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Amplitude Control Circuit for AD9834 Waveform Generator (DDS) (cn0156)

The circuit shown in Figure 1 provides a simple approach for controlling the amplitude of the output waveform of an AD9834 75 MHz low power (20 mW) waveform generator (DDS).

DDS (direct digital synthesis) devices are capable of producing sine wave, square wave, and triangular output waveforms and, therefore, serve as waveform generators.

Capability for phase modulation and frequency modulation is provided internally in the AD9834. 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. This determines the magnitude of the full-scale DAC current.

The DAC used in this example is the 12-bit AD5620, a member of the nanoDAC family. The AD5620 contains an on-chip 5 ppm/°C reference, has an SPI interface, and is available in an 8-lead SOT-23 or MSOP package. The low power (2.2 mW @ 3.3 V) and small size of the AD5620 (8-lead SOT-23) provide an attractive solution for generating an amplitude modulated output from the AD9834.

Figure 1. Low Power Amplitude Control Circuit for AD9834 DDS (Simplified Schematic: All Connections and Decoupling Not Shown)


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