CS5378 Datasheet, PDF (53/86 Pages) Cirrus Logic – Low-power Single-channel Decimation Filter

CS5378
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.
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