AD9833 from Analog Devices

Waveform-a-gogo!

While arbitrary waveform generators are among the most helpful tools in the laboratory, they tend to spend most of their time outputting sines, triangles, and rectangles intended to stimulate sensors or actors. Analog Devices AD9833 is a compact IC dedicated to precise waveform emission.

From a technical point of view, the part – its outline is shown in figure one – is but a simple implementation of the DDS principle.

the analog part of the AD9833 is pretty standardized

The analog part of the AD9833 is pretty standardized

The part has an internal pulldown resistor of 200 Ohms, and supplies a 0.6V pp output signal. Its 10 bit resolution is used fully, as the sinewave lookup table consists of 12bit wide quines. This leads to limited spectral purity – if you need an extremely clean sine, the AD9833 is not your part.

As for input voltages, levels from 2.3 to 5.5 volts pose no problems for the chip. Bringing the part up is not particularily difficult: an external clock frequency must be provided by the host circuit. Other than that, Analog Devices recommends heavy decoupling as the part can work at speeds of up to 25 MhZ.

Digital intelligence

Output frequency and desired waveform are set using an SPI interface. Two 16 bit words are needed for a complete parametrization, the datasheet provides further information on the actual word format. In theory, the part can change the desired frequency quite fast – a latency of between seven and eight MCLK ticks is required until new settings take effect.
Due to advancements in TMI, an AD9833-based function generator is unlikely to find many a buyer outside of education. The part should, instead, be used for sensor and TDR applications: if a detector works best with sines or triangles, Analog Devices saves you the effort of performing DDS.

Finally, let us give an honorable mention to the datasheet by clicking here. It provides a decently thorough discussion of the design, and might be interesting for process computer jockeys.

Is it worth it?

If your application needs sines or triangles with variable frequency (and can live with the limited resolution of the steps), the AD9833 definitely is a good choice. Using it purely as a squarewave oscillator, on the other hand, must be assessed carefully – a digital counter based solution can be cheaper.

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