AFE8220TPZPQ1 datasheet(22/49 Pages) TI1

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(z)=

EVEN

NCOEFF

n=0

COEFF

BASE_ADDR+ n

)

NCOEFF

(13)

(z)=

ODD

NCOEFF

n=0

COEFF

BASE_ADDR+ n

BASE_ADDR+ NCOEFF

)+COEFF

NCOEFF

(14)

(z)=

HALFBAND

NCOEFF

n=0

COEFF

BASE_ADDR+ n

BASE_ADDR+ NCOEFF

)+COEFF

NCOEFF

2n)

NCOEFF

(z)=

ARBITRARY

NCOEFF

n=0

COEFF

BASE_ADDR+ n

-n

(16)

GAIN

(z)=2

EVEN

-SHIFT

2 ´

NCOEFF

n=0

COEFF

BASE_ADDR+ n

(17)

GAIN

ODD

HALFBAND

=GAIN

-SHIFT

2 ´

NCOEFF

n=0

COEFF

+COEFF

BASE_ADDR+ n

BASE_ADDR+ NCOEFF

(18)

GAIN

(z)=2

ARBITRARY

-SHIFT

NCOEFF

n=0

COEFF

BASE_ADDR+ n

(19)

AFE8220-Q1

SBAS455A – JANUARY 2009 – REVISED JULY 2009....................................................................................................................................................... www.ti.com

Extended-Length Filter Mode

If FIR2A or FIR2B cannot provide enough filter taps to achieve the desired frequency response, setting control bit

ddc0_interleave puts the two filters into an interleaved mode that doubles the length of the filter that can be

realized. However, there are several limitations:

1. Only odd symmetrical filters may be realized;

2. The filter length M must be such that (M + 1)/4 is an integer; and

3. Only one filter can be realized (in ddc_interleave mode the A and B outputs are identical: IB = IA and

QB = QA).

In addition to setting the ddc0_interleave bit, FIR2A must be set to EVEN mode and FIR2A must be set to ODD

mode. ddc0_fir2a_ncoeff and ddc0_fir2b_ncoeff are both set to (M + 1)/4. ddc0_fir2a_shift and ddc0_fir2b_shift

should be identical. There are no restrictions on ddc0_fir2a_base_addr or ddc0_fir2b_base_addr.

The M-tap filter has (M + 1)/2 unique coefficients. The first, third, fifth, etc. coefficients are loaded into the FIR2A

coefficient memory; the second, fourth, sixth, etc. coefficients are loaded into the FIR2B memory. The center

coefficients of the filter end up as the last coefficient loaded into FIR2B.

FIR Filter Transfer Functions

Equation 13 to Equation 21 show transfer functions and dc gain for the various filter modes. Generic names for

the control variables are used; just substitute the appropriate variable (that is, ddc0_fir2a_ncoeff for NCOEFF) as

necessary. Also, note that SHIFT has a value of 0 for FIR1.

Basic Filter Modes

(15)

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