From the origin of conduction noise, it can be seen that the influence of conduction noise on the circuit can be reduced by reducing device noise or the interference of the surrounding environment. But in some occasions we need specific devices, and at the same time require to work in specific scenarios. For example: In an electrically isolated system, we need to isolate the power supply, and the DC/DC converter will generate a lot of switching noise. In actual operation, we generally reduce the influence of conduction noise in the circuit from the following three aspects.
(1) Use bypass capacitors to reduce conduction noise on the power supply line
Regardless of the power supply form of the circuit, always use bypass capacitors at the power supply, otherwise the power supply noise will affect the performance of the circuit. Generally, bypass capacitors are used in two places in circuit design: one is the power supply, and the other is at the power supply pins of all active devices, whether they are analog devices or digital devices. The specific bypass capacitor value used depends on the selected device. If the bandwidth of the device is less than or equal to 1MHz, a capacitor of 0.1μF can filter out noise well; when the bandwidth of the device is greater than 10MHz, a capacitor of 0.01μF is more appropriate; can be.
In addition, for a specific chip, the best choice is to refer to the chip data sheet to determine the value of the bypass capacitor. At the same time, the appropriate capacitance can be selected by the method described below.
As shown in the figure above, the figure on the right shows the corresponding curves of ceramic capacitors with different capacitance values. The horizontal axis is the frequency, and the vertical axis is the impedance shown by the capacitor. We can see that the impedance of the 0.1μf capacitor is lower than 1Ω at about 2MHz, that is, it has the best filtering effect on the signal around 2MHz; similarly, the 0.01μf capacitor has the best filtering effect on the signal around 10MHz . Combining the performance of the power supply voltage rejection ratio of the ADC in different frequency bands and the frequency of interference noise in the left figure, you can choose the most suitable capacitor size.
When placing the bypass capacitor, it should be as close as possible to the power supply pin of the device, and the trace should be as short as possible. At the same time, if two bypass capacitors are selected, the one with smaller capacitance should be closer to the chip pin.
(2) Reasonable PCB design reduces conduction noise on the ground plane.
In fact, the design and implementation of the ground and power planes are crucial to low-noise design. Generally speaking, the treatment of ground in all circuit designs should be copper laying treatment. In a circuit with analog or hybrid devices, it is very unwise to not have a ground plane. First of all, the analog signal is based on the ground, and the ground noise problem will have a greater impact on the circuit; second, the advantage of the ground plane is that it is in a low impedance state in a wide frequency domain, and the switching frequency of modern digital devices is in MHz. Above, when the digital device is working, there will be a large amount of high-frequency current returning to the ground layer; if there is no ground plane, and the ground wire is used to connect all the grounds, the parasitic inductance of the thin and long ground wire will be in the high-frequency signal. A large amount of voltage noise is generated under the impact, or the ground bounce as it is often said; again, the current path on the ground plane should be reasonably planned, and the current on the digital ground or power ground should absolutely avoid flowing into the analog ground; finally, try to maintain the integrity of the ground plane, divide A separate ground trace can disrupt the current return path, so avoid traces crossing split ground planes. In the design of the power layer, similarly, the power plane should be used as much as possible under the condition of cost, and at the same time, on the high-frequency and high-power traces (such as the power loop of the switching power supply), attention should be paid to ensure the line width by pouring copper, as far as possible. Possibly reduces parasitic inductance on power traces.
(3) Use a low-pass filter before the ADC to reduce the conduction noise in the signal chain.
In addition to adding bypass capacitors to the circuit, the signal before being sent to the ADC needs to be filtered first. The main purpose of the low-pass filter added before the ADC is to filter out the high-frequency components in the signal sent to the ADC. On the one hand, it is to avoid aliasing, and on the other hand, it is to reduce the broadband noise entering the ADC. We mentioned the thermal noise of the resistor, and the white noise of the op amp is an indicator related to the bandwidth. As shown in the figure below, after adding the second-order low Before the pass filter, the signal-to-noise ratio of the 12-bit ADC is 74dB (12×6.02+1.76=74dB) as the threshold, and the bandwidth of the signal chain formed by the selected amplifier exceeds 30MHz, which means that all broadband white noise within 30MHz is eliminated After integration, it will affect the performance of the ADC, and after we add a 10Hz second-order low-pass filter, only the noise within 1KHz will affect the ADC. This shows the great role of the low-pass filter:
To sum up, the methods to reduce conduction noise are:
(1) Replace devices with large noise figures
(2) Add decoupling capacitors next to the power supply pins
(3) Improve signal line routing and lay the ground
(4) Before entering the ADC, Filtering the Signal Chain