9.2.1.2.10.2.1.1 Isolation Capacitor

The differential signal lines are isolated between ICs by a DC blocking capacitor. The capacitor must be rated with a high enough voltage to provide standoff margin in the event of a fault in the system that exposes the device to a local hazardous voltage. Selecting a capacitor rated at a minimum of two times the stack voltage is the recommended practice. Ideally, only one 1000pF to 2500pF (2200pF typical) capacitor is sufficient for the normal operation of the device. However, two capacitors may be used (one at each end of the cable or PCB wiring) for an additional safety factor and proper coupling on both sides of the cable.

The capacitance on the daisy chain bus has a direct effect on performance. All parasitic capacitances from the support components and cabling must be taken into consideration when designing for communication robustness to EMC. Capacitance from the cables, ESD diodes, bypass capacitance, and chokes, form a capacitive divider with the isolation capacitors that may affect performance. Additionally, the amount of capacitance on the bus has a direct impact to the operating current during communication (the capacitor charging/ discharging).