Does impedance mismatch at the HMI (Human Machine Interface) interface affect EMC (Electronic Performance Management)?

Impedance mismatch at the HMI interface negatively impacts EMC, primarily affecting signal integrity and radiated emissions. When the driver's output impedance mismatches with the transmission line's characteristic impedance and the receiver's input impedance, signal reflection occurs, leading to overshoot, ringing, and edge degradation in the signal waveform. These ringing and overshoots are rich sources of high-frequency harmonics, significantly increasing radiated emissions from the interface. Simultaneously, reflections cause the signal eye diagram to close, reducing noise margin and making the interface more susceptible to external interference, i.e., decreased immunity. For differential interfaces such as USB and Ethernet, impedance mismatch also reduces the common-mode rejection ratio, making the interface more sensitive to common-mode noise.

To resolve impedance mismatch, the correct characteristic impedance must first be determined according to the interface standard, such as 90Ω for USB differential and 100Ω for Ethernet differential. Impedance control is achieved during PCB design by adjusting trace width, spacing, and dielectric thickness. Matching is also achieved by adding series or parallel terminating resistors at the driver or receiver ends. When using connectors and cables, their impedance must also be ensured to match the PCB. Impedance can be measured and optimized using a network analyzer or TDR. Good impedance matching is fundamental for achieving low EMI and high immunity in high-speed interfaces. Some of the interface protection devices from Etymotic also take impedance factors into account, such as low-capacitance TVS, to minimize the impact on matching.