Circuit functions and advantages

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 are capable of generating sine, square, and triangle output waveforms and therefore can be used as waveform generators.

AD9834 has built-in phase modulation and frequency modulation functions. However, 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 AD9834's FS ADJUST pin through a series resistor to determine the magnitude of the full-scale DAC current.

The DAC used in this example is the 12-bit AD5620 of the nano DAC series . It has a 5 ppm/°C on-chip voltage reference, an SPI interface, and is available in an 8-pin SOT-23 or MSOP package. The low power consumption (2.2 mW at 3.3 V supply) and small size (8-lead SOT-23) make the AD5620 ideally suited for generating the amplitude modulated output from the AD9834.

Circuit description

This circuit operates from a single 3 V to 5 V supply. Both DAC and DDS work using SPI interface. Many DDS devices have on-chip DACs that provide complementary current outputs IOUT and IOUTB to the AD9834 DDS.

The DAC's reference current is a function of the internal reference voltage V _REF and the external resistor R _SET , which is typically connected from the DAC FS ADJUST pin to ground. The reference current is equal to V _REF /R _SET , where V _REF is the AD9834's internal reference voltage, typically 1.20 V. The typical value of the R _SET resistor is 6.8 kΩ.

The DAC's full-scale current is a multiple of the reference current. For example, the full-scale current of the AD9834 is:

If FS ADJUST is connected to a variable voltage VDAC, the full-scale current is: 

Changing V _DAC will change the full-scale current and thus the voltage output of the DDS device. A voltage output DAC can be used to provide this variable voltage.

The AD5620 is a suitable low power, small size, and cost-effective solution. It belongs to the nano DAC series and has a built-in 5 ppm/°C on-chip reference voltage source. It is available in an 8-pin SOT-23 or MSOP package and has an output voltage of 0 V to +2.5 V.

When V _DAC = 0 V (zero scale), the maximum full-scale output current is reached, and the current of the AD9834 varies from approximately 0.16 mA to approximately 3.12 mA. With a load resistance of 200 Ω, the output voltage of the AD9834 varies from approximately 0.032 V to approximately 0.624 V. The output voltage on the AD9834 IOUT pin is shown in Figure 2, where the DDS output frequency is set to 1 MHz.

Increasing the voltage output of the AD5620 will reduce the full-scale output current of the AD9834. The minimum full-scale current is reached when the output voltage of the AD5620 is equal to V _{REF or 1.20 V.}

Figure 3 shows the AD9834 output voltage corresponding to half-scale output current, where V _DAC = 0.5 × V _REF or 0.6 V.

This circuit must be built on a multilayer circuit board with a large area ground plane. For optimal performance, proper layout, grounding, and decoupling techniques must be used (refer to Tutorial MT-031 and Tutorial MT-101 ).