Overview
Ideally, the voltage at a power supply output is smooth and stable, but in practice this is not the case. In switching power supplies, switching of the power device causes output voltage fluctuations. The frequency of these fluctuations matches the switching frequency. These fluctuations are known as ripple voltage. Excessive ripple can adversely affect downstream equipment. When designing and building prototypes, it is necessary to verify that the supply's output ripple meets design requirements. Incorrect measurement methods often cause large errors. The following describes correct methods for measuring ripple voltage.
Measurement guidelines
1. Bandwidth limit
Enable a 20 MHz bandwidth limit. The ripple frequency equals the switch frequency of the regulator. To prevent the probe from picking up high-frequency ambient noise, set the oscilloscope channel bandwidth used for ripple measurement to 20 MHz.
2. Coupling
Use AC coupling. Ripple is the AC component superimposed on a DC voltage, so use AC coupling to measure only the AC component on the DC signal.
3. Probe setup
Remove the probe ground clip. The probe ground clip can act like an antenna and pick up electromagnetic radiation, affecting the measurement. Do not use the ground clip during measurement. Instead, use the probe grounding spring or the ground connection at the probe tip and place the probe directly at the test point.
Using the non-recommended setup with the ground clip produced a measured ripple of 161 mV, while using the recommended direct probe method produced 133 mV. Therefore, placing the probe directly at the test point yields ripple measurements closer to the actual value.
