Complex Input

Spectra 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

M0017-01

DAC Gain and Offset Control

Dual-Channel Real Up-Conversion With NCO Set to fDAC/2

DAC5686

SLWS147F – APRIL 2003 – REVISED JUNE 2009 ............................................................................................................................................................ www.ti.com

Figure 22. Complex Input Spectrum and DAC Output Spectra

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.

The final interpolation filter in the DAC5686 can be converted from a low-pass filter to a high-pass filter by

multiplying the interpolation filter output by the (–1)N = 1, –1, 1, –1, … sequence generated by the NCO in

single-sideband mode. The high-pass filter selects a spectrally inverted image at fDAC/2 – fIF, where fIF is the

center frequency of the input spectrum. Note that in this mode fDAC is limited by the 320-MHz maximum

frequency for NCO operation.

20

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