AD6679 datasheet(52/82 Pages) AD

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Data Sheet

AD6679

Rev. B | Page 51 of 81

–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 70. 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 71 shows a simplified block diagram of the complex to

real conversion.

LOW-PASS

FILTER

REAL

HB1 FIR

LOW-PASS

FILTER

HB1 FIR

COMPLEX TO

REAL ENABLE

90°

0°

–

COMPLEX TO REAL CONVERSION

GAIN STAGE

cos(wt)

sin(wt)

fS/4

I/REAL

0dB

6dB

0dB

6dB

0dB

6dB

0dB

6dB

Figure 71. Complex to Real Conversion Block

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