DAC5687IPZP datasheet(60/79 Pages) TI

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Location of the Spurious Signals

fSIG/fDAC

−0.5

−0.4

−0.3

−0.2

−0.1

−0.0

0.1

0.2

0.3

0.4

0.5

0.0

0.1

0.2

0.3

0.4

0.5

G026

fSIG

fSIG − fDAC/2

(a) Complex Output in X2 Mode

(b) Real Output in X2 Mode

fSIG/fDAC

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.0

0.1

0.2

0.3

0.4

0.5

G027

fSIG

fSIG − fDAC/2

DAC5687

SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006

To calculate the nonharmonic clock-related spurious signals for a particular condition, we first determine the

location of the spurious signals and then the amplitude.

The location of the spurious signals is determined by the DAC5687 output frequency (fSIG) and whether the

output is used as a dual-output complex signal to be fed to an analog quadrature modulator (AQM) or as a real

IF signal from a single DAC output.

Figure 70 shows the location of spurious signals for X2 mode as a function of fSIG/fDAC. For complex outputs, the

spurious frequencies cover a range of –0.5

× f

DAC to 0.5 × fDAC, with the negative complex frequency indicating

that the spurious signal falls in the opposite sideband from the wanted signal at the output of the AQM. For the

real output, the phase information for the spurious signal is lost, and therefore what was a negative frequency for

the complex output is a positive frequency for a real output.

For the X2 mode, there is one spurious frequency with an absolute frequency less than 0.5

× f

DAC. For a complex

output in X2 mode, the spurious signal always is offset fDAC/2 from the wanted signal at fSIG – fDAC/2. For a real

output, as fSIG approaches fDAC/4, the spurious signal frequency falls at fDAC/2 – fSIG, which also approaches

fDAC/4.

Figure 70. Frequency of Clock Mixing Spurious Images in X2 Mode

Figure 71 shows the location of spurious signals for X4 and X4L mode as a function of fSIG/fDAC. The addition of

the fDAC/4 clock frequency for the first interpolation filter creates three new spurious signals. For a complex

output, the nearest spurious signals are fDAC/4 offset from fSIG. For a real output, the signal due to fSIG – fDAC/4

and fSIG – fDAC × 3/4 falls in band as fSIG approaches fDAC/8 and fDAC × 3/8. This creates optimum real output

frequencies fSIG = fDAC × N/16 (N = 1, 3, 5, and 7), where the minimum spurious product offset from fSIG is fDAC/8.

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