By Lucio Di Jasio
The first ingredient of these solutions is the integration of analogue functions required to perform the voltage and current product in a single analogue front-end (AFE) device. Microchip’s MCP3905A/6A and MCP3909 (Figure 1) metering AFEs feature:
- Two 16-bit delta-sigma analogue-to-digital (A/D) converters providing a dynamic range of up to 1000:1 (in the MCP3906A and MCP3909 models) and a typical measurement error of 0.1 percent.
- Available in minuscule 24-pin SSOP packages, they consume only 4 mA maximum current (consuming under 20 mW or just a fraction of the available 2 W budget) while operating in the entire -40 to +85OC temperature range, with a reference thermal drift of just 15 ppm/OC.
- A programmable gain amplifier, combined with the great sensitivity of the current sensing channel, allows for extremely small shunt values to be used (less than 200 µâ„¦).
- Tampering alert is provided automatically when a negative power condition is detected (inverted wiring).
The MCP3905A/6A and MCP3909 can be used with a few external components to implement a complete and inexpensive monophase meter (Figure 2), but its performance can be augmented by pairing it with advanced Flash microcontrollers, such as the PIC18F8490. Additionally, the MCP3909 can be used in 3-phase meters by accessing the A/D converter data directly and sending it to the microcontroller for additional post-processing (Figure 3). By using the Flash microcontroller as the computation engine to calculate power, the flexibility of the meter design increases dramatically. Automatic software calibration, flexibility in power calculation methods and additional security features can be implemented.
The PIC18 family of high-performance 8-bit microcontrollers has more than a hundred models based on Flash technology, and incorporates a variety of features and peripherals that are critical to energy metering applications. In the case of the PIC18F8490, the applicable features include:
- Input capture and compare logic to directly interface to the MCP3905/6 AFE
- Onboard LCD display driver, allowing low power and inexpensive control of complex LCD displays
- 16 kb of enhanced Flash program memory, allowing in-line (production) programming and calibration, as well as easy field firmware upgrades
- Flexible code-protection scheme, to provide the security required to implement the encryption used by billing and AMR applications
- nanoWatt technology for power management, allowing very low-power operation, but also high performance when required to perform AMR and billing
- Onboard 32 kHz oscillator for real time clock and calendar support of time-of-use and other advanced billing functions
- Flexible asynchronous serial communication port, capable of RS485 as well as DL/T645 infrared support of automated meter reading applications
- Flexible synchronous serial communication port, to support connection to external non-volatile memory devices (Serial EEPROMS) using I2C™ and/or SPI protocols
- Onboard 10-bit A/D converter, to assist in anti-tampering functionality and/or temperature sensing
- Brown-out detection and low voltage detection circuits for robustness and more anti-tampering features.
ENERGY METERING DESIGN CENTRE
Understanding the need to find all of the information necessary to create an energy-meter solution in one place, Microchip is offering a Design Centre completely dedicated to utility metering applications (www.microchip.com/meter). Microchip’s energy meter site presents the reader with a simple block diagram where each block links to a new page that is dedicated to a specific aspect of the design.