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

2.1.6 Power Supply and Protection

The EMC test board power supply circuits are intentionally designed to mimic typical industrial systems. By default, only one power supply between +6V and +12V on J9 is required to power the entire circuit board.

Non-Isolated Side

The TPS7A4700 (U18) low-dropout regulator (LDO) converts an externally-provided power supply (6V to 12V) to a steady 4.2V. This 4.2V output powers a low-noise, low-EMI push-pull transformer driver, SN6505 (U19). The PHI controller card directly provides the 3.3V supply for the ISO7721, ISO7730, and ISO7731 digital isolators on the non-isolated side of the test board.

The ADS125H18 EMC test board can also be powered by an external USB-C power supply using connector J14. This connection is intended to power the board during any non-EMC testing. The input LDO (U18) outputs 4.2V specifically to account for any losses in the USB voltage.

Isolated Side

Transformer T1 has a turns ratio of 1:3.45 and uses a bipolar configuration to produce a voltage gain of approximately 6.9 from the non-isolated to the isolated side of the board. Therefore, the 4.2V input voltage is approximately 29V at the transformer output. This voltage mimics the standard 24V isolated supply voltage in analog input module systems.

TPS7A4701 (U27) regulates the output of the transformer to generate a 24V output signal at connector J13. The unregulated transformer output voltage is also provided to two TPS7A4700 (U17 and U20) LDOs that generate the ADS125H18 AVDD (5V) and DVDD (3.3V) supplies, respectively. TVS diodes protect the outputs of each regulated supply

Figure 2-9 shows the power supply schematic.

ADS125H18 EMC Test Board Power Supply SchematicFigure 2-9 ADS125H18 EMC Test Board Power Supply Schematic