DAC5687IPZP datasheet(64/79 Pages) TI

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fSIG/fDAC

−20

−10

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

G040

DAC5687

SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006

Figure 75. Amplitude Adjustment Factor for fSIG

The steps for calculating the nonharmonic spurious signals are:

1. Find the spurious signal frequencies for the appropriate mode from Figure 70, Figure 71, or Figure 72.

2. Find the amplitude for each spurious frequency for the appropriate mode from Figure 73 or Figure 74.

3. Adjust the amplitude of the spurious signals for fSIG using the adjustment factor in Figure 75.

Consider Example 1 with the following conditions:

1. X4 Mode

2. PLL off

3. Complex output

4. fDAC = 500 MHz

5. fSIG = 160 MHz = 0.32 × fDAC

First, the location of the spurious signals is found for the X4 complex output in Figure 71(a). Three spurious

signals are present in the range –0.5

× f

DAC to 0.5 × fDAC: two from fDAC/4 (35 MHz and –215 MHz) and one from

fDAC/2 (–90 MHz). Consulting Figure 73(c), the raw amplitudes for fDAC/2 and fDAC/4 are 60 and 58 dBc,

respectively. From Figure 75, the amplitude adjustment factor for fSIG = 0.32 × fDAC is estimated at ~1 dB, and so

the fDAC/2 and fDAC/4 are adjusted to 61 and 59 dBc.

Table 16. Example # 1 for Calculating Spurious Signals

Spurious

Frequency

Frequency/fDAC

Raw Amplitude (dBc)

Adjusted Amplitude (dBc)

Signal

(MHz)

fDAC/4

0.07

fDAC/2

–0.18

–90

fDAC/4

–0.43

–215

Now consider Example 2 with the following conditions:

1. X2 Mode

2. PLL on

3. Real output

4. fDAC = 400 MHz

5. fSIG = 70 MHz = 0.175 × fDAC

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