set to logic 1, the converter can provide output word rates of 3200, 1600, 800, 400, 200, 100, 50,25, 12.5, and 6.25

Pagina 4 - -60 dB per Decade

4 AN311REV1AN311With the ADC is configured to output words at 50 Sps, one can determine the attenuation of this filter at 60 Hz asshown in the plot in

Pagina 5 - 735A-3.768-1

AN311REV1 5AN311Figure 5 illustrates the crystal oscillator circuitry of the CS5530/31/32/33/34 series A/D converters. The figure in-cludes the part

Pagina 6 - Revision History

6 AN311REV1AN311Revision HistoryRelease Date ChangesREV1 JUN 2007 Initial ReleaseContacting Cirrus Logic SupportFor all product questions and inquirie

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