SDAA162 July   2026 ADS125H18 , ISO7721 , ISO7730 , ISO7731 , SN6505B , SN74LVC1G17 , TUSB320 , TVS3301

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Design Overview and Measurement Performance (Normal Operation)
    1. 1.1 Design Overview
    2. 1.2 EMC Test Board Voltage Measurement Performance During Normal Operation
    3. 1.3 EMC Test Board Current Measurement Performance During Normal Operation
  5. 2EMC Test Board Circuit and PCB Layout Considerations
    1. 2.1 Circuit Design Considerations for EMC Compliance
      1. 2.1.1 High-Voltage Capacitors and Resistors on Every Input Connector Pin
      2. 2.1.2 TVS Diodes
      3. 2.1.3 Protecting the Current Shunt: PTC and Zener Diodes
      4. 2.1.4 Series Resistors on Digital Signals
      5. 2.1.5 Digital Isolation
      6. 2.1.6 Power Supply and Protection
      7. 2.1.7 High-Voltage Capacitors and Resistors for Discharging Path
    2. 2.2 PCB Layout Considerations for EMC Compliance
      1. 2.2.1 PCB Layer Stack-up and Ground Plane
      2. 2.2.2 Avoiding a Long Return Path
      3. 2.2.3 Avoiding 90-Degree Bends in PCB Traces
      4. 2.2.4 Using a Guard Ring to Isolate Interference Signals
      5. 2.2.5 Decoupling Capacitors
      6. 2.2.6 Differential Signal Routing
      7. 2.2.7 Stitching Vias
      8. 2.2.8 Layout for Isolation Barrier
      9. 2.2.9 Component Placement
  6. 3EMC Test System, Standards, and Results
    1. 3.1 EMC Test System
    2. 3.2 EMC Test Standards
    3. 3.3 EMC Test Results
      1. 3.3.1 Electrostatic Discharge (ESD)
      2. 3.3.2 Radiated Immunity (RI)
      3. 3.3.3 Electrical Fast Transients (EFT)
      4. 3.3.4 Surge Immunity (SI)
      5. 3.3.5 Conducted Immunity (CI)
  7. 4Schematic, PCB Layout and Bill of Materials
    1. 4.1 Schematic
    2. 4.2 PCB Layout
    3. 4.3 Bill of Materials (BOM)
  8. 5Summary
  9. 6References

2.1.1 High-Voltage Capacitors and Resistors on Every Input Connector Pin

Filtering signals directly at the input connector helps increase electrostatic discharge immunity, reduce radiated emissions, and increase immunity to coupled burst signals on the input. Every input signal that enters the PCB needs a filter element such as a ceramic capacitor. Connect the capacitor between the input connector and earth ground with a wide trace. Place this capacitor as close as possible to the connector pin.

Figure 2-2 shows the high-voltage capacitor circuit on the EMC test boards.

High-Voltage Capacitors on Input ConnectorFigure 2-2 High-Voltage Capacitors on Input Connector

Use a high-voltage rated capacitor because this component is exposed to high-energy transient signals including electrostatic discharge, electrical fast transients, or surge signals during the EMC test.

Use the input signal frequency to determine the capacitor value. This design measures low-frequency voltage and current signals such that larger value capacitors can be used. Therefore, this design uses a 1kV high voltage, X7R type, 0.022μF ceramic capacitor on each ADC channel as close as possible to the input terminal blocks so that the transient energy is discharged to earth ground through the shortest path.

Figure 2-2 also shows a 10MΩ resistor in parallel with the capacitor. These resistors help discharge the capacitor to avoid charge build up during repeated transients. Otherwise, this charge accumulates and reduces the effectiveness of the capacitor. Use a high-voltage rated resistor in the range of 1-10MΩ for these components.