# DDS output power

I am designing a circuit that uses a DDS (AD5932) and am having trouble finding its output power. I need to know the power because the output will be fed into a mixer with characteristics that depend on input power.

I have found that it is most likely determined by the resistance on the DAC's output (source - scroll down to Output Power Considerations), but the datasheet does not mention any equations for determining a resistor value.

Can anyone provide resources for determining the resistor value for a desired output power or if there is another way to determine output power?

• Many mixers are very non-linear loads. A mixer-maker might assume a sinusoidal drive waveform that delivers (say) +10 dBm to a linear 50 ohm load. When connected to the mixer's local oscillator port, currents and voltages change from the 50-ohm load case. Its a simple approach that avoids hair-pulling trying to deal with a non-linear load. So just use a 50 ohm load on your DDS output filter...if power is weak, you'll need a post-amplifier. An active mixer might have higher Z than 50 ohms at its L.O. port Commented Nov 12, 2021 at 1:33

From the datasheet:

So the output looks like a current source across 200 ohms to ground. I'd buffer it before feeding it to a mixer, but then I don't know what mixer you are contemplating.

It comes with a DC offset which may or may not be an issue.

• Thank you. I will need to look more into output buffering. Commented Nov 12, 2021 at 15:43

The datasheet says it outputs a peak to peak voltage of 0.58V and the typical current consumption of the analog section is 3.8 mA. This means the output power would be less than 0.58 x 3.8e-3 = 2.2 milliwatts which is 3.42 dBm.

I would suggest you use an amplifier after the DDS since you don’t guarantee your load’s impedance. You’ll need an opamp with enough bandwidth for your application.

You can use the BALUN as from schematics, then you get approx. 1mW output power, 0.22 Vrms.

Note that without appropriate lowpass (or even bandpass) filter the signal from DDS is almost useless.

Source of project

Can anyone provide resources for determining the resistor value for a desired output power or if there is another way to determine output power?

I do think it is almost fixed value, just little variation. Read the datasheet, it is explained.

• The AD5932 has a single ended output. This project was for a AD9851 which has a balanced differential current source output. Commented Nov 12, 2021 at 9:28
• @Tesla23 my bad :( Commented Nov 12, 2021 at 9:37
• This will be helpful if I use a differential DDS in the future. Thank you! Commented Nov 12, 2021 at 15:23