Datasheet ADE7769 (Analog Devices) - 10

link to page 10 link to page 10
ADE7769 FUNCTIONAL DESCRIPTION THEORY OF OPERATION Power Factor Considerations
The two ADCs in the ADE7769 digitize the voltage signals from The method used to extract the real power information from the current and voltage sensors. These ADCs are 16-bit Σ-Δs the instantaneous power signal, that is, by low-pass filtering, is with an oversampling rate of 450 kHz. This analog input still valid even when the voltage and current signals are not in structure greatly simplifies sensor interfacing by providing a phase. Figure 16 shows the unity power factor condition and a wide dynamic range for direct connection to the sensor and by displacement power factor (DPF) = 0.5, that is, current signal simplifying the antialiasing filter design. A high-pass filter in lagging the voltage by 60°. Assuming that the voltage and the current channel removes any dc component from the current waveforms are sinusoidal, the real power component of current signal. This eliminates any inaccuracies in the real the instantaneous power signal (that is, the dc term) is given by power calculation due to offsets in the voltage or current signals. ⎛ V × I ⎞ ⎜ ⎟ × cos (60°) (1) ⎝ 2 ⎠ The real power calculation is derived from the instantaneous power signal. The instantaneous power signal is generated by This is the correct real power calculation. a direct multiplication of the current and voltage signals. To
INSTANTANEOUS INSTANTANEOUS REAL
extract the real power component (the dc component), the
POWER SIGNAL POWER SIGNAL POWER
instantaneous power signal is low-pass filtered. Figure 15 illustrates the instantaneous real power signal and shows how the real power information can be extracted by low-pass
V
×
I 2
filtering the instantaneous power signal. This scheme correctly calculates real power for sinusoidal current and voltage
0V TIME
waveforms at all power factors. All signal processing is carried out in the digital domain for superior stability over temperature
CURRENT VOLTAGE
and time.
INSTANTANEOUS REAL POWER INSTANTANEOUS POWER SIGNAL POWER SIGNAL DIGITAL-TO- FREQUENCY F1 CH1 ADC F2 HPF V
×
I COS (60
°
) 2 MULTIPLIER DIGITAL-TO- LPF FREQUENCY 0V TIME CH2 ADC CF VOLTAGE CURRENT 60
°
INSTANTANEOUS INSTANTANEOUS REAL
05332-006
POWER SIGNAL – p(t) POWER SIGNAL
Figure 16. DC Component of Instantaneous Power Signal Conveys Real Power Information, PF < 1
Nonsinusoidal Voltage and Current
The real power calculation method also holds true for
TIME TIME
05332-005 nonsinusoidal current and voltage waveforms. All voltage and current waveforms in practical applications have some Figure 15. Signal Processing Block Diagram harmonic content. Using the Fourier transform, instantaneous The low frequency outputs (F1 and F2) are generated by voltage and current waveforms can be expressed in terms of accumulating this real power information. This low frequency their harmonic content. inherently means a long accumulation time between output pulses. Consequently, the resulting output frequency is propor- ∞ v t ( ) = V + 2 × ∑V × sin hωt + α ) (2) 0 ( tional to the average real power. This average real power h h h ≠ 0 information is then accumulated (by a counter) to generate real energy information. Conversely, due to its high output frequen- where: cy and shorter integration time, the CF output frequency is v(t) is the instantaneous voltage. proportional to the instantaneous real power. This is useful for V0 is the average value. system calibration, which can be done faster under steady load Vh is the rms value of voltage harmonic h. conditions. αh is the phase angle of the voltage harmonic. 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