Evaluation board combination computes metrology and power-quality data
The EvalSTPM-3Phiso evaluation board from STMicroelectronics combines the STPMS2 metering front-end IC and the STISO621 digital isolator, with customisable turn-key firmware running on an STM32 microcontroller to compute metrology and power-quality data. The sensing circuitry and PCB layout are optimised to ensure robustness against electromagnetic interference and a strong signal-to-noise ratio, for high-accuracy measurement and post-processing computation.
This evaluation board, 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 synchronised 4MHz clock distributed by the microcontroller and multiplexes voltage and current sigma-delta bitstreams on a single output pin. Three STPMS2 are used in the 3-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-micro seconds. 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 6kV thick-oxide galvanic-isolation technology to transfer data between isolated domains in a variety of industrial applications. The isolator features two independent channels with Schmitt-trigger inputs that ensure high noise immunity and keeps pulse distortion below 3ns. There is a maximum data rate of 100 Mbits per second, 6000V impulse withstand voltage and 1200V maximum repeating isolation voltage.
ST’s metering evaluation board meets EN 50470-x, IEC 62053-2x, ANSI12.2x standards for AC watt meters. The application provides active wideband, active fundamental, reactive and apparent power and energy data, both per-phase and cumulative, reaching accuracy Class 0.5 according to IEC 62053-22 on 3-phase active/apparent power measurement and accuracy Class 1 according to IEC 62053-21 on 3-phase reactive power measurement. It performs RMS and optional distortion calculations for each voltage and current signal, as well as DC measurements, and line period, phase shift, and phase-voltage delay for each phase.