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AD9713B Arkusz danych(PDF) 8 Page - Analog Devices |
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AD9713B Arkusz danych(HTML) 8 Page - Analog Devices |
8 / 12 page AD9712B/AD9713B REV. B –8– NUMERICALLY-CONTROLLED OSCILLATOR TUNING WORD 12 PHASE ACCUMULATOR 14 32 OUTPUT SINE DATA TTL REGISTER 12 SYSTEM CLOCK LATCH ENABLE PHASE-TO-AMPLITUDE CONVERSION D D 1 12 AD9712B AD9713B D/A CONVERTER Figure 6. Direct Digital Synthesizer Block Diagram 90 100 10 0% 5mV/div 5ns/div Figure 7. AD9712B/AD9713B Glitch Impulse 1ns/div 90 100 10 0% 200mV/div Figure 8. Rise and Fall Characteristics When the analog frequency (fA) is exactly fC/N and N is an even integer, the DDS continually uses a small subset of the available DAC codes. The DNL of the converter is effectively the DNL error of the codes used, and is typically worse than the error measured against all available DAC codes. This increase in DNL is translated into higher harmonic and noise levels at the output. Glitch impulse, often considered a figure of merit in DDS appli- cations, is simply the initial transient response of the DAC as it moves between two output levels. This nonlinearity is com- monly associated with external data skew, but this effect is mini- mized by using the on-board registers of the AD9712B/AD9713B converters (see Digital Inputs/Timing section). The majority of the glitch impulse, shown below, is produced as the current in the R-2R ladder network settles, and is fairly constant over the full-scale range of the DAC. The fast transients which form the glitch impulse appear as high-frequency spurs in the output spectrum. While it is difficult to predict the effects of glitch on the output waveform, slew rate limitations translate directly into harmonics. This makes slew rate the dominant effect in ac linearity of the DAC. Applications in which the ratio of analog frequency (fA) to clock frequency (fC) is relatively high will benefit from the high slew rate and low output capacitance of the AD9712B/ AD9713B devices. Another concern in DDS applications is the presence of aliased harmonics in the output spectrum. Aliased harmonics appear as spurs in the output spectrum at frequencies which are deter- mined by: MfA ± Nf C where M and N are integers. The effects of these spurs are most easily observed in applica- tions where fA is nearly equal to an integer fraction of the clock rate. This condition causes the aliased harmonics to fold near the fundamental output frequency (see Performance Curves.) |
Podobny numer części - AD9713B_15 |
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Podobny opis - AD9713B_15 |
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