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

1.1 Design Overview

Modern industrial control systems often use process-level voltage (±10V) or current (4-20mA) signaling to monitor and operate a factory or plant. An analog input module receives these signals and converts them to digital so the control system can make efficient and precise decisions. Therefore, designing high-accuracy voltage and current measurement systems that are also protected against electromagnetic interference (EMI), overvoltages, and other interference signals is critical.

The EMC test board discussed in this application note uses the ADS125H18, a precision, 24-bit, 1MSPS delta-sigma (ΔΣ) analog-to-digital converter (ADC) designed for use in analog input modules. The ADS125H18 can measure up to eight fully differential analog inputs or up to 16 single-ended high voltage input signals. Each input comprises a high-impedance voltage divider with integrated precision matched resistors to scale down the input voltage to the input range of the ADC. The ADS125H18 is equipped with a channel autosequencer and a FIFO (first-in, first-out) buffer. The power-scalable architecture provides four speed modes to optimize data rate, resolution, and power consumption. Figure 1-1 shows the ADS125H18 functional block diagram:

ADS125H18 Functional Block DiagramFigure 1-1 ADS125H18 Functional Block Diagram

The ADS125H18 EMC test board includes external isolated power supplies and a digital isolator. The digital isolator provides galvanic isolation between the ADC serial peripheral interface (SPI) and the precision host interface (PHI) controller card that monitors conversion data from the ADS125H18. The PCB is designed to satisfy the IEC 61000-4-x standards for systems operating in a harsh electromagnetic environment.

Figure 1-2 shows an overall view of the EMC test board layout:

ADS125H18 EMC Test BoardFigure 1-2 ADS125H18 EMC Test Board