CS5376 View Datasheet(PDF) - Cirrus Logic

Part Name

Description

MFG CO.

CS5376

Low Power Multi-Channel Decimation Filter

Cirrus Logic 

CS5376

filter output bypasses the 2nd order stage, while se-

lection of a 2nd order IIR filter output bypasses the

1st order stage. Selection of a 3rd order IIR filter

output runs both the 1st and 2nd order IIR filter

stages.

8.4.1 1st Order IIR

The 1st order IIR filter stage is a direct form filter

with three coefficients: a11, b10, and b11. Coeffi-

cients of a 1st order IIR are inherently normalized

to one, and should be scaled to 24-bit 2’s comple-

ment full scale, 0x7FFFFF (decimal 8388607).

The characteristic equations for the 1st order IIR

include an input value, X, an output value, Y, and

two intermediate values, W1 and W2, separated by

a delay element (z-1).

W2 = W1

W1 = X + (-a11 * W2)

Y = (W1 * b10) + (W2 * b11)

8.4.2 2nd Order IIR

The 2nd order IIR filter stage is a direct form filter

with five coefficients: a21, a22, b20, b21, and b22.

Coefficients of a 2nd order IIR are inherently nor-

malized to two, and should be scaled to 24-bit 2’s

complement full scale, 0x7FFFFF (decimal

8388607). Normalization effectively divides the

2nd order coefficients in half relative to the input

samples, and requires a modification to the charac-

teristic equations.

The characteristic equations for the 2nd order IIR

include an input value, X, an output value, Y, and

three intermediate values, W3, W4, and W5, each

separated by a delay element (z-1). The following

characteristic equations are used to model the oper-

ation 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 CS5376 uses normalized coeffi-

cients to perform the 2nd order IIR filter calcula-

tion, which changes the algorithm slightly. The

following characteristic equations are used to mod-

el the operation of the 2nd order IIR filter when us-

ing normalized coefficients.

W5 = W4

W4 = W3

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

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

8.4.3 3rd Order IIR

The 3rd order IIR filter is implemented by running

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

This filter can be modeled by cascading the charac-

teristic equations of the 1st order and 2nd order IIR

stages, with the 1st order output passed as an input

to the 2nd order stage.

8.4.4 IIR coefficient upload

Custom IIR coefficients are uploaded through the

SPI 1 serial port using the ‘Write IIR Coefficients’

command to perform a burst write of both the 1st

order and 2nd order IIR coefficients. See “Serial

Peripheral Interface 1” on page 21 for information

about how to upload custom IIR coefficients.

8.4.5 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 requires significant time to settle out. The ex-

act settling time depends on the filter coefficient

characteristics.

DS256PP1

62

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