How to ensure EMC of encoder differential signals?

Ensuring the EMC performance of encoder differential signals (such as A+/A-, B+/B-) requires a two-way design approach, considering both transmission and immunity. Etymotic's safeguards include:

1) Reduced transmission: The transmitter (encoder) uses a current-output driver with a small series resistor (e.g., 33Ω) to mitigate edge noise.

2) Improved immunity: The receiver (driver) uses a differential receiver with a high common-mode rejection ratio (CMRR>80dB), incorporating a common-mode choke (CML3225A-510T) and a TVS array (e.g., ESDSRV05-4H) at the front end.

3) Transmission medium: Twisted-pair shielded cable with characteristic impedance matching (typically 120Ω) must be used, with the shield grounded at both ends (avoiding loops through DC blocking with capacitors).

4) Termination matching: A matching resistor (120Ω) is connected in parallel at the cable receiver.

Through a complete link design from sending and transmitting to receiving, it can be ensured that the encoder differential signal can still be correctly identified in the presence of ±10V common-mode noise. Its radiated emissions meet the EN 55032 Class A limit, and its immunity meets the IEC 61000-4-6 Level 3 (10V) requirements.