REV. A

–8–

ADE7757

THEORY OF OPERATION

The two ADCs digitize the voltage signals from the current

and voltage sensors. These ADCs are 16-bit

-

with an

oversampling rate of 450 kHz. This analog input structure

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 chan-

nel 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. Because the HPF is

always enabled, the IC will operate only with ac input (see HPF

and Offset Effects section).

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. In order

to extract the real power component (i.e., the dc component),

the instantaneous power signal is low-pass filtered. Figure 3

illustrates the instantaneous real power signal and shows how

the real power information can be 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.

TIME

TIME

ADC

ADC

CH1

CH2

MULTIPLIER

LPF

F1

F2

DIGITAL-TO-

FREQUENCY

CF

DIGITAL-TO-

FREQUENCY

INSTANTANEOUS REAL

POWER SIGNAL

INSTANTANEOUS

POWER SIGNAL – p(t)

HPF

Figure 3. Signal Processing Block Diagram

The low frequency outputs (F1, F2) of the ADE7757 are gener-

ated 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 (e.g., 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, which 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 (i.e., by low-pass filtering) is still

valid even when the voltage and current signals are not in phase.

Figure 4 displays the unity power factor condition and a DPF

(displacement power factor) = 0.5, i.e., current signal lagging the

voltage by 60

°. If we assume the voltage and current waveforms

are sinusoidal, the real power component of the instantaneous

power signal (i.e., the dc term) is given by

V

×









×°

()

1

2

60

cos

This is the correct real power calculation.

V

I

2

0V

POWER

CURRENT

VOLTAGE

POWER

TIME

TIME

VOLTAGE

CURRENT

V

I

2

COS (60 )

0V

INSTANTANEOUS

POWER SIGNAL

INSTANTANEOUS REAL

POWER SIGNAL

INSTANTANEOUS

POWER SIGNAL

INSTANTANEOUS REAL

POWER SIGNAL

60

Figure 4. DC Component of Instantaneous Power

Signal Conveys Real Power Information, PF < 1