AD71056ARZ Просмотр технического описания (PDF) - Analog Devices

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AD71056ARZ

Energy Metering IC with Integrated Oscillator and Reverse Polarity Indication

Analog Devices 

AD71056

THEORY OF OPERATION

The two ADCs in the AD71056 digitize the voltage signals from

the current and voltage sensors. These ADCs are 16-bit, Σ-Δ

with an oversampling rate of 450 kHz. This analog input struc-

ture greatly simplifies sensor interfacing by providing a wide

dynamic range for direct connection to the sensor and also

simplifies 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 power

calculation due to offsets in the voltage or current signals.

The real power calculation is derived from the instantaneous

power signal. The instantaneous power signal is generated by

a direct multiplication of the current and voltage signals. To

extract the real power component (that is, the dc component),

the instantaneous power signal is low-pass filtered. Figure 15

illustrates the instantaneous real power signal and shows how

the real power information is extracted by low-pass filtering the

instantaneous power signal. This scheme correctly calculates

real power for sinusoidal current and voltage waveforms at all

power factors. All signal processing is carried out in the digital

domain for superior stability over temperature and time.

CH1

ADC

HPF

DIGITAL-TO-

FREQUENCY

F1

F2

MULTIPLIER

LPF

DIGITAL-TO-

FREQUENCY

CH2

ADC

CF

INSTANTANEOUS

POWER SIGNAL – p(t)

INSTANTANEOUS REAL

POWER SIGNAL

TIME

TIME

Figure 15. Signal Processing Block Diagram

The low frequency outputs (F1, F2) of the AD71056 are

generated by accumulating this real power information. This

low frequency inherently means a long accumulation time

between output pulses. Consequently, the resulting output

frequency is proportional to the average real power. This

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

integration time, the CF output frequency is proportional to the

instantaneous real power. This is useful for system calibration

that can be done faster under steady load conditions.

POWER FACTOR CONSIDERATIONS

The method used to extract the real power information from

the instantaneous power signal (that is, by low-pass filtering) is

valid even when the voltage and current signals are not in

phase. Figure 16 displays the unity power factor condition and

a displacement power factor (DPF) = 0.5; that is, the current

signal lagging the voltage by 60°. Assuming the voltage and

current waveforms are sinusoidal, the real power component of

the instantaneous power signal (the dc term) is given by

⎜⎛ V × I ⎟⎞ × cos (60°)

(1)

⎝2⎠

This is the correct real power calculation.

INSTANTANEOUS

POWER SIGNAL

POWER

INSTANTANEOUS REAL

POWER SIGNAL

V× I

2

0V

CURRENT

AND

VOLTAGE

POWER INSTANTANEOUS

POWER SIGNAL

INSTANTANEOUS REAL

POWER SIGNAL

TIME

V×

2

I

COS

(60°)

0V

TIME

VOLTAGE

CURRENT

60°

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 non-

sinusoidal current and voltage waveforms. All voltage and

current waveforms in practical applications have some

harmonic content. Using the Fourier transform, instantaneous

voltage and current waveforms can be expressed in terms of

their harmonic content.

v (t) = V0 +

2×

∞

∑Vh

× sin (hω

t

+ αh)

(2)

h≠0

where:

v(t) is the instantaneous voltage.

V0 is the average value.

Vh is the rms value of Voltage Harmonic h.

αh is the phase angle of the voltage harmonic.

Rev. A | Page 10 of 20

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