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

Signal Chain Error Analysis

The conditioning can be done through the calculation tool Isolated Amplifier Voltage Sensing Excel Calculator. The tool offers a detailed worst-case error analysis across temperature considering gain error and drift, and offset error and drift, and nonlinearity drift. Note that the error analysis is output referenced.

Hence, the errors in the analog front end, come mainly from the offset voltages of the AMC ( V o f f s e t A M C ) and the TLV6001 ( V o f f s e t d i f f 2 s i n g l e ), and the gain error in the internal gain of the AMC ( e A M C ) and in the gain set by the resistors in the differential-to-single-ended conversion ( e d i f f 2 s i n g ):

Equation 15. V i s o l a t i o n = V i n A D C - V o f f s e t d i f f 2 s i n g l e - V r e f G a i n d i f f 2 s i n g l e + G a i n d i f f 2 s i n g × e d i f f 2 s i n g - V o f f s e t A M C × R i n A M C + R s t R i n A M C × 1 G a i n A M C + G a i n A M C × e A M C

The results in Section 3.4 show that with the actual signal chain and resistive bridge, asymmetrical faults are detected with less than 3% error, symmetrical faults with less than 5% error, asymmetrical warnings with less than 3% error and symmetrical warnings with less than 3% error. If higher accuracy is desirable, precision op amps can be used for the differential-to-single-ended conversion, such as the TLV6001 pin-to-pin compatible OPA320 (Voffset 0.15 mV).

If even higher accuracy on the isolation voltage readings is needed, a stand-alone precision ADC out of TI’s family can be used for this application. A differential input ADC could be used for this purpose, as there would be no need for a differential-to-single-ended pre-stage. Moreover, in this reference design the internal 12-bit ADC in the TMS320F280049C was used, which leads to a step size of 0.8 mV. Higher accuracy ADCs with 16-bits could be used for better precision.

The aim of this reference design was to show that good accuracy can be achieved with a cost-efficient solution.