DAC5687MPZPEP datasheet(63/75 Pages) TI1

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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-EP

SGLS333 – JUNE 2006

Figure 73. 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 68, Figure 69, or Figure 70.

2. Find the amplitude for each spurious frequency for the appropriate mode from Figure 71 or Figure 72.

3. Adjust the amplitude of the spurious signals for fSIG using the adjustment factor in Figure 73.

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 69(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 71, the raw amplitudes for fDAC/2 and fDAC/4 are 60 and 58 dBc,

respectively. From Figure 73, 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 15. Example 1 for Calculating Spurious Signals

Spurious

Frequency/fDAC

Frequency

Raw Amplitude (dBc)

Adjusted Amplitude (dBc)

Signal

(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 × fDAC

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