Datasheet

Q(t)

I(t)

qmc_gaina(10:0)

qmc_gain b(10:0)

qmc _phase (9:0)

qmb_offset

{-4096, - 4095, … ,4095}

qma_offset

{-4096, - 4095, … ,4095}

DAC5688

SLLS880C –DECEMBER 2007–REVISED AUGUST 2010

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Quadrature Modulator Correction (QMC)

The Quadrature Modulator Correction (QMC) block provides a means for adjusting the gain and phase of the

complex signal. At a quadrature modulator output, gain and phase imbalances result in an undesired sideband

signal.

The block diagram for the QMC is shown in Figure 30. The QMC block contains 3 programmable parameters:

qmc_gaina(10:0), qmc_gainb(10:0) and qmc_phase(9:0).

Registers qmc_gaina(10:0) and qmc_gainb(10:0) control the I and Q path gains and are 11 bit values with a

range of 0 to approximately 2. This value is used to scale the signal range. Register qmc_phase(9:0) controls the

phase imbalance between I and Q and is a 10-bit value that ranges from –1/8 to approximately +1/8. This value

is multiplied by each Q sample then summed into the I sample path. This operation is a simplified approximation

of a true phase rotation and covers the range from –7.5 to +7.5 degrees in 1024 steps.

Figure 30. QMC Block Diagram

DAC Offset Control

The qmc_offseta(12:0) and qmc_offsetb(12:0) values can be used to independently adjust the I and Q path DC

offsets. Both offset values are in represented in 2s-complement format with a range from –4096 to 4095.

The offset value adds a digital offset to the digital data before digital-to-analog conversion. Since the offset is

added directly to the data it may be necessary to back off the signal to prevent saturation. Both data and offset

values are LSB aligned.

Figure 31. DAC Offset Block

Product Folder Link(s): DAC5688