DAC5687_15 Datasheet, PDF (70/80 Pages) Texas Instruments – 16-BIT, 500 MSPS 2´–8´ INTERPOLATING DUAL-CHANNEL DIGITAL-TO-ANALOG CONVERTER (DAC)

DAC5687
SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006
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Application Example: CMTS/VOD Transmitter
The exceptional SNR of the DAC5687 enables a dual-cable modem termination system (CMTS) or video on
demand (VOD) QAM transmitter in excess of the stringent DOCSIS specification, with > 74 dBc and 75 dBc in
the adjacent and alternate channels.
A typical system using the DAC5687 for a cost-optimized dual-channel two-QAM transmitter is shown in
Figure 81. A GC5016 would take four separate symbol rate inputs and provide pulse shaping and interpolation to
~128 MSPS. The four QAM carriers would be combined into two groups of two QAM carriers with intermediate
frequencies of approximately 30 MHz to 40 MHz. The GC5016 would output two real data streams to one
DAC5687. The DAC5687 would function as a dual-channel device and provide 2× interpolation to increase the
frequency of the second Nyquist zone image. The two signals are then output through the two DAC outputs,
through a transformer and to an RF upconverter.
QAM1
QAM2
DUC
Ch2
y2
DUC
DAC5687
DAC
QAM3
QAM4
DUC
Ch1
y2
DUC
DAC
GC5016
CDC7005
Figure 81. Dual-Channel Two-QAM CMTS Transmitter System Using DAC5687
B0044-01
The DAC5687 output for a two-QAM256 carrier signal at 33-MHz and 39-MHz IF with the signal and system
properties listed in Table 19 is shown in Figure 82. The low DAC5687 noise floor provides better than 75 dBc
(equal bandwidth normalized to one QAM256 power) at > 6-MHz offset.
Table 19. Signal and System Properties for Complex IF System Example in Figure 82
Signal
IF frequencies
DAC5687 input rate
DAC5687 output rate
DAC5687 mode
QAM256, 5.36 MSPS, α = 0.12
33 MHz and 39 MHz
5.36 MSPS × 24 = 128.64 MSPS
257.28 MSPS (2× interpolation)
X2
70
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