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

Differential to Single-Ended Conversion

As stated, the isolation voltage is monitored with an AMC3330 across the isolation barrier. The AMC3330 is really convenient for this application as it offers an integrated DC/DC converter and high CMTI bipolar input (VINP – VINN) in the range from –1 V to 1 V.

The output of the AMC3330 is a fully-differential analog signal comprised of the OUTP and OUTN pins centered around a common-mode voltage of 1.44 V that can be fed directly to a stand-alone analog-to-digital converter (ADC).


GUID-484203D6-9550-4A3D-8E2E-3F7395D0A141-low.gif

Figure 2-9 AMC3330 Output Behavior

The MSP430 and C2000 family of processors have embedded single-ended input ADCs. The addition of a differential to single-ended amplifier output stage, illustrated in Figure 2-10, allows the full output range of the AMC3330 to be served to the 3.3-V range single-ended embedded ADC. The signal range is amplified, and the common-mode voltage is set to half of the ADC range with the REF2033, dual-output Vref and Vref / 2 voltage reference.



Figure 2-10 Differential to Single-Ended Conversion - Output AMC3330
Equation 13. Vout=VoutP×R4R3+VoutN×R1R2+VRef

The TLV6001 operational amplifier for cost-sensitive systems is used for this purpose. Further details on the TI isolated amplifier family differential output conversion to singled ended are found in the Interfacing a Differential-Output (Isolated) Amplifier to a Single-Ended Input ADC application brief.

At ideal conditions, when the isolation barrier is unbroken and one of the resistive branches is switched in, the current flowing through the isolation barrier is 0 V and Vref / 2 should be present at the input of the ADC. The opposite occurs when the isolation barrier is fully broken, as the full DC bus voltage is seen at the isolation voltage. The voltage drop at the input of the AMC3330 is –1 V for the negative side switching or +1 V for the positive side switching. This translates into the 0 V and 3.3 V at the single-ended 0-V to 3.3-V range ADC.

Hence, a gain of 0.825 is set with the ratio of R1/R2 and R4/R3. Common-mode voltage of 1.65 V is set with the Vref / 2 output of the REF2033 voltage reference.