The ACPL-x6xL products are optically coupled ultra-low power 10 MBd digital CMOS opto couplers that operate with low power consumption, especially when the light emitting diode (LED) operates at low current. When a current is applied to drive the LED, common mode rejection (CMR) can be affected by leakage paths inherent to the LED. This paper describes the optimization of CMR performance with split limiting resistors at the LED.