A system including circuitry to communicate data across an isolation barrier of a switch driver circuit. For switch driver circuits with galvanic isolation, the circuitry of this disclosure uses the unavoidable common mode voltages caused by the coupling capacitances of the data transfer circuit to evaluate the common mode voltage characteristics, such as the slew rate of a switching event. The switch driver circuit of this disclosure may include a common mode voltage detector to detect and measure features of the unavoidable common mode voltage during a switching event, such as voltage amplitude and slew rate. The common mode voltage detector may couple to a communication interface that provides the common mode voltage information to a controller for the switch driver circuit. In some examples, based on the received information, the controller may adjust the operation of the switching circuit.