TIDUF98 October   2024

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 End Equipment
      1. 1.1.1 Electricity Meter
    2. 1.2 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Highlighted Products
      1. 2.2.1 ADS131M03
      2. 2.2.2 MSPM0L2228
      3. 2.2.3 THVD1400
      4. 2.2.4 ISO6731
      5. 2.2.5 DRV5032
    3. 2.3 Design Considerations
      1. 2.3.1 Design Hardware Implementation
        1. 2.3.1.1 Analog Inputs
          1. 2.3.1.1.1 Voltage Measurement Analog Front End
          2. 2.3.1.1.2 Current Measurement Analog Front End
      2. 2.3.2 Energy Metrology Software
        1. 2.3.2.1 Software Architecture
        2. 2.3.2.2 Setup
          1. 2.3.2.2.1 Clocking Scheme
          2. 2.3.2.2.2 SPI
          3. 2.3.2.2.3 UART Setup for GUI Communication
          4. 2.3.2.2.4 Real-Time Clock
          5. 2.3.2.2.5 LCD Controller
          6. 2.3.2.2.6 Direct Memory Access
    4. 2.4 Hardware, Software, Testing Requirements, and Test Results
      1. 2.4.1 Required Hardware and Software
        1. 2.4.1.1 Cautions and Warnings
        2. 2.4.1.2 Hardware
          1. 2.4.1.2.1 Connections to the Test Setup
          2. 2.4.1.2.2 Power Supply Options and Jumper Settings
        3. 2.4.1.3 Calibration
      2. 2.4.2 Testing and Results
        1. 2.4.2.1 Test Setup
          1. 2.4.2.1.1 Viewing Metrology Readings and Calibration
            1. 2.4.2.1.1.1 Viewing Results From LCD
            2. 2.4.2.1.1.2 Viewing Results From PC GUI
        2. 2.4.2.2 Electricity Meter Metrology Accuracy Testing
        3. 2.4.2.3 Electricity Meter Metrology Accuracy Results
  9. 3Design Files
    1. 3.1 Schematics
    2. 3.2 Bill of Materials
    3. 3.3 PCB Layout Recommendations
      1. 3.3.1 Layout Prints
    4. 3.4 Altium Project
    5. 3.5 Gerber Files
    6. 3.6 Assembly Drawings
  10. 4Related Documentation
    1. 4.1 Trademarks
  11. 5About the Authors
2.3.1.1.2 Current Measurement Analog Front End

Analog Front End for Shunt Current Input shows the analog front end for the current input, where the positive and negative leads from the external current SHUNT sensor (not shown) are connected to pins 1 and 2 of header J14.

TIDA-010940 Analog Front End for Shunt Current InputFigure 2-5 Analog Front End for Shunt Current Input

The analog front end for current input consists of footprints for electromagnetic interference filter beads (R74 and R81), and an RC low-pass filter (C30, C34, R34, R35, and C38) that functions as an anti-alias filter.

Equation 2 shows how to calculate the range of differential voltages fed to the current ADC channel for a given maximum current and shunt value.

Equation 2. TIDA-010940

The shunt sensor value for an electricity meter is selected based on the tradeoff between accuracy and shunt power dissipation. If the shunt value is decreased, less power is dissipated through the shunt; however, the decreased shunt value means a smaller output voltage from the shunt, which leads to worse accuracies at lower currents, even if a higher PGA gain is used to boost the shunt output.

Based on the VADC, Current,Shunt range, select the proper PGA gain by looking at the full-scale range table in Table 2-1 to find the two gain ranges that VADC, Current,Shunt fits between. From these two gain values, select the lower one as the PGA gain setting as this maximizes the utilized ADC range without saturation occurring at higher currents. As an example, suppose a 100A maximum RMS current and a 200µΩ shunt is used. Based on these values, VADC,shunt RMS varies between ±28.3mV and this voltage range lies between the maximum ±37.5mV voltage at a gain of 32 and ±18.75mV at a PGA gain of 64; so the PGA gain setting of the shunt channel is set for 32 (the lower gain value).

Table 2-1 Full-Scale Range of ADS131M03
GAIN SETTINGFSR
1±1.2V
2±600mV
4±300mV
8±150mV
16±75mV
32±37.5mV
64±18.75mV
128±9.375mV