Power supply noise rejection (PSNR) is a measurement of how well a circuit rejects noise from various frequencies which are coupled into the power supply. In actual high speed analog and digital circuitry, the power supply pins are vulnerable to random noise. Most customer designs use linear voltage regulators or switching voltage regulators as the power supply for ICs. Linear regulators will almost always get input voltage from a switching DC/DC converter. Therefore, power supply noises in a customer board typically come from the switching noise of the power supply and coupling from other high-frequency sections of the circuit board. Figure 1 below is a simplified 3-component power supply filtering circuit that is recommended for a power rail. It consists of a 0.1μF capacitor (C1) a ferrite bead (FB1) and a 10μF capacitor (C2). While C1, FB1 and C2 are for the power rail, C3 is used for each power pin supplied by this rail. If a power rail supplies multiple power pins, each power pin will have a 0.1μF capacitor for decoupling. PSNR is measured by injecting a sinusoidal signal of a known amplitude and frequency onto the various power supply pins of the devices. The core power pin primarily supplies voltage to all PLL peripherals while the analog power pin supplies the voltage to the PLL. Each power pin should be analyzed independently, and noise should be applied to one power pin at a time. The technique used for applying the injected frequency into the device supply uses an inductor which presents a high AC impedance back to the supply. Then a signal from a generator is injected through a capacitor after the inductor at the pin of the DUT. Refer to Figure 2. Both the input and output should be monitored with an oscilloscope and Spectrum analyzer. The oscilloscope is used to ensure the signal amplitude while the spectrum analyzer is used to measure the deterministic jitter. Refer to application note AN-806 for more details. For other questions not addressed by the Knowledge Base, please submit a technical support request.




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Application Notes & White Papers
AN-806 Power Supply Noise Rejection Application Note PDF 438 KB