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The World Leader in High Performance Signal Processing Solutions

di/dt Current Sensor and

Digital Integrator Technology ADE7759

Energy Measurement GroupPrecision Converters (PRC) Division

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Current Induces Magnetic Field

When current passes through a conductor, magnetic field is formedaround the conductor. The magnitude of magnetic field is directlyproportional to the current:

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Voltage induced in a wire loop byChanging Magnetic Field The changes in magnetic field induces a electromotive force ( EMF )within a wire loop. The EMF is a voltage signal and is proportional to

the changes in the magnitude field inside the loop.

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Voltage Output Proportional to di/dt

Combining the two formulas:

The output voltage of the loop is therefore proportionalto the time-differentiation (di/dt) of the current

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An example of di/dt sensor

EMF (in volt) outputs from a aircore coil around a long straightwire is*:

e.g. a Current Transformer (CT)

without the ferrite core can beused as a di/dt sensor

* the coil is assumed to be rectangular shape and has N-turn

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di/dt sensor has all advantages of CT

di/dt sensor has the same advantages of CT:

Electrical isolation

The di/dt sensor detects current without any contact to the conductor

Capable of handling high current

di/dt is capable of handling very high current

Low power consumption

No significant power consumption needed for current sensing

Low temperature shift The output varies very little with changes in temperature

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But better

di/dt sensor outperforms CT in many ways:

No DC or high current saturation

The saturation point of air is extremely high comparing with ferrite core Light weight

No need to use heavy ferrite core

Phase characteristics

Air core has linear phase response (90 phase shift for sinusoidal signal)

Low costEven a small di/dt sensor can accurately detect hundreds of Amperes of current

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Why a digital integrator?Advantages of digital implementation:

Stability of performance The operating condition of an energy meter is very hostile. Digital processingprovides reliable performance over the long operating life required. Analogcomponents are susceptible to drift over time and temperature which will resultin large measurement error.

Outstanding phase characteristicsAnalog components can introduce slight phase variation and would require

phase calibration. A digital integrator has consistent and accurate phaseresponse.

Reduced costNo external component (high performance op-amp) is needed to implement the

integrator.

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Building an integrator: using op-amp

An analog integrator using an op-amp:

Error includes: offset error, leakage current, long-termstability of op-amp and temperature drift

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Building an integrator: digital integrator

Time domain Frequency domain

Integration in time domain is equivalent to the following in the frequency domain:

Magnitude: -20dB/decade Phase: 90 phase shift for all frequency

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Digital Integrator in ADE7759

Digital Integrator in ADE7759:

Magnitude Response: -20dB/decade Phase Response: less than 0.05deviation from -90

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ADE7759 Block Diagram

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Block Diagram: A meter based on the

energy metering IC

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ADE7759

Overview of Functionalities Incorporated digital integrator and PGA simplify choice for current sensors

Surpasses IEC1036 requirement. Less than 0.1% error over a largecurrent dynamic range

Provides the following information:

kW, kWHrInstantaneous Power, Voltage, and Current waveforms

Temperature measurement

Voltage SAG warning

Full digital calibration through serial port

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ADE7759 Functional Block Diagram