TIDUEZ8 May   2021

 

  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Description
    1. 1.1 Insulation Monitoring
    2. 1.2 Isolation Capacitance
    3. 1.3 IEC 61557-8 Standard for Industrial Low-Voltage Distribution Systems
    4. 1.4 Key System Specifications
  7. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Highlighted Products
      1. 2.2.1 TPSI2140
      2. 2.2.2 AMC3330
      3. 2.2.3 TPS7A24
      4. 2.2.4 REF2033
      5. 2.2.5 TLV6001
    3. 2.3 Design Considerations
      1. 2.3.1 Resistive Bridge
      2. 2.3.2 Isolated Analog Signal Chain
        1. 2.3.2.1 Differential to Single-Ended Conversion
        2. 2.3.2.2 High-Voltage Measurement
        3. 2.3.2.3 Signal Chain Error Analysis
      3. 2.3.3 PCB Layout Recommendations
  8. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
      1. 3.1.1 Connectors
      2. 3.1.2 Default Jumper Configuration
      3. 3.1.3 Prerequisites
    2. 3.2 Software Requirements
    3. 3.3 Test Setup
    4. 3.4 Test Results
  9. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Documentation Support
    3. 4.3 Support Resources
    4. 4.4 Trademarks
  10. 5About the Authors

Isolated Analog Signal Chain

As previously stated, the AMC3330 is used as an amplification phase and reinforced isolation barrier of the analog front end presented in this reference design.

The value of the isolation resistances is given by: RisoN=RstDC-Vn-VpVp; RisoP=RstDC-Vn-VpVn

So the isolation voltages must be served scaled down and isolated to the inputs of the ADC on the MCU computing the isolation resistances and leakage currents.

Figure 2-7 Isolated Analog Signal Chain
GUID-20220401-SS0I-84FZ-16PJ-SCPBPQX90BVD-low.pngFigure 2-8 Schematic Isolated Analog Signal Chain + Reference

Assuming the DC bus voltage stays the same when switching the resistive branch on the positive or negative side, and no tolerance on the resistive branch value, the isolation voltages (Vn and Vp), computed from the ADC readings are given by Equation 12:

Equation 12. Visolation=VinADC-VrefGaindiff2single×RinAMC+RstRinAMC×1GainAMC

where

  • Visolation is the actual Isolation Voltage monitored
  • VinADC is the voltage at the input of the ADC
  • Vref is the voltage reference coming from REF2033 (or REF1933)
  • Gaindiff2single is the differential-to-single ended gain set with the resistor divider, as explained in Section 2.3.2.1
  • GainAMC is the internal gain of the AMC3330
  • Rst is the switched Resistive Bridge equivalent resistance
  • And RinAMC is the resistor that serves the scaled down isolation voltage to the input of the AMC3330

As mentioned in Section 2.3.2.1, the common-mode voltage is generated by using REF2033 – 3.3-V Vref, low-drift, low-power, dual-output Vref and Vref / 2 voltage reference. Also, to have an I/O interface signaling voltage of 3.3-V compliant with the input range of the MSP and C2000 family, the 3.3-V rail is used to power the differential to single-ended operational amplifiers.