EVAL-ADM2582EEMIZ uses C19 high-voltage capacitors to reduce interference to the power supply. This way, interference on the 485 line may enter the MCU system through C19. What is the impact of interference into the MCU system?What is the impact of iCoupler on the MCU through RxD and TxD?
C19 on the PCB is to reduce radiated emissions from the PCB and isolator. During operation common mode (CM) currents can capactively couple through the isoPower onto the GND2 side. The isolation barrier and the CM current form a dipole and radiate. In order to reduce these emissions a stitching cap or HV cap is recommended to allow the CM currents to return to the GND1 side through a low impedance path. This in turn reduces the level of emissions.
The stitching capacitor also has some draw backs. The system is more susceptible to AC leakage and it may allow transients to flow through the cap to the GND1 side. This is dependent on your grounding. If transients flow to the GND1 side make sure you get these currents off the PCB as soon as possible and removing the currents from the ground plane. There is a possibility that this may cause ground bounce on the digital lines and may cause errors, but proper grounding should greatly reduce the possibility of this.
Another option is to use our next generation isolated RS485, the ADM2867E, This device does not require HV or stitching capacitors in order to reduce the level of emissions. It is designed to meet the toughest EMI standards with a 2 layer PCB and no capacitance across the barrier. This will make your system more immune to AC leakage and High Voltage Transients and less susceptible to any errors.
Please see the following link for more information on the ADM2867E
Any more questions, please ask