Datasheet

NCO
Frequency
Register
32
Accumulator
32
RESETCLK
PHSTR
32
32
Phase
Register
16
16
16
Look-Up
Table
sin
cos
Σ
Σ
16
16
f
DAC
B0026-01
f
NCO
+
freq f
DAC
2
32
DAC5686
www.ti.com ............................................................................................................................................................ SLWS147F – APRIL 2003 – REVISED JUNE 2009
Table 5. Filter Taps for FIR1 – FIR5 (continued)
FIR1 FIR2 FIR3 FIR4 FIR5 (INVSINC)
0
58
0
– 24
0
8
The DAC5686 uses a numerically controlled oscillator (NCO) with a 32-bit frequency register and a 16-bit phase
register. The NCO is used in quadrature-modulation and single-sideband modes to provide sin and cos for
mixing. The NCO tuning frequency is programmed in registers 0x1 through 0x4. Phase offset is programmed in
registers 0x5 and 0x6. A block diagram of the NCO is shown in Figure 44 .
Figure 44. Block Diagram of the NCO
The NCO accumulator is reset to zero when the PHSTR pin is high and remains at zero until PHSTR is set low.
Frequency word freq in the frequency register is added to the accumulator every clock cycle. The output
frequency of the NCO is:
While the maximum clock frequency of the DACs is 500 MSPS, the maximum clock frequency the NCO can
operate at is 320 MHz; mixing at DAC rates higher than 320 MSPS requires using the f
DAC
/4 mixing option.
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