Datasheet

I (t)
IN
Q (t)
IN
I (t)
OUT
Q (t)
OUT
16
16
16
16
16
16
cosine sine
DAC3484
SLAS749C –MARCH 2011–REVISED AUGUST 2012 www.ti.com
With the complex mixer enabled, the two channels in the mixer path are treated as complex vectors of the form
I
IN
(t) + j Q
IN
(t). The complex signal multiplier (shown in Figure 75) will multiply the complex channels with the sine
and cosine terms generated by the NCO. The resulting output, I
OUT
(t) + j Q
OUT
(t), of the complex signal multiplier
is:
I
OUT
(t) = (I
IN
(t)cos(2πf
NCO
t + δ) – Q
IN
(t)sin(2πf
NCO
t + δ)) × 2
(mixer_gain – 1)
Q
OUT
(t) = (I
IN
(t)sin(2πf
NCO
t + δ) + Q
IN
(t)cos(2πf
NCO
t + δ)) × 2
(mixer_gain – 1)
where t is the time since the last resetting of the NCO accumulator, δ is the phase offset value and mixer_gain is
either 0 or 1. δ is given by:
δ = 2π × phase_offsetAB/CD(15:0)/2
16
The mixer_gain option allows the output signals of the multiplier to reduce by half (6dB). See Mixer Gain section
for details.
Figure 75. Complex Signal Multiplier
COARSE COMPLEX MIXER
In addition to the full complex mixer, the DAC3484 also has a coarse mixer block capable of shifting the input
signal spectrum by the fixed mixing frequencies ±n×f
S
/8. Using the coarse mixer instead of the full mixer lowers
power consumption.
The output of the fs/2, fs/4, and –fs/4 mixer block is:
I
OUT
(t) = I(t)cos(2πf
CMIX
t) – Q(t)sin(2πf
CMIX
t)
Q
OUT
(t) = I(t)sin(2πf
CMIX
t) + Q(t)cos(2πf
CMIX
t)
Since the sine and the cosine terms are a function of fs/2, fs/4, or –fs/4 mixing frequencies, the possible resulting
value of the terms will only be 1, -1, or 0. The simplified mathematics allows the complex signal multiplier to be
bypassed in any one of the modes, thus mixer gain is not available. The fs/2, fs/4, and –fs/4 mixer blocks
performs mixing through negating and swapping of I/Q channel on certain sequence of samples. Table 8 shows
the algorithm used for those mixer blocks.
Table 8. Fs/2, Fs/4, and –Fs/4 Mixing Sequence
MODE MIXING SEQUENCE
Iout = {+I1, +I2, +I3, +I4…}
Normal (mixer bypassed)
Qout = {+Q1, +Q2, +Q3, +Q4…}
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