A new evaluation board from ST Microelectronics speeds design of three-phase AC Watt meters that meet the most stringent international quality and accuracy standards using low-cost, electromagnetic-immune shunt sensors and advanced galvanic-isolation technology for superior reliability and robustness.
The EVALSTPM-3PHISO evaluation board combines the high-accuracy STPMS2 metering front-end IC and the advanced STISO621 digital isolator with customizable turn-key firmware running on an STM32 microcontroller to compute metrology and power-quality data. The sensing circuitry and PCB layout are 
Source: STMicroelectronics optimized to ensure robustness against electromagnetic interference and a strong signal-to-noise ratio for high-accuracy measurement and post-processing computation.
The STPMS2, a two-channel 24-bit second-order sigma-delta modulator, measures voltage and current for each phase through an on-board voltage divider and a shunt current sensor. It then oversamples the signal using a synchronized 4 MHz clock distributed by the microcontroller and multiplexes voltage and current sigma-delta bitstreams on a single output pin. Three STPMS2 are used in the three-phase system to collect voltage and current data from each phase.
The provided firmware leverages digital filters for sigma-delta modulators integrated in the chosen STM32 microcontroller to convert the six bitstreams into 24-bit voltage and current values and computes all metrology data in real-time every 200 µs. The firmware also implements a virtual com port that provides access to internal parameters for reading metrology data, modifying the internal configuration and calibrating the board.
The STISO621 dual-channel digital isolator is the first in a new series of ICs that leverage ST’s 6 kV thick-oxide galvanic-isolation technology to transfer data between isolated domains in a variety of industrial applications. The STISO621 features two independent channels with Schmitt-trigger inputs that ensure high noise immunity and keeps pulse distortion below 3 ns. With a maximum data rate of 100 Mbit/s, 6000 V impulse withstand voltage and 1200 V maximum repeating isolation voltage, the STISO621 ensures faster data transmission, longer lifetime and higher reliability than conventional optical isolators.
The EVALSTPM-3PHISO can be also used as a reference for multi-phase industrial metering applications beyond utility metering, such as electric vehicle charging, servers and solar inverters.
