DAC37J82_15 Datasheet, PDF (39/119 Pages) Texas Instruments – DAC3xJ82 Dual-Channel, 16-Bit, 1.6/2.5 GSPS, Digital-to-Analog Converters with 12.5 Gbps JESD204B Interface

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DAC37J82, DAC38J82
SLASE16B – JANUARY 2014 – REVISED MAY 2014
With mixer_gain = 0 in config2, the gain through FMIX is sqrt(2)/2 or –3 dB. This loss in signal power is in most
cases undesirable, and it is recommended that the gain function of the QMC block be used to increase the signal
by 3 dB to compensate. With mixer_gain = 1, the gain through FMIX is sqrt(2) or +3 dB, which can cause
clipping of the signal if I_IN(t) and Q_IN(t) are simultaneously near full scale amplitude and should therefore be used
with caution.
7.3.12 Coarse Mixer
In addition to the full complex mixers the DAC37J82/DAC38J82 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
mixers will result in lower power consumption.
Treating the two complex channels as complex vectors of the form I(t) + j Q(t), the outputs of the coarse mixer,
I_OUT(t) and Q_OUT(t) are equivalent to:
I_OUT(t) = I(t)cos(2πf_CMIXt) – Q(t)sin(2πf_CMIXt)
Q_OUT(t) = I(t)sin(2πf_CMIXt) + Q(t)cos(2πf_CMIXt)
where f_CMIX is the fixed mixing frequency selected by cmix=(fs8, fs4, fs2, fsm4). The mixing combinations are
described in Table 14.
cmix(3:0)
0000
0001
0010
0100
1000
1010
1100
1110
All others
Table 14. Coarse Mixer Combinations
Fs/8 MIXER
cmix(3)
Disabled
Disabled
Disabled
Disabled
Enabled
Enabled
Enabled
Enabled
—
Fs/4 MIXER
cmix(2)
Disabled
Disabled
Disabled
Enabled
Disabled
Disabled
Enabled
Enabled
—
Fs/2 MIXER
cmix(1)
Disabled
Disabled
Enabled
Disabled
Disabled
Enabled
Disabled
Enabled
—
-Fs/4 MIXER
cmix(0)
Disabled
Enabled
Disabled
Disabled
Disabled
Disabled
Disabled
Disabled
—
MIXING MODE
No mixing
–Fs/4
Fs/2
+Fs/4
+Fs/8
–3Fs/8
+3Fs/8
–Fs/8
Not recommended
7.3.13 Dithering
The DAC37J82/DAC38J82 supports the addition of a band limited dither to the DAC output after the complex
mixer. This feature is enabled by set dither_ena to “1” and can be useful in reducing the high order harmonics.
The generated dithering sequence can be optionally up-converted to an offset of Fs/2 by setting
dither_mixer_ena to “1”. The added dithering sequence has variable amplitude in 6 dB steps via
dither_sra_sel.
7.3.14 Complex Summation
The DAC37J82/DAC38J82 has a complex summation block which is to sum channel A with channel C, channel
B with Channel D, and the resulted complex summation are divided by 2 and sent via channel A and channel B.
This feature is enabled by set output_sum to “1” and can be useful for multi-band application.
7.3.15 Quadrature Modulation Correction (QMC)
7.3.15.1 Gain and Phase Correction
The DAC37J82/DAC38J82 includes a Quadrature Modulator Correction (QMC) block. The QMC blocks provide a
mean for changing the gain and phase of the complex signals to compensate for any I and Q imbalances present
in an analog quadrature modulator. The block diagram for the QMC block is shown in Figure 71. The QMC block
contains 3 programmable parameters.
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