Electronic Design

Boost Instrumentation Accuracy With Direct Digital Synthesis

Direct digital synthesis (DDS) generates arbitrary frequency sinewaves with amazing accuracy and spectral purity. The ability to generate spectrally pure sinewaves at programmable arbitrary frequencies has application in instrumentation and communications products, among other uses.

Today’s DDS components provide extremely accurate, spectrally pure sinewave generation with reasonable power requirements. Modern DDS components have added communications features such as “chirp programming,” FM modulation, AM modulation, programmable gain settings, and the ability to generate period digital data strobes. All are terrific features for an electronic engineer’s bag of tricks.

Orchid Technologies recently applied two 48-bit data-word DDS components from Analog Devices in one of its new instrument designs. These components replaced an older product generation’s 32-bit DDS system (Fig. 1).

The new 48-bit data-word device made it possible to combine the previous discrete waveform generation and gain functions into a single higher-performance component. Removing the external multiplying digital-to-analog converter (DAC) eliminated a significant source of spectral noise and impurity while simplifying the digital data signal path.

New DDS component technology utilizes a 48-bit data word to synthesize output sinewave frequency. The use of a 48-bit data word reduces the output sinewavefrequency error by a factor of 104. Calculation of 48-bit DDS output frequency is:

Output F = (programmed data word × base frequency)/ 281474976710656.0 Older 32-bit DDS devices operated with a base frequency of about 75 MHz, while today’s counterparts operate at 300 MHz. The base frequency increase permits easier low-pass filter design for modern applications. The table compares 32-bit, 75-MHz devices with 48-bit, 300-MHz devices. Qualitatively, both DDS devices have increased frequency error at lower frequencies.

At higher target frequencies, the 48-bit DDS is approximately 105 times more accurate than its older 32-bit counterpart. Of course, when it comes to the design of precision instrumentation, the reduced frequency error results in far better instrument performance. Careful circuit board layout and routing execution was essential in squeezing the most out of our 48-bit DDS components (Fig. 2).

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