Designed to measure the current and voltage (electric power) of circuits, an energy meter is a useful tool to monitor the power consumption (kilowatt-hour units) in homes, factories, offices, and more.

Today, these smart meters are increasingly in demand in view of the ever-growing number of energy-saving and eco-friendly initiatives and regulations.

The first complete 3-phase meter design with shunt current sensors

This solution provides a full set of metrology parameters: active wideband, active fundamental, reactive and apparent power and energy data, both per-phase and cumulative. It performs RMS and optional distortion (THD) calculations for each voltage and current signal, as well as DC measurements, and line period, phase shift, and phase-voltage delay for each phase.

Leveraging the STM32F4 MCU’s six independent digital filters for sigma-delta modulators (DFSDM), it can process in real time and in parallel all the voltage and current SD bitstreams, guaranteeing a 2.5 kHz bandwidth on each signal and high timing resolution for power and energy measurements. The STM32F4 MCU can also interface with the STPMS2’s modulator using only two wires for isolation via the STISO621W dual-channel digital isolator which ensures a 6000V impulse withstand voltage (V_IOTM), and a 1200V maximum repeating isolation voltage (V_IORM).

How does the solution work?

The STM32F4 high-performance microcontroller based on an Arm® Cortex®-M4 32-bit core distributes a synchronized clock to the three STPMS2 smart sensors by configuring the dedicated DFSDM.

The STPMS2, a low-cost dual-channel 24-bit second-order sigma-delta modulator, measures the circuit’s voltage and current at each phase through a voltage divider and a shunt current sensor, respectively. The modulator oversamples the signal using the STM32F4’s 4-MHz clock and multiplexes voltage and current sigma-delta bitstreams on a single output pin. Thanks to its programmable chopper-stabilized, low noise, and low offset amplifier, the STPMS2 smart sensor ensures a high-precision signal conversion in a wide signal range. It also offers very low power consumption in High-precision mode (4 mA at 4 MHz), which can be further reduced in Low power mode (1.2 mA at 1 MHz).

The STM32F4’s DFSDM demultiplexes the voltage and current streams and uses a dedicated filter to remove the quantization noise and provide a “slower” 24-bit digital signal.

From the converted voltage and current signals, the STM32F4’s embedded digital signal processor (DSP) and floating-point unit (FPU) computes all metrology data in real-time every 200 µs, and still leaves room for an additional metrology application.

Suitable for harsh environments, the STISO621W dual-channel digital isolators include with a Schmitt trigger input, providing robustness to noise and high-speed input/output switching time. This solution uses three STISO621W devices to effectively isolate the clock and data signals between each STPMS2 and the STM32F4 for safe and fast (up to 100 Mbit/s) data transfers between domains as well as keeps insulation integrity over a long lifetime and during any possible faults.