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Part NameDescriptionManufacturer
DAC5687 16-BIT, 500 MSPS 2?8 INTERPOLATING DUAL-CHANNEL DIGITAL-TO-ANALOG CONVERTER (DAC) Texas-Instruments
Texas Instruments Texas-Instruments
DAC5687 Datasheet PDF : 80 Pages
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DAC5687
SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006
www.ti.com
Dual-Channel Real Upconversion
The DAC5687 can be used in a dual-channel mode with real upconversion by mixing with a 1, –1, … sequence
in the signal chain to invert the spectrum. This mixing mode maintains isolation of the A and B channels. There
are two points of mixing: in X4L mode, the FIR1 output is inverted (high-pass mode) by setting registers hpla and
hplb to 1, and the FIR3 output is inverted by setting CMIX to fDAC/2. In X8 mode, the output of FIR1 is inverted
by setting hpla and hplb to 1, and the FIR3 output is inverted by setting CMIX to fDAC/2. In X2 and X4 modes,
the output of FIR3 is inverted by setting CMIX to fDAC/2.
The wide bandwidth of FIR3 (40% passband) in X4L mode provides options for setting four different frequency
ranges, listed in Table 9. For example, with fDATA = 125 MSPS (fDAC = 500 MSPS), setting FIR1/FIR3 to High
Pass/High Pass, respectively, upconverts a signal between 25 MHz and 50 MHz to a signal between 150 MHz
and 175 MHz. With the High Pass/Low Pass and Low Pass/High Pass settings, the upconvertered signal is
spectrally inverted.
FIR1
Low pass
High pass
High pass
Low pass
Table 9. X4L Mode High-Pass/Low-Pass Options
FIR3
Low pass
Low pass
High pass
High pass
Input Frequency
0–0.4 × fDATA
0.2 to 0.4 × fDATA
0.2 to 0.4 × fDATA
0–0.4 × fDATA
Output Frequency
0–0.4 × fDATA
0.6–0.8 × fDATA
1.2–1.4 × fDATA
1.6–2 × fDATA
Bandwidth
0.4 × fDATA
0.2 × fDATA
0.2 × fDATA
0.4 × fDATA
Inverted?
No
Yes
No
Yes
Limitations on Signal BW and Final Output Frequency in X4L and X8 Modes
For very wide-bandwidth signals, the FIR3 pass band (0–0.4 × fDAC/2) can limit the range of the final output
frequency. For example, in X4L FMIX CMIX mode (4× interpolation with FMIX after FIR1), at the maximum input
data rate of fIN = 125 MSPS, the input signal can be ±50 MHz before running into the transition band of FIR1.
After 2× interpolation, FIR3 limits the signal to ±100 MHz (0.4 × 250 MHz). Therefore, at the maximum signal
bandwidth, FMIX can mix up to 50 MHz and still fall within the pass band of FIR3. This results in gaps in the final
output frequency between FMIX alone (0 MHz to 50 MHz) and FMIX + CMIX with fDAC/4 (75 MHz to 175 MHz)
and FMIX + CMIX with fDAC/2 (200 MHz to 250 MHz).
In practice, it may be possible to extend the signal into the FIR3 transition band. Referring to Figure 36 in the
preceding FIR Filters section, if 0.5 dB of attenuation at the edge of the signal can be tolerated, then the signal
can be extended up to 0.44 × fIN. This would extend the range of FMIX in the example to 60 MHz.
Fine Mixer (FMIX)
The fine mixer block FMIX uses a numerically controlled oscillator (NCO) with a 32-bit frequency register
freq(31:0) and a 16-bit phase register phase(15:0) to provide sin and cos for mixing. The NCO tuning frequency
is programmed in registers 0x09 through 0x0C. Phase offset is programmed in registers 0x0D and 0x0E. A block
diagram of the NCO is shown in Figure 39.
32
Frequency 32
Register
Σ
32 Accumulator 32 16
Σ
16
CLK RESET
fNCO_CLK
PHSTR
16
Phase
Register
Look-Up
Table
16
sin
16
cos
Figure 39. Block Diagram of the NCO
B0026-02
36
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