AD9779A with ADL5385

Hi srdjan1 ,

There are four parts to consider when designing an interface circuit between the AD9779A and ADL5385:

1. Bias-Setting Resistors. These resistors establish the bias level from the DAC to the modulator. Thus, their value should be based on the input dc bias required by the ADL5385 (500 mV) and the nominal midscale current of the AD9779A ( ~10 mA). It can be determined from these two that a 50-ohm resistor at each DAC output to ground should be used.
2. AC Swing-Limiting Resistor. This resistor is added to the interface to decrease the ac voltage swing for a given DAC output current and so depends on the desired voltage swing (see formula and graph obtained from the ADL5375 datasheet below for when a 50-ohm bias-setting resistors are used). To input a 1-Vpp differential swing to the ADL5385 from the AD9779A with a 50-ohm bias-setting resistor at each of its outputs, a 100-ohm limiting resistor should be placed between each side of the differential pair.
   
   
3. LO Feedthrough Compensation. The auxiliary DACs of the AD9779A can be used to compensate for the LO feedthrough when a quadrature modulator such as the ADL5385 is connected to the DAC output. This can be achieved by connecting each auxiliary output through a 500-ohm resistor to ground, and through a 250-ohm resistor to one of the modulator signal inputs.
4. Low-Pass Filter. When a DAC is used to drive a modulator, a low-pass filter is employed to eliminate noise and images while preserving the dc bias level between the devices. This filter is typically placed between the bias-setting resistors and the limiting resistor to conveniently establish its input and output impedances. Moreover, the filter design depends on the requirements of the application. The ADL5385 and ADL5375 datasheets provide a few examples such as a five-pole Chebyshev filter with a corner frequency of 39 MHz (see ADL5385 datasheet p. 18); a third-order Bessel filter with a 3-dB frequency of 10 MHz (see ADL5375 datasheet p. 25); and a fifth-order Butterworth filter with a 300-MHz corner frequency (see ADL5375 datasheet p. 26). The AD9779A evaluation board (AD9739-DPG2-EBZ) uses the ADL5375 quadrature modulator and has adopted the third filter design.

For the working set of components, I recommend following the circuit below (Figure 79 in AD9779A datasheet) for the bias-setting, limiting, and LO feedthrough compensation resistors. As for the filter, I suggest choosing from the examples in the datasheet and modifying the design as necessary to fit your requirements.

Hope this helps.

Regards,
Zaeefa

Hi srdjan1 ,

There are four parts to consider when designing an interface circuit between the AD9779A and ADL5385:

1. Bias-Setting Resistors. These resistors establish the bias level from the DAC to the modulator. Thus, their value should be based on the input dc bias required by the ADL5385 (500 mV) and the nominal midscale current of the AD9779A ( ~10 mA). It can be determined from these two that a 50-ohm resistor at each DAC output to ground should be used.
2. AC Swing-Limiting Resistor. This resistor is added to the interface to decrease the ac voltage swing for a given DAC output current and so depends on the desired voltage swing (see formula and graph obtained from the ADL5375 datasheet below for when a 50-ohm bias-setting resistors are used). To input a 1-Vpp differential swing to the ADL5385 from the AD9779A with a 50-ohm bias-setting resistor at each of its outputs, a 100-ohm limiting resistor should be placed between each side of the differential pair.
   
   
3. LO Feedthrough Compensation. The auxiliary DACs of the AD9779A can be used to compensate for the LO feedthrough when a quadrature modulator such as the ADL5385 is connected to the DAC output. This can be achieved by connecting each auxiliary output through a 500-ohm resistor to ground, and through a 250-ohm resistor to one of the modulator signal inputs.
4. Low-Pass Filter. When a DAC is used to drive a modulator, a low-pass filter is employed to eliminate noise and images while preserving the dc bias level between the devices. This filter is typically placed between the bias-setting resistors and the limiting resistor to conveniently establish its input and output impedances. Moreover, the filter design depends on the requirements of the application. The ADL5385 and ADL5375 datasheets provide a few examples such as a five-pole Chebyshev filter with a corner frequency of 39 MHz (see ADL5385 datasheet p. 18); a third-order Bessel filter with a 3-dB frequency of 10 MHz (see ADL5375 datasheet p. 25); and a fifth-order Butterworth filter with a 300-MHz corner frequency (see ADL5375 datasheet p. 26). The AD9779A evaluation board (AD9739-DPG2-EBZ) uses the ADL5375 quadrature modulator and has adopted the third filter design.

For the working set of components, I recommend following the circuit below (Figure 79 in AD9779A datasheet) for the bias-setting, limiting, and LO feedthrough compensation resistors. As for the filter, I suggest choosing from the examples in the datasheet and modifying the design as necessary to fit your requirements.

Hope this helps.

Regards,
Zaeefa