Datasheet ADE7768 (Analog Devices) - 10

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ADE7768 FUNCTIONAL DESCRIPTION THEORY OF OPERATION
The two ADCs in the ADE7768 digitize the voltage signals from the current and voltage sensors. These ADCs are 16-bit Σ-Δs with an oversampling rate of 450 kHz. This analog input
MAGNITUDE ACTIVE ENERGY
structure greatly simplifies sensor interfacing by providing a
0Wh
wide dynamic range for direct connection to the sensor and by simplifying the antialiasing filter design. A high-pass filter in the current channel removes any dc component from the current signal. This eliminates any inaccuracies in the real
MAGNITUDE INSTANTANEOUS
power calculation due to offsets in the voltage or current
POWER 0W
signals. The real power calculation is derived from the instantaneous 05331-028 power signal. The instantaneous power signal is generated by Figure 16. Positive-Only Energy Accumulation a direct multiplication of the current and voltage signals. To extract the real power component (the dc component), the
Power Factor Considerations
instantaneous power signal is low-pass filtered. Figure 15 The method used to extract the real power information from illustrates the instantaneous real power signal and shows how the instantaneous power signal (that is, by low-pass filtering) is the real power information can be extracted by low-pass still valid even when the voltage and current signals are not in filtering the instantaneous power signal. In the ADE7768, phase. Figure 17 displays the unity power factor condition and a this signal is compared to 0 and only positive real power is displacement power factor (DPF) = 0.5 (a current signal lagging accumulated for F1, F2, and CF pulse outputs. This scheme the voltage by 60°). Assuming that the voltage and current correctly calculates real power for sinusoidal current and waveforms are sinusoidal, the real power component of the voltage waveforms at all power factors. All signal processing instantaneous power signal (that is, the dc term) is given by is carried out in the digital domain for superior stability over temperature and time. ⎛ V × I ⎞ ⎜ ⎟ × cos ( ° 60 ) (1)
DIGITAL-TO-
⎝ 2 ⎠
FREQUENCY F1 CH1 ADC F2
This is the correct real power calculation.
HPF MULTIPLIER
≥
0 DIGITAL-TO- LPF FREQUENCY INSTANTANEOUS INSTANTANEOUS REAL POWER SIGNAL POWER SIGNAL CH2 ADC POWER CF INSTANTANEOUS INSTANTANEOUS REAL POWER SIGNAL – p(t) POWER SIGNAL V
×
I 2 0V TIME CURRENT VOLTAGE TIME TIME
05331-005
INSTANTANEOUS REAL POWER INSTANTANEOUS POWER SIGNAL
Figure 15. Signal Processing Block Diagram
POWER SIGNAL
The low frequency outputs (F1 and F2) are generated by accumulating positive-only real power information. This low
V
×
I COS (60
°
)
frequency inherently means a long accumulation time between
2
output pulses. Consequently, the resulting output frequency is
0V TIME
proportional to the average positive-only real power. This average positive-only real power information is then accumu-
VOLTAGE CURRENT 60
° lated (by a counter) to generate real energy information (see 05331-006 Figure 16). Conversely, due to its high output frequency and Figure 17. DC Component of Instantaneous Power Signal Conveys shorter integration time, the CF output frequency is propor- Real Power Information, PF < 1 tional to the instantaneous positive-only real power. This is useful for system calibration, which can be done faster under steady load conditions. Rev. A | Page 10 of 20 Document Outline FEATURES GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM SPECIFICATIONS TIMING CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS ESD CAUTION TERMINOLOGY PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS FUNCTIONAL DESCRIPTION THEORY OF OPERATION Power Factor Considerations Nonsinusoidal Voltage and Current ANALOG INPUTS Channel V1 (Current Channel) Channel V2 (Voltage Channel) Typical Connection Diagrams POWER SUPPLY MONITOR HPF and Offset Effects Digital-to-Frequency Conversion Connecting to a Microcontroller for Energy Measurement Power Measurement Considerations INTERNAL OSCILLATOR (OSC) TRANSFER FUNCTION Frequency Outputs F1 and F2 Example Frequency Output CF SELECTING A FREQUENCY FOR AN ENERGY METER APPLICATION Frequency Outputs NO-LOAD THRESHOLD NEGATIVE POWER INFORMATION EVALUATION BOARD AND REFERENCE DESIGN BOARD OUTLINE DIMENSIONS ORDERING GUIDE