TIDUFE3 July   2025

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
    3. 2.3 Highlighted Products
      1. 2.3.1 TPS7A03
      2. 2.3.2 REF35
      3. 2.3.3 TVS3301
      4. 2.3.4 OPA391
      5. 2.3.5 AFE881H1
      6. 2.3.6 AFE882H1
      7. 2.3.7 SN74LV8T165
      8. 2.3.8 TMUX1219
  9. 3System Design Theory
  10. 4Hardware, Software, Testing Requirements, and Test Results
    1. 4.1 Hardware Requirements
    2. 4.2 Test Setup
    3. 4.3 Test Results
      1. 4.3.1 Linearity Tests
        1. 4.3.1.1 Linearity Tests Summary
      2. 4.3.2 Noise Tests and Current Histogram
        1. 4.3.2.1 Noise Tests and Current Histogram Summary
      3. 4.3.3 Step Response
        1. 4.3.3.1 Step Response Summary
      4. 4.3.4 Start-Up
      5. 4.3.5 MCU Current
        1. 4.3.5.1 MCU Current Summary
      6. 4.3.6 System Currents
        1. 4.3.6.1 Summary of System Currents
  11. 5Design and Documentation Support
    1. 5.1 Design Files
      1. 5.1.1 Schematics
      2. 5.1.2 BOM
      3. 5.1.3 PCB Layout Recommendations
        1. 5.1.3.1 Layout Prints
    2. 5.2 Tools and Software
    3. 5.3 Documentation Support
    4. 5.4 Support Resources
    5. 5.5 Trademarks
  12. 6About the Author

1 System Description

This reference design implements the 4–20mA current loop interface for loop powered sensors. The design incorporates a 16-bit DAC, either the AFE881H1 or AFE882H1. The DAC output voltage drives an OPA391 op amp converting the voltage to current.

Power supplies with 1.8V or 3.3V output options are integrated into this design. Configuration settings dictate whether 1.8V or 3.3V is used for powering the analog circuit on the board and the connected MCU. Low dropout (LDO) regulators (TPS7A0333 and TPS7A0318) with low IQ are used to maintain a stable and efficient power output keeping the available power for the system high.

The design incorporates a discrete P-channel Metal-Oxide-Semiconductor field-effect transistor (PMOSFET) CSD25404 power switch for an MCU. This power switch controls the 1.8V and 3.3V rail on a connector attached to the MCU. Additional bulk capacitance supports start-up currents exceeding loop limits. The capacitor powers MCU initialization prior to entering low-power mode.