DAC5686_15 Datasheet, PDF (20/53 Pages) Texas Instruments – 16-BIT, 500-MSPS, 2×–16× INTERPOLATING DUAL-CHANNEL DIGITAL-TO-ANALOG CONVERTER

DAC5686
SLWS147F – APRIL 2003 – REVISED JUNE 2009 ............................................................................................................................................................ www.ti.com
Complex Input
Spectrum
to DAC5686
0
ω
–ω_c
0
ω_c
ω
rspect = 0
ssb = 0
Complex Input
Spectra to
–ω_c
0
Quadrature
Modulator
–ω_c
0
ω_c
ω
ω_c
ω
rspect = 0
ssb = 1
rspect = 1
ssb = 1
–ω_c
0
ω_c
ω
Figure 22. Complex Input Spectrum and DAC Output Spectra
rspect = 1
ssb = 0
M0017-01
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.
DAC Gain and Offset Control
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.
Dual-Channel Real Up-Conversion With NCO Set to f_DAC/2
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 f_DAC/2 – f_IF, where f_IF is the
center frequency of the input spectrum. Note that in this mode f_DAC is limited by the 320-MHz maximum
frequency for NCO operation.
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