DAC5687IPZP datasheet(69/79 Pages) TI

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Application Example: Wide-Bandwidth Direct Baseband-to-RF Conversion

B0043-01

DAC

Processing

TRF3750

DAC5687

DAC

Phase/

Gain/

Offset

Adjust

TRF3701

TRF3702

TRF3703

GC1115

and DPD

Processor

DAC5687

SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006

Table 18. Signal and System Properties for Complex IF System Example in Figure 79

Signal

Three WCDMA carriers, test model 1

Baseband carrier offsets

–7.5 MHz, 2.5 MHz, 7.5 MHz

DAC5687 input rate

122.88 MSPS

DAC5687 output rate

491.52 MSPS (4

× interpolation)

DAC5687 mode

X4 CMIX

DAC5687 complex IF

122.88 MHz (fDAC/4)

LO frequency

2140 MHz

The complex IF has several advantages over the real IF architecture such as:

• Uncalibrated sideband suppression, ~35 dBc compared to 0 dBc for real IF architecture.

• Direct DAC–complex-mixer connection—no amplifiers

• Nonharmonic clock-related spurious signals fall out-of-band

• DAC second Nyquist zone image is offset f

DAC compared with fDAC – 2 × IF for a real IF architecture, reducing

the need for filtering at the DAC output.

• Uncalibrated LO feedthrough for AQM is ~35 dBc and calibration can reduce or completely remove the LO

feedthrough.

A system example of the DAC5687 used in a wide-bandwidth direct baseband-to-RF conversion is shown in

Figure 80. The DAC input would typically be generated by a crest factor reduction processor such as Texas

Instruments GC1115 and digital predistortion processor. With a complex baseband input, the DAC5687 would be

used to increase the data rate through interpolation. In addition, phase, gain, and offset correction of the IQ

imbalance is possible using the QMC block, DAC gain, and DAC offset features. The correction could be done

one time during manufacturing (see the TRF3701 data sheet (SLWS145) and the TRF3702 data sheet

(SLWS149) for the variation with temperature, supply, LO frequency, etc., after calibration at nominal conditions)

or during operation with a separate feedback loop measuring imbalance in the RF signal.

Figure 80. Direct Conversion System Using DAC5687 in X4L Mode

Operating at baseband has the advantage that the DAC5687 output is insensitive to DAC sample clock phase

noise, so using the DAC PLL clock mode has similar spectral performance to the external clock mode. In

addition, the nonharmonic clock-related spurious signals are small due to the low DAC output frequency.

With a complex input rate specified up to 250 MSPS, the DAC5687 is capable of producing signals with up to

200-MHz bandwidth for systems such as digital predistortion (DPD).

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