How to correctly obtain the output ripple noise value of the power supply

Output ripple noise plays an important role in designing power supply circuits, but how much is appropriate? If the requirements are relaxed, the ripple noise is too large, the circuit cannot work properly, and it will take time to debug and modify; if the requirements are high, the cost of the filter will naturally increase, and it is very likely to become an over-design of "killing a chicken with a sledgehammer". After reading this article shared by the editor, I believe this problem will no longer bother you.

Based on some common applications, some values ​​that can tolerate ripple noise are recommended below for reference.

(1) Ordinary digital I/O isolation: The ripple noise tolerance of the power supply is relatively large, about 100mV is no problem;

Figure 1 IO isolation

(2) Power supply isolation of relay output and optocoupler output : Similar to digital I/O, it can tolerate ripple noise up to 100mV;

(3) Power supply of industrial communication ports: Bus-type power supply isolation such as RS-232, RS-485, CAN, etc. is a digital signal. RS-485 and CAN are transmitted in differential form and are not so sensitive to power supply ripple noise. The power supply ripple noise can generally be controlled at around 75mV.

Figure 2 Industrial communication power isolation

(4) Power supply circuit with linear regulator at the back stage: The linear regulator is similar to the LM7805 produced by Zhiyuan Electronics, which has an internal negative feedback system and has a certain attenuation ability for the ripple noise of the power supply. Therefore, the ripple noise of the power supply circuit with linear regulator at the back stage can generally meet the requirements if it is controlled at around 75mV;

(5) Low-speed, low-precision data acquisition system: The requirements for accuracy and speed are not high, and the ripple noise is controlled at 50mV, which can generally meet the needs of data acquisition;

Figure 3 Low-speed data acquisition

(6) Power supply for low-voltage CPU: CPU power supply systems such as 1.2V and 0.8V are sensitive to power supply ripple noise. When the ripple noise is large, it is easy to affect the normal operation of the CPU and even burn out the CPU. It is generally required to be controlled within 30mV;

(7) High-speed, high-precision data acquisition system: It has high requirements for accuracy and speed, and is extremely sensitive to the ripple noise of the power supply. In addition to requiring the ripple noise of the power supply to be small, it is also necessary to use some high-precision, common-mode and power supply rejection ratio operational amplifiers to cooperate. The ripple noise of the power supply generally needs to be controlled within 10mV.

Figure 4 High-precision data acquisition isolation