DAC3484 Datasheet, PDF (58/82 Pages) Texas Instruments – Quad-Channel, 16-Bit, 1.25 GSPS Digital-to-Analog Converter (DAC)

DAC3484
SLAS749A – MARCH 2011 – REVISED JULY 2011
www.ti.com
MODE
fs/2
fs/4
-fs/4
Table 8. Fs/2, Fs/4, and –Fs/4 Mixing Sequence (continued)
MIXING SEQUENCE
Iout = {+I1, -I2, +I3, -I4…}
Qout = {+Q1, -Q2, +Q3, -Q4…}
Iout = {+I1, -Q2, -I3, +Q4…}
Qout = {+Q1, +I2, -Q3, -I4…}
Iout = {+I1, +Q2, -I3, -Q4…}
Qout = {+Q1, -I2, -Q3, +I4…}
The fs/8 mixer can be enabled along with various combinations of fs/2, fs/4, and –fs/4 mixer. Since the fs/8 mixer
uses the complex signal multiplier block with fixed fs/8 sine and cosine term, the output of the multiplier is:
I_OUT(t) = (I_IN(t)cos(2πf_NCOt + δ) – Q_IN(t)sin(2πf_NCOt + δ)) × 2^{(mixer_gain – 1)}
Q_OUT(t) = (I_IN(t)sin(2πf_NCOt + δ) + Q_IN(t)cos(2πf_NCOt + δ)) × 2^{(mixer_gain – 1)}
where f_CMIX is the fixed mixing frequency selected by cmix(3:0). The mixing combinations are described in
Table 9. The mixer_gain option allows the output signals of the multiplier to reduce by half (6dB). See Mixer Gain
section for details.
cmix(3:0)
0000
0001
0010
0100
1000
1010
1100
1110
All others
Fs/8 Mixer
cmix(3)
Disabled
Disabled
Disabled
Disabled
Enabled
Enabled
Enabled
Enabled
–
Table 9. Coarse Mixer Combinations
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
MIXER GAIN
The maximum output amplitude out of the complex signal multiplier (i.e., FMIX mode or CMIX mode with fs/8
mixer enabled) occurs if I_IN(t) and Q_IN(t) are simultaneously full scale amplitude and the sine and cosine
arguments are equal to 2π x f_MIXt + δ (2N-1) x π/4, where N = 1, 2, 3, etc....
cosine
sine
Max output occurs when both
sine and cosine are 0.707
M0221-01
Figure 74. Maximum Output of the Complex Signal Multiplier
With mixer_gain = 1 and both I_IN(t) and Q_IN(t) are simultaneously full scale amplitude, the maximum output
possible out of the complex signal multiplier is 0.707 + 0.707 = 1.414 (or 3dB). This configuration can cause
clipping of the signal and should therefore be used with caution.
With mixer_gain = 0 in config2, the maximum output possible out of the complex signal multiplier is 0.5 x (0.707
+ 0.707) = 0.707 (or -3dB). 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.
58
Copyright © 2011, Texas Instruments Incorporated