How to Design with Capacitive Digital Isolators 3

Due to the 1 to 2ns rise and fall time of digital isolators, they are often prone to signal reflection in long signal routing situations, and their characteristic impedance does not match the source impedance output by the isolator. Therefore, we recommend installing an isolator near its corresponding data receiving device and data source (such as controllers, drivers, receivers, and transceivers). If this cannot be done in the design, then controlled impedance transmission lines must be used.  diode

PCB Design Guide

For digital circuit boards, standard FR-4 epoxy glass should be used as the PCB material because compared to cheaper materials, it not only meets UL94-V0 requirements, but also has less high-frequency dielectric loss, lower moisture absorption, greater strength/hardness, and higher flame retardancy.

To achieve low electromagnetic interference (EMI) PCB design, we recommend a design example with at least four layers (see Figure 3), from top to bottom: high-speed signal layer, ground layer, power layer, and low-frequency signal layer.

Recommended four layer laminated board in Figure 3

The high-speed wiring arranged at the top provides a clear connection for the isolator and its corresponding driver. The high-speed wiring should be short and avoid using vias to ensure the lowest inductance of the high-speed wiring.

Next, a balanced ground layer is placed on the high-speed signal layer to ensure strong electrical coupling between the ground layer and the signal routing. This establishes a controlled impedance for transmission line interconnection and greatly reduces EMI. In the end, the balanced surface ground layer provides a very good low inductance path for reflow.diode