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AD6684

Data Sheet

Rev. 0 | Page 44 of 107

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 81 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(ωt)

sin(ωt)

fS/4

I/REAL

0dB

6dB

0dB

6dB

0dB

6dB

0dB

6dB

Figure 81. Complex to Real Conversion Block

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