CS5530 datasheet(27/36 Pages) CIRRUS

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CS5530

DS742F3

2.8 Digital Filter

The CS5530 has a linear phase digital filter which

is programmed to achieve a range of output word

rates (OWRs) as stated in the Configuration Regis-

ter Description section. The ADC uses a Sinc5 dig-

ital filter to output word rates at 3200 Sps and 3840

Sps (MCLK = 4.9152 MHz). Other output word

rates are achieved by using the Sinc5 filter followed

by a Sinc3 filter with a programmable decimation

rate.Figure 13 shows the magnitude response of the

60 Sps filter, while Figures 14 and 15 show the

magnitude and phase response of the filter at 120

Sps. The Sinc3 is active for all output word rates

except for the 3200 Sps and 3840 Sps (MCLK =

4.9152 MHz) rate. The Z-transforms of the two fil-

ters are shown in Figure 16. For the Sinc3 filter,

“D” is the programmable decimation ratio, which is

equal to 3840/OWR when FRS = 0 and 3200/OWR

when FRS = 1.

The converter’s digital filters scale with MCLK.

For example, with an output word rate of 120 Sps,

the filter’s corner frequency is at 31 Hz. If MCLK

is increased to 5.0 MHz, the OWR increases by

1.0175 percent and the filter’s corner frequency

moves to 31.54 Hz. Note that the converter is not

specified to run at MCLK clock frequencies greater

than 5 MHz.

Figure 13. Digital Filter Response (Word Rate = 60 Sps)

-120

-80

-40

120

180

240

300

Frequency (Hz)

FRS = 0

-120

-80

-40

040

120

Frequency (Hz)

Flatness

Frequency

2-0.01

4-0.05

6-0.11

8-0.19

-0.30

-0.43

-0.59

-0.77

-1.09

-3.13

Figure 14. 120 Sps Filter Magnitude Plot to 120 Hz

-180

-90

180

120

Frequency (Hz)

Figure 15. 120 Sps Filter Phase Plot to 120 Hz

Note:

See the text regarding the Sinc3 filter’s

decimation ratio “D”.

Sinc5

z 80

–

()5

z 16

–

()5

--------------------------

z 16

–

()3

z 4

–

()3

--------------------------

z 4

–

()2

z 2

–

()2

-----------------------

z 2

–

()3

z 1

–

()3

-----------------------

Sinc3

z D

–

()3

z 1

–

()3

-------------------------

Figure 16. Z-Transforms of Digital Filters

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