How should the signal ground and shielded ground be separated in a Human-Machine Interface (HMI)?

The separation of signal ground and shield ground in an HMI aims to prevent noise current from contaminating sensitive signal loops while providing a discharge path for interference. Signal ground serves as the reference ground for circuit operation and should be kept clean and low-impedance. Shield ground typically refers to the grounding of the chassis, shielding cover, or cable shielding layer, primarily used to discharge common-mode interference and provide shielded return current. During separation, the signal ground plane is physically isolated from the chassis connection area on the PCB using copper-free areas or trenches. The two are connected through specific connection points.

Common connection methods include: direct single-point connection via a 0Ω resistor or ferrite bead; high-frequency connection via safety-certified Y capacitors (such as 1nF/2kV Y2 capacitors); and connection at I/O ports via a hybrid circuit consisting of high-voltage capacitors and discharge tubes. The choice of connection method depends on the signal type and safety regulations. For example, for floating ground systems, a connection via only Y capacitors may be necessary; for protective grounding systems, a low-impedance direct connection may be required. Routing of the separated ground plane requires caution; high-speed signal lines or sensitive analog signal lines must not cross the separation gap, otherwise the return path will be cut off, leading to EMI problems. Etymotic's safety-certified Y capacitors and specific models of ferrite beads provide a reliable choice for a safe and effective connection between signal ground and shielding ground.