How to avoid loops in mixed grounding of I/O modules?

When using hybrid grounding for I/O modules, clearly defining high-frequency and low-frequency grounding paths is crucial to avoid ground loops. For low-frequency circuits and safety grounding, a single-point grounding strategy is employed, with all ground wires converging at a single power input point. For high-frequency circuits and EMC, a multi-point grounding strategy is used, connecting the circuit ground to a complete ground plane via a via matrix, which is then connected to the chassis ground at multiple high-frequency points. The key is to establish an impedance between the single-point grounding point and the high-frequency ground plane, such as a 10-100nH inductor or ferrite bead.

This impedance presents low impedance at DC and low frequencies, ensuring safe grounding, and high impedance at high frequencies, blocking high-frequency ground loops. The shielding layer of the signal cable should overlap the chassis ground 360 degrees at the module end. If the other end of the cable is also grounded, a loop may form. In this case, a ferrite ring can be placed on the shielding layer to increase high-frequency impedance. For floating ground modules, a Y-capacitor is used to connect to the chassis ground. The capacitor value must be chosen between discharging high-frequency noise and limiting low-frequency ground loops. Measuring the noise current on the ground wire verifies whether the loop is effectively suppressed.