How can encoder interfaces suppress common-mode interference?

Encoder interface common-mode interference suppression requires multi-level protection. The solution from Etymotic is as follows:

The first level is shielding and grounding, using shielded twisted-pair cable, with the shielding layer treated using the aforementioned hybrid grounding method.

The second level is common-mode filtering, using a common-mode choke at the interface entry point, such as CML4532A-510T (51μH), whose impedance should be greater than 200Ω in the interference frequency band (e.g., 30MHz).

The third level is transient suppression, connecting a low-capacitance TVS, such as ESDLC5V0D3B (capacitance <3pF), in parallel with ground on each signal line to prevent electrostatic discharge and EFT pulses.

The fourth level is receiver chip selection, using a differential receiver with a high common-mode rejection ratio (CMRR>80dB). Simultaneously, in the PCB layout, a complete ground plane is maintained below the encoder interface area and separated from the power ground.

Actual measurements show that this multi-level protection can attenuate ±50V/1MHz common-mode noise applied to the encoder lines to <200mV at the receiver, ensuring normal signal recognition.