TIDA-010243 Block Diagram, Three-Phase + Neutral Configuration depicts the block diagram of the high-level interface used for an ADS131M08-based three-phase energy measurement application. For each phase, the line-to-neutral voltage is directly measured, as well as the current for each line (3 phases) and the current thru the N (neutral) wire.

In the TIDA-010243 block diagram, a current transformer (CT) connects to each current channel and a simple voltage divider is used for dividing down the corresponding voltage of each channel. Each CT has an associated burden resistor that must be connected at all times to protect the measuring device. The selection of the CT and the burden resistor is made based on the manufacturer and current range required for energy measurements.

The choice of voltage divider resistors for the voltage channel is selected to make sure the Mains voltage is divided down to adhere to the normal input ranges of the ADS131M08 device. Since the ADS131M08 ADCs have a large dynamic range and a large dynamic range is not needed to measure voltage, the voltage front-end circuitry is purposely selected so that the maximum voltage seen at the inputs of the voltage channel ADCs are only a fraction of the full-scale voltage.

Figure 2-1 TIDA-010243 Block Diagram, Three-Phase + Neutral Configuration

By reducing the voltage fed to the three ADS131M08 voltage ADC channels, voltage to current crosstalk, which actually affects metrology accuracy more than voltage ADC accuracy, is reduced at the cost of voltage accuracy, thereby resulting in more accurate energy measurements at lower currents.

The ADS131M08 device interacts with the MSPM0+ MCU in the following manner:

1. The CLKIN clock used by the ADS131M08 device is provided from the M0_CLKOUT clock signal output of the MSPM0G3507 MCU.
2. The ADS131M08 device divides the clock provided on the CLKIN pin by two and uses this divided clock as the delta-sigma modulation clock.
3. When new ADC samples are ready, the ADS131M08 device asserts the DRDY pin, which alerts the MSPM0+ MCU that new samples are available.
4. After being alerted of new samples, the MSPM0+ MCU uses one of the SPIs and two of the DMA channels in the DMA module to get the voltage and current samples from the ADS131M08 device.

The optional TPS3840 device is used as an external SVS for the MSPM0+ MCU. Although the MSPM0+ MCU has internal Power-on reset (POR) and POR as well as a Brownout reset (BOR) supply monitor with four configurable threshold voltages, the external TPS3840 standalone SVS adds redundancy in case of a power failure.

Other signals of interest in Figure 2-1 are the active and reactive energy pulses used for accuracy measurement and calibration. The ISO6720 device provides an isolated connection for these pulses for connecting to non-isolated equipment. This design also supports isolated RS-232 communication through the use of the TPS70933, ISO6731B, and TRS3232E devices. The hardware uses a push switch to select between the RS-485 interface, with ISO6731 and THVD1400 devices, or the RS-232 interface with TRS3232E.

The design can be powered either by applying 3.3 V at TP6 and GND at TP1 directly or by connecting 3.3 V and GND to the Application Board Connector J13. See Header Names and Jumper Settings for more details on the proper jumper connections for powering the board for both options.