How to effectively filter out differential mode interference in HMI (Human-Machine Interface) delivery?

Filtering differential-mode interference from HMIs primarily relies on inserting differential-mode filters along the interference conduction path. Differential-mode interference exists between power lines or signal lines. At the AC power input, safety X capacitors are key components for filtering differential-mode noise; their capacitance value selection must comprehensively consider both filtering effect and leakage current requirements. On the DC power supply side, differential-mode filtering is typically constructed using LC circuits, such as connecting a differential-mode inductor and capacitor in parallel between the input Vin and GND of a switching power supply. For board-level power supplies, placing sufficient and appropriately sized decoupling capacitors on the chip's power pins is the most direct method for differential-mode filtering; multiple capacitance values should be used in parallel to cover a wide bandwidth, such as 10μF, 0.1μF, and 100pF.

Ferrite beads also have a good attenuation effect on differential-mode noise at high frequencies; for example, a PBZ1608E102Z0T ferrite bead can be connected in series at the power input of a digital IC. For differential-mode interference on signal lines, such as the A/B lines of RS485, a low-pass filter can be constructed by connecting a resistor or ferrite bead in series on the line and a capacitor in parallel between the lines, but care must be taken not to affect signal quality. Optimizing the PCB layout and reducing the power loop area can reduce the radiation efficiency of differential-mode noise at the source. By properly configuring the safety X capacitors, power inductors, and ferrite beads from Audiotech, effective filtering networks can be designed for differential-mode interference in different frequency bands, ensuring the power quality of the HMI system.