AD9540PCB datasheet(20/32 Pages) AD

20 / 32 page

AD9540

Rev. 0 | Page 20 of 32

DDS AND DAC

The precision frequency division within the device is accom-

plished using DDS technology. The DDS can control the digital

phase relationships by clocking a 48-bit accumulator. The

incremental value loaded into the accumulator, known as the

frequency tuning word, controls the overflow rate of the

accumulator. Similar to a sine wave completing a 2π radian

revolution, the overflow of the accumulator is cyclical in nature

and generates a fundamental frequency according to

)

( s

FTW

}

≤

≤ FT

The instantaneous phase of the sine wave is therefore the output

of the phase accumulator block. This signal may be phase-offset

by programming an additive digital phase that is added to each

phase sample coming out of the accumulator.

These instantaneous phase values are then piped through a

phase-to-amplitude conversion (sometimes called an angle-to-

amplitude conversion or AAC) block. This algorithm follows a

cos(x) relationship, where x is the phase coming out of the

phase offset block, normalized to 2π.

Finally, the amplitude words are piped to a 10-bit DAC. Because

the DAC is a sampled data system, the output is a reconstructed

sine wave that needs to be filtered to take high frequency im-

ages out of the spectrum. The DAC is a current steering DAC

that is AVDD referenced. To get a measurable voltage output,

the DAC outputs must be terminated through a load resistor to

AVDD, typically 50 Ω. At positive full scale, IOUT sinks no cur-

rent and the voltage drop across the load resistor is 0. However,

the IOUT output sinks the DAC’s programmed full-scale output

current, causing the maximum output voltage drop across the

load resistor. At negative full-scale, the situation is reversed and

IOUT sinks the full-scale current (and generates the maximum

drop across the load resistor), while IOUT sinks no current

(and generates no voltage drop). At midscale, the outputs sink

equal amounts of current, generating equal voltage drops.

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