Maxim's Chipset Eliminates Costly Current Transformers in Polyphase Energy Meters

The DS8102 contains two high-precision second-order delta-sigma modulators with programmable gains up to 32x. The modulators' outputs are encoded into a single bit stream to minimize the isolation and data-coupling cost. The data processing is carried out in the MAXQ3108, a dual-core microcontroller that accepts three DS8102 output bit streams, then decodes and decimates that data to produce raw ADC samples across a wide dynamic range. The ADC samples are fed into the MAXQ3108's integrated digital signal processor that computes the power, energy, power factors, and RMS voltage and current parameters required for a multifunction electricity meter.

The MAXQ3108 and DS8102 chipset thus reduces total system cost and delivers high performance and flexibility for polyphase electricity meters. The MAXQ3108's DSP is also programmable, making the chipset suitable for a wide variety of industrial data-acquisition applications where isolation between the physical signals and the data-acquisition instrument is required. Reference designs are available for assistance.

The electricity metering industry's requirements are evolving rapidly and becoming more challenging for both meter designers and suppliers of electricity measurement circuits. The electric utility and energy consumers, moreover, want accurate metering across a wide range of loads. With huge amounts of power measured constantly, each kWh that is monitored inaccurately represents a significant monetary loss to the power companies or to the consumers.

A typical class 1 meter design today must measure current over a 1000:1 range or better. This means that the electricity meter must measure power/energy with less than 1% error of the quantity measured, all while the quantity itself varies by a factor of 1000 to 1. An even wider dynamic range is coming for the industry, with 2000:1 or even 5000:1 being discussed now. It is also important to note that the error budget for the metering IC is normally much smaller than the total meter error budget. With a class 1 meter, for example, the typical error budget for the metering IC is < 0.3%.

Since the AC voltage supply in a power system rarely varies more than 10%, the key to achieving precision power measurement is to measure the AC current accurately. There are three critical elements for acquiring high-precision current signal data: the ADC, the current sensor, and the meter's printed circuit board. The ADC and current sensor both need to be accurate across the target dynamic range. There are additional demands for the current sensors, and linearity is the most critical specification. A resistive shunt has been popular for single-phase meters due to its low cost and excellent linearity. But for polyphase meters, there are traditionally two types of current sensors: current transformers and Rogowski coils (i.e., di/dt sensors). Because of the inherent voltage differentials between the three phases, the voltage and current signals must be isolated from the data-acquisition device (the metering IC). Admittedly, current transformers and di/dt sensors both provide that needed isolation, while also coupling the analog signals to the data-acquisition device through a magnetic field. However, that precise analog signal coupling is expensive, and the cost increases significantly with the signal's dynamic range and the requirement for high linearity.

The DS8102 modulator and encoder and the MAXQ3108 microcontroller form a unique solution to the isolation and data-coupling challenge for today's polyphase energy meters. A three-phase meter uses three DS8102s and one MAXQ3108. Each DS8102 floats on the respective phase that it is measuring. A DS8102 converts the voltage and current inputs into a high-frequency digital bit stream that is then coupled to the MAXQ3108 through a low-cost capacitor. The MAXQ3108 is DC isolated from the three DS8102s, but AC coupled to them to accept the digital data bit streams from each. With each DS8102 isolated from the other phases and from the MAXQ3108, each phase can use a shunt resistor for its current sensing. The expensive current transformer is thus eliminated and BOM costs reduced significantly.