11.3.1.2.1 Common Mode Signal

A common-mode signal is an average voltage of the two signal wires that the differential receiver rejects. The common-mode signal comes from the CAN driver, ground noise, and coupled bus noise. Since the bias voltage of the recessive state of the device is dependent on V_CC, any noise present or variation of V_CC will have an effect on this bias voltage seen by the bus. The SN65HVD23x family has the recessive bias voltage set higher than 0.5*V_CC to comply with the ISO 11898-2 CAN standard which states that the recessive bias voltage must be between 2 V and 3 V. The caveat to this is that the common mode voltage will drop by a couple hundred millivolts when driving a dominant bit on the bus. This means that there is a common mode shift between the dominant bit and recessive bit states of the device. While this is not ideal, this small variation in the driver common-mode output is rejected by differential receivers and does not effect data, signal noise margins or error rates.