Maxim : Metering chips reduce space and BOM cost

Maxim Integrated Products introduces a new chipset, the MAXQ3108 and DS8102, that reduces the build of materials (BOM) cost of a polyphase energy meter. Together, the MAXQ3108 and DS8102 replace three high-cost current transformers with three low-cost shunts to save both cost and space.

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.

Electricity Metering Challenge: High Accuracy and Low Cost

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 totalmeter 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.

Finally, with dynamic ranges moving to 2000:1 or greater, one quickly understands the industry drive for a lower cost solution.

Chipset Reduces Cost and Adds Flexibility for Polyphase Energy Meters

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.

There is yet another advantage to this solution: the data coupling is now digital, suffering no loss of fidelity across any analog input range.

MAXQ3108 Key Specifications

The dual-core MAXQ3108 contains two 16-bit RISC processors. The user core has the following resources:

• 64KB flash program memory
• 2kB data SRAM
• 16 bytes battery-backed (VBAT) data SRAM
• Digitally trimmable real-time clock
• SPI™, I2C, dual USART ports
• Hardware multiplier, three Manchester decoders, and three cubic-sinc filters
• 10MHz FLL with 32kHz input

The DSP core contains:

• 8KB user-loadable SRAM code memory
• 1kB data SRAM
• Hardware multiplier

If the DSP processor is disabled, the DSP code memory can be configured as data memory for access by the user core. In this configuration the user core has access to 10KB of data memory. The ADC samples (from the decimators) can be made available to either the DSP core or the user core.

DS8102 Key Specifications

• Two second-order delta-sigma modulators
• Programmable gain to 32x
• Manchester encoder
• Internal 8MHz oscillator
• Internal reference

The MAXQ3108 is packaged in a 28-pin TSSOP and the DS8102 in a 16-pin TSSOP. Both devices operate over the -40°C to +85°C temperature range. Prices are $2.43 for the MAXQ3108 and $1.17 for the DS8102 (1000-up, FOB USA).

Note: The above text is the public part of the press release obtained from the manufacturer (with minor modifications). EETimes Europe cannot be held responsible for the claims and statements made by the manufacturer. The text is intended as a supplement to the new product presentations in EETimes Europe magazine.

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