DAC5687IPZP datasheet(62/72 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

SLWS164B – FEBRUARY 2005 – REVISED JUNE 2005

Figure 72. Amplitude Adjustment Factor for fSIG

The steps for calculating the non-harmonic spurious signals are:

1. Find the spurious signal frequencies for the appropriate mode from Figure 67, Figure 68, or Figure 69.

2. Find the amplitude for each spurious frequency for the appropriate mode from Figure 70 or Figure 71.

3. Adjust the amplitude of the spurious signals for f

SIG using the adjustment factor in Figure 72.

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 x fDAC

First, the location of the spurious signals is found for the x4 complex output in Figure 68(a). Three spurious

signals are present in the range -0.5 x fDAC to 0.5 x fDAC: two from fDAC/4 (35 MHz and -215 MHz) and one from

fDAC/2 (-90 MHz). Consulting Figure 70©), the raw amplitudes for fDAC/2 and fDAC/4 are 60 and 58 dBc,

respectively. From Figure 72, the amplitude adjustment factor for fSIG = 0.32 x fDAC is estimated at ~ 1 dB and so

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

Table 15. Example # 1 for Calculating Spurious Signals

Spurious Sig-

Frequency/fDAC

Frequency

Raw Amplitude (dBc)

Adjusted Amplitude (dBc)

nal

(MHz)

fDAC/4

0.7

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 x fDAC

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