The Sonoma (MAXREFDES14#) energy measurement subsystem reference design provides galvanic isolation from the system with a single pulse transformer while using resistors as the sensing elements. The result is a small, cost-optimized board. AC measurement applications often require galvanic isolation to protect the system and user from high voltages. This is typically accomplished by either using bulky voltage/current transformers for sensors or by isolating the data and power interface to the measurement subsystem. These approaches, however, consume a considerable amount of space and come with hidden costs and design challenges.
The Sonoma design utilizes an isolated energy measurement processor (MAX78615+LMU); a multichannel, precision analog-to-digital converter (ADC) (MAX78700); a pulse transformer; optional 20MHz crystal oscillator, and the appropriate sense resistors for converting AC voltage and current into measurable signals. With the embedded load monitoring unit (LMU) firmware and nonvolatile storage of calibration and configuration data, Sonoma is a complete measurement subsystem ready for integration into any design.
Figure 1. The Sonoma subsystem design block diagram.