The choice of voltage divider resistors for the voltage channel is selected to ensure the mains voltage is divided
down to adhere to the normal input ranges of the ADS131M04 device. Since the ADS131M04 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. By reducing the voltage fed to the ADS131M04 voltage ADC, 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.
In this design, the ADS131M04 device interacts with the MSP432
™
MCU in the following manner:
1. The CLKIN clock used by the ADS131M04 device is provided from the SMCLK clock signal output of the
MSP432 MCU.
2. The ADS131M04 device divides the clock provided on its CLKIN pin by two and uses this divided clock as its
delta-sigma modulation clock.
3. When new ADC samples are ready, the ADS131M04 device asserts its DRDY pin, which alerts the MSP432
MCU that new samples are available.
4. After being alerted of new samples, the MSP432 MCU uses one of its SPI interfaces and its DMA to get the
voltage and current samples from the ADS131M04 device.
In this design, a TPS3840 device is used as an external SVS for the MSP432 MCU. Although the MSP432 MCU
has an internal SVS that suffices for this application, the TPS3840 standalone SVS is used because there is
additional security in having a SVS that is independent of the MCU.
are the active and reactive energy pulses used for
accuracy measurement and calibration. The ISO7720 device provides an isolated connection for these pulses
for connecting to non-isolated equipment. This is especially needed for the one-voltage configuration since the
system is referenced with respect to one of the two high-voltage line voltages for this configuration.
In addition to isolated pulses, the design supports isolated RS-232 communication through the use of the
TPS70933, ISO7731B, and TRS3232E devices. The design can be configured to use RS-485 as well instead of
RS-232 through the use of the ISO7731B and THVD1500 devices on the board.
The design also supports communication using external radios or radio modules that are connected to the J11
and J12 headers of the board. The J11 header connects the P7.0, P7.1, P7.2, and P7.3 port pins, which can be
configured as port mappable GPIO pins, if the LCD is disabled. Software can be added to the MSP432 MCU to
communicate to an external radio or radio module by configuring any of these four pins as communication
pins(SPI or UART) and appropriately writing and reading packets to and from them. The J12 header is the power
rail and GND connection to the radio. The power rail on J12 is the current-limited version of the 5V_IN rail at J4.
The J12 power rail is specifically current limited to 1.6 A by the TPS25921L device. If there is a short on the
radio module or the current exceeds 1.6 A for some reason, the power to this rail will be limited to prevent the
power supply from affecting the power to the metrology-relevant portions of this board. If it is a transient
overcurrent event, the module continues to receive power and operate without interruption. However, if there is a
persistent fault on the module, the TPS25921L device will enter thermal shutdown and power is disconnected.
After this thermal shutdown event, the power can only be reconnected to the radio module manually by driving
the P5.2 GPIO pin. In addition, if the voltage fed to the 5V_IN rail at J4 is below approximately 4.0 V or above
5.8 V, the TPS25921L device is configured to disconnect the power rail on J12. To protect the TPS25921L
device from any transients due to switching, a TVS1800 transient protection device is connected to this device.
The design can be powered either by applying 3.3 V at the DVCC header(J9) directly or through the TPS70933
device by applying 5 V at the 5V_IN header(J4). See
for more details on the proper jumper
connections for powering the board for both options.
2.2 Highlighted Products
2.2.1 ADS131M04
The ADS131M04 device is a four-channel, simultaneously-sampling, 24-bit, 2nd order delta-sigma (ΔΣ), analog-
to-digital converter (ADC) that offers wide dynamic range, and internal calibration features making it well-suited
for energy metering, power quality, and protection applications. The ADC inputs can be directly interfaced to a
resistor-divider network, a transformer to measure voltage or current, or a Rogowski coil to measure current.
System Overview
6
High-Accuracy Split-Phase CT Electricity Meter Reference Design Using
Standalone ADCs
TIDUEM8B – MARCH 2019 – REVISED FEBRUARY 2021
Copyright © 2021 Texas Instruments Incorporated
