DAC5687IPZP datasheet(38/79 Pages) TI

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Quadrature Modulator Correction (QMC)

DAC5687

SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006

Table 10. Coarse Mixer Sequences

cm_mode(3:0)

Mixing Mode

Sequence

00XX

No mixing

0100

fDAC/2

DAC A = {–A +A –A +A …}

DAC B = {–B +B –B +B …}

0101

fDAC/2

DAC A = {–A +A –A +A …}

DAC B = {+B –B +B –B …}

0110

fDAC/2

DAC A = {+A –A +A –A …}

DAC B = {–B +B –B +B …}

0111

fDAC/2

DAC A = {+A –A +A –A …}

DAC B = {+B –B +B –B …}

1000

fDAC/4

DAC A = {+A –B –A +B …}

DAC B = {+B +A –B –A …}

1001

fDAC/4

DAC A = {+A –B –A +B …}

DAC B = {–B –A +B +A …}

1010

fDAC/4

DAC A = {–A +B +A –B …}

DAC B = {+B +A –B –A …}

1011

fDAC/4

DAC A = {–A +B +A –B …}

DAC B = {–B –A +B +A …}

1100

–fDAC/4

DAC A = {+A +B –A –B …}

DAC B = {+B –A –B +A …}

1101

–fDAC/4

DAC A = {+A +B –A –B …}

DAC B = {–B +A +B –A …}

1110

–fDAC/4

DAC A = {–A –B +A +B …}

DAC B = {+B –A –B +A …}

1111

–fDAC/4

DAC A = {–A –B +A +B …}

DAC B = {–B +A +B –A …}

The output of CMIX is complex. For a real output, either DACA or DACB can be used and the other DAC slept,

the difference being the phase sequence.

The quadrature modulator correction (QMC) block provides a means for changing the phase balance of the

complex signal to compensate for I and Q imbalance present in an analog quadrature modulator. The QMC block

is limited in operation to a clock rate of 320 MSPS.

The block diagram for the QMC block is shown in Figure 40. The QMC block contains three programmable

parameters. Registers qmc_gain_a and qmc_gain_b control the I and Q path gains and are 11-bit values with a

range of 0 to approximately 2. Note that the I and Q gain can also be controlled by setting the DAC full-scale

output current (see following). Register qmc_phase controls the phase imbalance between I and Q and is a

10-bit value with a range of –1/2 to approximately 1/2.

LO feedthrough can be minimized by adjusting the DAC offset feature described as follows.

An example of sideband optimization using the QMC block and gain adjustment is shown in Figure 41. The QMC

phase adjustment in combination with the DAC gain adjustment can reduce the unwanted sideband signal from

~40 dBc to > 65 dBc.

Note that mixing in the CMIX block after the QMC correction destroys the I and Q phase compensation

information from the QMC block.

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