Power ripple measurement settings

Definition of power ripple
Power supply ripple is an intuitive manifestation of power supply performance. DC stabilized power supply is generally formed by AC power supply through rectification and stabilization. It is inevitable that there will be some AC components in the DC stabilized voltage. This superposition is superimposed on the DC power supply. The AC component on the voltage regulator is called ripple.
Set up the oscilloscope
1. First of all, the probe should select the appropriate gear. If the voltage is relatively large, or the bandwidth requirement is relatively high, the X10 gear can be used. Under normal circumstances, it is recommended to use the X1 gear to avoid unnecessary noise attenuation affecting the ripple measurement. At the same time, remember to also adjust the attenuation ratio of the oscilloscope channel to X1.
2. Ripple is an AC component, so the "channel coupling" method should use AC coupling to limit the input of DC signals. In addition, the adjustable range of the vertical scale of the oscilloscope is limited, so when the DC signal is too large, the ripple may not be seen. Select AC coupling to display only the AC ripple signal, which is convenient for observing the waveform.
3. Using the bandwidth limit function, the ripple frequency of the general switching power supply output is in the range of 0~20MHz. The noise caused by high-frequency synchronous switching noise and signal reflection is in the range of 0~1GHz. Therefore, it is recommended to select a 20MHz bandwidth limit to filter out unnecessary high-frequency noise.
4. In order to avoid electromagnetic radiation and other interference to the signal, the ground wire of the oscilloscope probe should be as short as possible, and the ground spring that comes with the probe is usually used for grounding.
5. Through the FFT function, the ripple waveform can be analyzed in the frequency domain, so that the noise at each frequency point and the ripple caused by the switch can be accurately known.
To sum up: try to use the X1 gear for the probe, use AC coupling for channel coupling, limit the open bandwidth to 20M low-pass, and use the ground spring pin to make the ground wire as short as possible.
Measurement result
After adjusting the vertical and horizontal directions of the waveform to a suitable position, turn on the measurement option of the oscilloscope and select the peak-to-peak value to directly obtain the peak-to-peak value of the ripple. So what is the appropriate peak-to-peak value of the ripple?
Ordinary digital I/O: The ripple noise tolerance of the power supply is relatively large, about 100mV is no problem; the power supply of relay output and optocoupler output: can tolerate up to 100mV ripple noise; the power supply of the industrial communication port: like RS- Bus-type power supplies such as 232, RS-485, CAN, etc., are digital signals themselves, such as RS-485, CAN or differential transmission, which are not so sensitive to the ripple noise of the power supply. The ripple noise of the power supply is generally controlled at about 75mV. Yes; low-speed, low-speed data acquisition system: low speed requirements, ripple noise control at 50mV generally can meet the needs of data acquisition; power supply for low-voltage CPU: like a 1.2V, 0.8V CPU power supply The system is more sensitive to the ripple noise of the power supply. When the ripple noise is large, it will easily affect the normal operation of the CPU, and even burn the CPU. Generally, it is required to be controlled within 30mV; high-speed, high-data acquisition system: high requirements for and speed , It is very sensitive to the ripple noise of the power supply. In addition to the low ripple noise of the power supply, some high common-mode rejection ratio operational amplifiers must be selected to cooperate. The ripple noise of the power supply generally needs to be controlled within 10mV .