ADE7752/ADE7752A

Rev. C | Page 12 of 24

THEORY OF OPERATION

The six voltage signals from the current and voltage transducers

are digitized with ADCs. These ADCs are 16-bit second-order

∑-Δ with an oversampling rate of 833 kHz. This analog input

structure greatly simplifies transducer interface by providing a

wide dynamic range for direct connection to the transducer and

also by simplifying the antialiasing filter design. A high-pass

filter in the current channel removes the dc component from

the current signal. This eliminates any inaccuracies in the real

power calculation due to offsets in the voltage or current

signals. See the

The low frequency output of the ADE7752 is generated by

accumulating the total real power information. This low

frequency inherently means a long accumulation time between

output pulses. The output frequency is therefore proportional to

the average real power. This average real power information

can, in turn, be accumulated (by a counter, for example) to

generate real energy information. Because of its high output

frequency and therefore shorter integration time, the CF output

is proportional to the instantaneous real power. This pulse is

useful for system calibration purposes that would take place

under steady load conditions.

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 of each

phase. In order to extract the real power component (the dc

component), the instantaneous power signal is low-pass filtered

on each phase.

POWER FACTOR CONSIDERATIONS

Low-pass filtering, the method used to extract the real power

information from the individual instantaneous power signal, is

still valid when the voltage and current signals of each phase are

not in phase.

Figure 15 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 method is used to extract the real power information on

each phase of the polyphase system. The total real power

information is then obtained by adding the individual phase

real power. This scheme correctly calculates real power for

nonsinusoidal 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.

Figure 16 displays the unity power factor

condition and a DPF (displacement power factor) = 0.5, or

current signal lagging the voltage by 60°, for one phase of the

polyphase. Assuming that the voltage and current waveforms

are sinusoidal, the real power component of the instantaneous

power signal, or the dc term, is given by

()

°

×

⎟

⎠

⎞

⎜

⎝

⎛ ×

60

cos

2

1

V

TIME

IAP

IAN

VAP

HPF

LPF

IBP

IBN

VBP

ICP

ICN

VCP

VN

DIGITAL-TO-

FREQUENCY

DIGITAL-TO-

FREQUENCY

F1

F2

CF

INSTANTANEOUS

REAL POWER SIGNAL

INSTANTANEOUS

POWER SIGNAL - p(t)

INSTANTANEOUS

TOTAL

POWER SIGNAL

VA

× IA + VB × IB +

VC

×IC

2

ABS

|X|

Σ

Σ

Σ

LPF

LPF

|X|

|X|

p(t) = i(t)

× v(t)

WHERE:

V

× I

2

{1+ cos (2

ωt)}

v(t) = V

× cos (ωt)

i(t) = I

× cos (ωt)

p(t) =

V

× I

2

V

× I

V

× I

2

MULTIPLIER

MULTIPLIER

MULTIPLIER

HPF

HPF

ADC

ADC

ADC

ADC

ADC

ADC

Figure 15. Signal Processing Block Diagram