CS5378 datasheet(53/88 Pages) CIRRUS

53 / 88 page

CS5378

DS639F2

characteristic equations model the operation of the

2nd order IIR filter with unnormalized coefficients.

W5 = W4

W4 = W3

W3 = X + (-a21 * W4) + (-a22 * W5)

Y = (W3 * b20) + (W4 * b21) + (W5 * b22)

Internally, the CS5378 uses normalized coeffi-

cients to perform the 2nd order IIR filter calcula-

tion, which changes the algorithm slightly. The

following characteristic equations model the oper-

ation of the 2nd order IIR filter when using normal-

ized coefficients.

W5 = W4

W4 = W3

W3 = 2 * [(X / 2) + (-a21 * W4) + (-a22 * W5)]

Y = 2 * [(W3 * b20) + (W4 * b21) + (W5 * b22)]

14.4 IIR3 Filter

The 3rd order IIR filter is implemented by running

both the 1st order and 2nd order IIR filter stages. It

can be modeled by cascading the characteristic

equations of the 1st order and 2nd order IIR stages.

14.5 On-Chip IIR Coefficients

Five sets of on-chip coefficients are available for

IIR1 and IIR2, each providing a 3 Hz high-pass

Butterworth response at different output word

rates. Characteristics of the on-chip coefficient sets

are described in Figure 31 and Table 16.

Which on-chip coefficient set to use is selected by

a data word following the ‘Write ROM Coeffi-

cients’ configuration command. See “Filter Coef-

ficient Selection” on page 38 for information about

selecting on-chip coefficient sets.

14.6 Programmable IIR Coefficients

A maximum of 3 + 5 coefficients can be pro-

grammed into IIR1 and IIR2 to create a custom fil-

ter response. Custom filter sets should normalize

the coefficients to 24-bit two’s complement full

scale, 0x7FFFFF. To maintain maximum internal

dynamic range, the CS5378 IIR filter performs

double precision calculations with an automatic

gain correction to scale the final output.

Custom IIR coefficients are uploaded using the

‘Write IIR Coefficients’ configuration command.

See “EEPROM Configuration Commands” on

page 27 or “Microcontroller Configuration Com-

mands” on page 33 for information about writing

custom IIR coefficients.

14.7 IIR Filter Synchronization

The IIR filter is not synchronized to the external

system directly, only indirectly through the syn-

chronization of the SINC and FIR filters. Because

IIR filters have ‘infinite’ memory, a discontinuity

in the input data stream from a synchronization

event can require significant time to settle out. The

exact settling time depends on the size of the dis-

continuity and the filter coefficient characteristics.

CS5378 Datasheet(PDF) 53 Page - Cirrus Logic

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