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Complex Input

Spectrums

to Quadrature

Modulator

0

ω

ω

c

–

ω

c

0

ω

0

ω

0

ω

0

ω

rspect = 0

ssb = 0

rspect = 1

ssb = 0

rspect = 1

ssb = 1

rspect = 0

ssb = 1

Complex Input

Spectrum

to DAC5686

ω

c

–

ω

c

ω

c

–

ω

c

ω

c

–

ω

c

DAC Gain and Offset Control

Quadrature Modulation Mode

DAC5686

SLWS147B – APRIL 2003 – REVISED AUGUST 2004

DETAILED DESCRIPTION (continued)

Figure 22. Spectrum After First and Second Up-Converson for Complex Input

To compensate for the sinx/x rolloff of the zero-order hold of the DACs, the DAC5686 provides an inverse sinc

FIR, which provides high-frequency boost. The magnitude spectral response of this filter is presented in the

Digital Filters section.

Unmatched gains and offsets at the RF quadrature modulator result in unwanted sideband and local-oscillator

feedthrough. Gain and offset imbalances between the two DACs are compensated for by programming

daca_gain, dacb_gain, daca_offset, and dacb_offset in registers 0x0A through 0x0F (see the following

register descriptions). The DAC gain value controls the full-scale output current. The DAC offset value adds a

digital offset to the digital data before digital-to-analog conversion. Care must be taken when using the offset by

restricting the dynamic range of the digital signal to prevent saturation when the offset value is added to the

digital signal.

In quadrature modulation mode, on-chip mixing of complex I and Q inputs provides the final baseband-to-IF

up-conversion. Quadrature modulation mode is selected by mode[1:0] = 10 in the config_lsb register. Figure 23

shows the data path architecture in quadrature modulation mode. Complex baseband I and Q from the

ASIC/FPGA are input to the DAC5686, which in turn quadrature modulates I and Q to produce the final IF

single-sideband spectrum. DAC A is held constant, while DAC B presents the DAC5686 quadrature modulator

mode output.

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