What is the appropriate output ripple noise of a power supply?

When designing or applying a power supply, everyone will be concerned about the output ripple noise of the power supply, but how much is appropriate? If the requirements are relaxed, the ripple noise will be too large, the circuit will not 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 may become an over-design that is overkill.

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 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 itself. 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 in the back stage: similar to the linear regulator of LM7805 produced by Zhiyuan Electronics, it has a negative feedback system inside, which has a certain attenuation ability for the ripple noise of the power supply. Therefore, the power supply circuit with linear regulator in the back stage, the ripple noise is controlled at about 75mV, which can generally meet the requirements. 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 like 1.2V and 0.8V are sensitive to power ripple noise. When the ripple noise is large, it is easy to affect the normal operation of the CPU and even burn 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