Datasheet AD71056 (Analog Devices) - 10

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AD71056 THEORY OF OPERATION
The two ADCs in the AD71056 digitize the voltage signals from
POWER FACTOR CONSIDERATIONS
the current and voltage sensors. These ADCs are 16-bit, Σ-Δ The method used to extract the real power information from with an oversampling rate of 450 kHz. This analog input struc- the instantaneous power signal (that is, by low-pass filtering) is ture greatly simplifies sensor interfacing by providing a wide valid even when the voltage and current signals are not in dynamic range for direct connection to the sensor and also phase. Figure 16 displays the unity power factor condition and simplifies the antialiasing filter design. A high-pass filter in the a displacement power factor (DPF) = 0.5; that is, the current current channel removes any dc component from the current signal lagging the voltage by 60°. Assuming the voltage and signal. This eliminates any inaccuracies in the real power current waveforms are sinusoidal, the real power component of calculation due to offsets in the voltage or current signals. the instantaneous power signal (the dc term) is given by The real power calculation is derived from the instantaneous ⎛ V × I ⎞ power signal. The instantaneous power signal is generated by ⎜ ⎟ × cos ( ° 60 ) (1) ⎝ 2 ⎠ a direct multiplication of the current and voltage signals. To extract the real power component (that is, the dc component), This is the correct real power calculation. the instantaneous power signal is low-pass filtered. Figure 15 illustrates the instantaneous real power signal and shows how
INSTANTANEOUS INSTANTANEOUS REAL POWER SIGNAL POWER SIGNAL
the real power information is extracted by low-pass filtering the
POWER
instantaneous power signal. This scheme correctly calculates real power for sinusoidal current and voltage waveforms at all
V × I
power factors. All signal processing is carried out in the digital
2
domain for superior stability over temperature and time.
0V TIME DIGITAL-TO- FREQUENCY CURRENT F1 AND CH1 ADC F2 VOLTAGE HPF POWER INSTANTANEOUS REAL INSTANTANEOUS POWER SIGNAL MULTIPLIER DIGITAL-TO- POWER SIGNAL LPF FREQUENCY CH2 ADC CF V × I 2 COS (60°) INSTANTANEOUS INSTANTANEOUS REAL 0V TIME POWER SIGNAL – p(t) POWER SIGNAL
6
VOLTAGE CURRENT
-01 6
60°
63 05 Figure 16. DC Component of Instantaneous Power Signal Conveys 5 01 Real Power Information, PF < 1 6- 63
TIME TIME
05 Figure 15. Signal Processing Block Diagram
NONSINUSOIDAL VOLTAGE AND CURRENT
The real power calculation method also holds true for non- The low frequency outputs (F1, F2) of the AD71056 are sinusoidal current and voltage waveforms. All voltage and generated by accumulating this real power information. This current waveforms in practical applications have some low frequency inherently means a long accumulation time harmonic content. Using the Fourier transform, instantaneous between output pulses. Consequently, the resulting output voltage and current waveforms can be expressed in terms of frequency is proportional to the average real power. This their harmonic content. average real power information is then accumulated (for example, by a counter) to generate real energy information. ∞ Conversely, due to its high output frequency and, hence, shorter v t ( ) = V + 2 × ∑V × sin hω t + α ) (2) 0 h ( h integration time, the CF output frequency is proportional to the h≠0 instantaneous real power. This is useful for system calibration where: that can be done faster under steady load conditions. v(t) is the instantaneous voltage. V0 is the average value. Vh is the rms value of Voltage Harmonic h. α is the phase angle of the voltage harmonic. h Rev. A | Page 10 of 20 Document Outline FEATURES GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS TIMING CHARACTERISTICS Timing Diagram ABSOLUTE MAXIMUM RATINGS ESD CAUTION TERMINOLOGY PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION POWER FACTOR CONSIDERATIONS NONSINUSOIDAL VOLTAGE AND CURRENT APPLICATIONS 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 Frequency Output CF SELECTING A FREQUENCY FOR AN ENERGY METER APPLICATION Frequency Outputs NO LOAD THRESHOLD NEGATIVE POWER INFORMATION OUTLINE DIMENSIONS ORDERING GUIDE