CS5378 datasheet(44/88 Pages) CIRRUS

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CS5378

DS639F2

13. FIR FILTER

The finite impulse response (FIR) filter block con-

sists of two cascaded stages, FIR1 and FIR2. It

compensates for SINC filter droop and creates a

low-pass corner to block aliased components of the

input signal.

On-chip linear phase or minimum phase coeffi-

cients can be selected using a configuration com-

mand, or the coefficients can be programmed for a

custom filter response.

13.1 FIR1 Filter

The FIR1 filter stage has a decimate by four archi-

tecture. It compensates for SINC filter droop and

flattens the magnitude response of the pass band.

The on-chip linear and minimum phase coefficient

sets are 48-tap, with a maximum 255 programma-

ble coefficients. All coefficients are normalized to

24-bit two’s complement full scale, 0x7FFFFF.

The characteristic equation for FIR1 is a convolu-

tion of the input values, X(n), and the filter coeffi-

cients, h(k), to produce an output value, Y.

Y = [h(k)*X(n-k)] + [h(k+1)*X(n-(k+1))] + ...

13.2 FIR2 Filter

The FIR2 filter stage has a decimate by two archi-

tecture. It creates a low-pass brick wall filter to

block aliased components of the input signal.

The on-chip linear and minimum phase coefficient

sets are 126-tap, with a maximum 255 programma-

ble coefficients. All coefficients are normalized to

24-bit two’s complement full scale, 0x7FFFFF.

The characteristic equation for FIR2 is a convolu-

tion of the input values, X(n), and the filter coeffi-

cients, h(k), to produce an output value, Y.

Y = [h(k)*X(n-k)] + [h(k+1)*X(n-(k+1))] + ...

13.3 On-Chip FIR Coefficients

Two sets of on-chip coefficients, linear phase and

minimum phase, are available for FIR1 and FIR2.

Performance of the on-chip coefficient sets is very

good, with excellent ripple and stop band charac-

teristics as described in Figure 26 and Table 14.

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.

FIR1 Filter - decimate by 4

FIR2 Filter - decimate by 2

Figure 25. FIR Filter Block Diagram

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