Datasheet

DETAILED DESCRIPTION
Dual-Channel Mode
FIR1 FIR2 FIR3 FIR4
IOUTB1
IOUTB2
IOUTA1
IOUTA2
16-Bit
DAC
FIR5
x
sin(x)
DA[15:0]
DB[15:0]
2f
DATA
2y–16y f
DATA
8f
DATA
4f
DATA
16f
DATA
f
DATA
y2
A Gain
A
Offset
FIR1 FIR2 FIR3 FIR4
16-Bit
DAC
FIR5
x
sin(x)
2f
DATA
8f
DATA
4f
DATA
16f
DATA
y2
B
Offset
B Gain
DEMUX
B0020-01
Single-Sideband Mode
DAC5686
www.ti.com ............................................................................................................................................................ SLWS147F – APRIL 2003 – REVISED JUNE 2009
In dual-channel mode, interpolation filtering increases the DAC update rate, thereby reducing sinx/x rolloff and
enabling relaxed analog post-filtering, and is useful for baseband I and Q modulation or two-channel low-IF
signals. The dual-channel mode is set by mode[1:0] = 00 in the config_lsb register. Figure 18 shows the data
path architecture in dual-channel mode. The A- and B-data paths, which are independent, consist of four
cascaded half-band interpolation filters, followed by an optional inverse sinc filter. Interpolation filtering is
selected as 2 × , 4 × , 8 × , or 16 × by sel[1:0] in the config_lsb register. Magnitude spectral responses of each filter
are presented following in the section on digital filtering. Full bypass of all the interpolation filters is selected by
fbypass in the config_lsb register. The inverse sinc filter is intended for use in single-sideband and quadrature
modulation modes and is of limited benefit in dual-channel mode.
Figure 18. Data Path in Dual-Channel Mode
Single-sideband (SSB) mode provides optimum interfacing to analog quadrature modulators. The SSB mode is
selected by mode[1:0] = 01 in the config_lsb register. Figure 19 shows the data path architecture in
single-sideband mode. Complex baseband I and Q are input to the DAC5686, which in turn performs a complex
mix, resulting in Hilbert transform pairs at the outputs of the DAC5686 ' s two DACs. NCO mixing frequencies are
programmed through 32-bit freq (4 registers); 16-bit phase adjustments are programmed through phase (2
registers). The NCO operates at the DAC update rate; thus, increased amounts of interpolation allow for higher
IFs. More details for the NCO are provided as follows. For mixing to f
DAC
/4, DAC5686 provides a specific
architecture that exploits the { … – 1 0 1 0 … } resultant streams from sin and cos; the NCO is shut off in this mode
to conserve power. f
DAC
/4 mix mode is implemented by deasserting nco in register config_msb while in
single-sideband or quadrature modulation mode.
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