Converter architecture for metering apps

Today's energy management and industrial automation applications often require multiple sensor readings to be handled in real-time by local embedded processors optimized for application-specific requirements.

Whether the system is tasked with antitamper residential electricity metering, the basic architectural challenges are quite similar. Effective solutions require a combination of efficient analog signal processing, precision A/D conversion, programmable simultaneous processing of mixed-signal inputs from multiple channels and integration with microprocessor-based higher level functions, such as I/O, flash memory and LCD display interfaces.

The evolution of advanced, mixed-signal SoC designs has helped fuel the utility industry's migration to solid-state metering, while driving down costs and improving antitampering capabilities through polyphase multichannel processing. Now, these same fundamental mixed-signal SoC architectures are also fueling more effective solutions for lower-cost, higher-performance energy management, process control and industrial automation solutions.

Early solid-state electricity metering designs used multiple ICs to perform the required combination of functions. For example, an MCU handled the management and control tasks, while multiple ADCs handled the metrology functions. The next level of integration came about when large manufacturers of meters independently developed proprietary ASICs to handle the A/D conversion. However, the continued evolution of new metering requirements and competitive pressures exposed the shortcomings and inflexibility of relying on fixed-function ASICs with general purpose MCUs.

The next leap forward came when highly integrated mixed-signal SoC designs were introduced. These gave meter manufacturers a single-chip, programmable solution for implementing high-performance, low-cost metering applications while still providing ample opportunity for product differentiation.

Besides the inherent benefits of single-chip SoC integration, a critical element for success is the design of the mixed-signal A/D conversion and multichannel integration functionality. For example, the patented Single Converter Technology in Teridian's 71M651x architecture uses a 21bit second-order ΔΣ ADC with up to seven multiplexed analog inputs and a programmable compute engine (CE). The 32bit CE receives and processes all sensor data from the 21bit ADC, while running independently of the on-chip 8bit MCU core that handles higher-level system management and external interface tasks. This division of functionality allows the mixed-signal metrology subsystem to deliver high speed, reliability and dynamic range, without the burden of external interruptions or unnecessary processing overhead.

Industry experience has shown that a multiplexed system typically offers the lowest cost compared to architectures that dedicate separate ADCs to each channel. Instead, a multiplexed system uses switching circuitry to scan through a number of input channels to sample each one in rotation for processing by the single ADC. Multiplexing provides the system designer with gain uniformity, offset uniformity, reduced channel to channel crosstalk, design flexibility and a lower-cost solution.

A multiplexed approach is particularly well-suited for applications with separate signals that are similar in nature, such as power measurement and many industrial-automation applications including process control sensor measurement and motor control. A key requirement is the preservation of phase information between the channels, which enables the CE in a multiplexed system to perform simultaneous measurements across different channels. In addition, single converter technology provides lower channel-to-channel crosstalk compared with architectures that dedicate separate ADCs to each channel, a key benefit in power-measurement applications.

Division of functionality allows the metrology subsystem to deliver high speed, reliability and dynamic range, without the burden of external interruptions or unnecessary processing overhead.

Flexibility
The inherent flexibility of a mixed-signal, multichannel SoC architecture can be leveraged for a wide range of advanced metering functions—from single-phase, two-input basic residential power metering to three-phase, seven-input high-end commercial power meters. It can be programmed to compensate for internal or external temperature or both variations. It can also be programmed to support active power, reactive power, rms and other measurement functions with virtually any combination of sensor inputs, including resistive shunts, current transformers or Rogowski coils.

This flexibility enables meter manufacturers and utilities to further reduce overall costs by adapting the intelligence metrology engine to leverage lower-cost sensor technology. For example, current transformers have been commonly used in polyphase metering for neutral-current measurement to detect tampering. However, Rogowski coils can be a preferred solution because they don't use a metallic core, are relatively immune to magnetic tampering and approximately 20 percent less expensive than shielded current transformers. The Rogowski coils' drawback is that its differentiated output must be electronically integrated to provide the required current flow readings. In the past, this called for a costly additional external circuitry.

With the programmable CE's ability to handle these functions internally, the industry is reducing cost by implementing antitampering functions with Rogowski coils. This can be a major advantage for utilities operating in developing markets like China, India, Russia, Eastern Europe and South America, where power usage is rapidly growing and power theft must be addressed simultaneously.

The inherent flexibility of the programmable, multichannel, mixed-signal SoC architecture also makes it suitable for implementing a variety of other applications. An automated feedback loop can be easily established by connecting inputs from one or more sensors for measuring power, pressure, position, vibration, flow, temperature or humidity. It can also connect the output to a process controller, such as a programmable logic controller, motion controller or other control system found commonly in industrial automation systems.

The SoC's internal compute engine can also be programmed to compensate for specific variations in sensor inputs, allowing the system to be optimized for specialized measurements. By connecting the device to a load cell, it can function as a weigh scale, addressing commercial and industrial costs, and precision requirements. Another important application is using the device as an "electronic trip unit" by programming trip threshold and delay attributes and monitoring power transients. The device can be used in industrial circuit breaker applications, protecting industrial manufacturing and packaging equipment.

In summary, the use of intelligent mixed-signal, multichannel SoC-based programmable devices has already spurred new levels of functionality, performance and cost reduction in metering applications. Now, the same fundamental chip-level architectures are poised to open innovative opportunities for enhancing industrial automation and control.

- Steve McClure
VP of Marketing, Teridian Semiconductor Corp.