Use the LC filter skillfully to improve the phase noise of the high-speed DAC power supply!

For high-speed DAC power supply selection, LDOs are proven regulators, especially for high-quality noise performance. For technical information, see the article: Choosing an Ultra-Low Noise LDO to Improve Phase Noise.

Q&A

Q: Improve high-speed DAC power supply phase noise with LC filters

For high-speed DAC power supply selection, LDOs are proven regulators, especially for high-quality noise performance. For technical information, see the article: Choosing an Ultra-Low Noise LDO to Improve Phase Noise.

But it’s not just LDOs that can’t be used with other power supplies. Depending on the overall system requirements, with proper LC filtering, switching regulators can also provide a power solution that simplifies the power solution.

But because of the LC filter, care should be taken with series resonance, otherwise the noise may get worse. Resonance can be achieved by lowering the Q of the circuit – by adding lossy components to the circuit, it can be controlled.

The figure below shows an example design using the AD9162 DAC, also clocked from the ADP1740 LDO, but followed by an LC filter.

Use the LC filter skillfully to improve the phase noise of the high-speed DAC power supply!
Figure 1: LC filter and de-Q network

The schematic shows the filter under consideration, with the RL model representing the Inductor and the RC model representing the main filter capacitor (C1+R1). The red circle is the original LC filter circuit, and the blue circle is the additional lossy component to reduce the Q value.


Figure 2: LC filter response

The filter response is shown in the figure below, the red line is the original LC circuit response curve, and the blue line is the improved response curve. We see that the Q value decreases.


Figure 3: Phase Noise Response

Let’s take a look again. For the phase noise response, the blue line is the original LC circuit response curve, and the orange line is the improved response curve. It can be seen that the phase noise is improved.

In summary, the noise will not only vary due to different power supply choices, but may also be affected by output capacitance, output voltage, and load. These factors should therefore be carefully considered when designing, especially for sensitive supply rails.

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