AD6674 datasheet(48/91 Pages) AD

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AD6674

Data Sheet

–120

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0.1

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NORMALIZED FREQUENCY (× π RAD/SAMPLE)

Figure 105. HB2 Filter Response

HB1 Filter

The fourth and final decimate by 2, half-band, low-pass, FIR

filter (HB1) uses a 55-tap, symmetrical, fixed coefficient filter

implementation that is optimized for low power consumption.

The HB1 filter is always enabled and cannot be bypassed.

Table 22 and Figure 106 show the coefficients and response of

the HB1 filter.

Table 22. HB1 Filter Coefficients

HB1 Coefficient

Number

Normalized

Coefficient

Decimal

Coefficient (21-Bit)

C1, C55

−0.000023

−24

C2, C54

C3, C53

0.000097

102

C4, C52

C5, C51

−0.000288

−302

C6, C50

C7, C49

0.000696

730

C8, C48

C9, C47

−0.0014725

−1544

C10, C46

C11, C45

0.002827

2964

C12, C44

C13, C43

−0.005039

−5284

C14, C42

C15, C41

0.008491

8903

C16, C40

C17, C39

−0.013717

−14,383

C18, C38

C19, C37

0.021591

22,640

C20, C36

C21, C35

−0.033833

−35,476

C22, C34

C23, C33

0.054806

57,468

C24, C32

C25, C31

−0.100557

−105,442

C26, C30

C27, C29

0.316421

331,792

C28

0.500000

524,288

–120

–100

–80

–60

–40

–20

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

NORMALIZED FREQUENCY (× π RAD/SAMPLE)

Figure 106. HB1 Filter Response

DDC GAIN STAGE

Each DDC contains an independently controlled gain stage.

The gain is selectable as either 0 dB or 6 dB. When mixing a real

input signal down to baseband, it is recommended that the user

enable the 6 dB of gain to recenter the dynamic range of the

signal within the full scale of the output bits.

When mixing a complex input signal down to baseband, the

mixer has already recentered the dynamic range of the signal

within the full scale of the output bits, and no additional gain is

necessary. However, the optional 6 dB gain compensates for low

signal strengths. The downsample by 2 portion of the HB1 FIR

filter is bypassed when using the complex to real conversion

stage.

DDC COMPLEX TO REAL CONVERSION

Each DDC contains an independently controlled complex to

real conversion block. The complex to real conversion block

reuses the last filter (HB1 FIR) in the filtering stage along with

an fS/4 complex mixer to upconvert the signal. After upconvert-

ing the signal, the Q portion of the complex mixer is no longer

needed and is dropped.

Figure 107 shows a simplified block diagram of the complex to

real conversion.

Rev. B | Page 48 of 91

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