Circuit Function and Benefits The circuit shown in Figure 1 provides a simple method to control the output waveform amplitude of the AD9834, a 75 MHz low power (20 mW) waveform generator (DDS). DDS (direct digital synthesizer) devices can generate sine, square, and triangle output waveforms, so they can be used as waveform generators. The AD9834 has built-in phase modulation and frequency modulation functions. However, in order to modulate the amplitude of the output signal, a low power DAC or digital potentiometer is required to set the full-scale current. A voltage output DAC can be used to drive the FS ADJUST pin of the AD9834 through a series resistor to determine the amplitude of the full-scale DAC current. The DAC used in this example is the 12-bit AD5620 from the nanoDAC series. It has an internal 5 ppm/℃ on-chip reference voltage source, an SPI interface, and an 8-lead SOT-23 or MSOP package with an output voltage of 0 V to +2.5 V. The low power consumption (2.2 mW at 3.3 V) and small size (8-lead SOT-23) make the AD5620 ideal for generating the amplitude modulated output from the AD9834.
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