Datasheet

M0221-01
Max output occurs when both
sine and cosine are 0.707
cosine
sine
DAC3484
www.ti.com SLAS749C –MARCH 2011–REVISED AUGUST 2012
Table 8. Fs/2, Fs/4, and –Fs/4 Mixing Sequence (continued)
MODE MIXING SEQUENCE
Iout = {+I1, -I2, +I3, -I4…}
fs/2
Qout = {+Q1, -Q2, +Q3, -Q4…}
Iout = {+I1, -Q2, -I3, +Q4…}
fs/4
Qout = {+Q1, +I2, -Q3, -I4…}
Iout = {+I1, +Q2, -I3, -Q4…}
-fs/4
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
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 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.
Table 9. Coarse Mixer Combinations
Fs/8 Mixer Fs/4 Mixer Fs/2 Mixer –Fs/4 Mixer
cmix(3:0) Mixing Mode
cmix(3) cmix(2) cmix(1) cmix(0)
0000 Disabled Disabled Disabled Disabled No mixing
0001 Disabled Disabled Disabled Enabled –Fs/4
0010 Disabled Disabled Enabled Disabled Fs/2
0100 Disabled Enabled Disabled Disabled +Fs/4
1000 Enabled Disabled Disabled Disabled +Fs/8
1010 Enabled Disabled Enabled Disabled –3Fs/8
1100 Enabled Enabled Disabled Disabled +3Fs/8
1110 Enabled Enabled Enabled Disabled –Fs/8
All others – – – – 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
MIX
t + δ (2N-1) x π/4, where N = 1, 2, 3, etc....
Figure 76. 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.
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