ADE7760

Rev. 0 | Page 13 of 24

SHAPED NOISE

HIGH RESOLUTION

OUTPUT FROM

DIGITAL LFP

NOISE

SIGNAL

NOISE

SIGNAL

0

1kHz

225kHz

450kHz

FREQUENCY (Hz)

0

1kHz

225kHz

450kHz

FREQUENCY (Hz)

DIGITAL FILTER

ANTIALIAS FILTER (RC)

SAMPLING FREQUENCY

Figure 15. Noise Reduction Due to Oversampling and

Noise Shaping in the Analog Modulator

Antialias Filter

Figure 15 also shows an analog low-pass filter (RC) on input to

the modulator. This filter is present to prevent aliasing. Aliasing

is an artifact of all sampled systems, which means that fre-

quency components in the input signal to the ADC that are

higher than half the sampling rate of the ADC appear in the

sampled signal frequency below half the sampling rate.

Figure 16 illustrates the effect.

In Figure 16, frequency components (arrows shown in black)

above half the sampling frequency (also known as the Nyquist

frequency), that is, 225 kHz get imaged or folded back down

below 225 kHz (arrows shown in gray). This happens with all

ADCs no matter what the architecture. In the example shown, it

can be seen that only frequencies near the sampling frequency

(450 kHz) move into the band of interest for metering (40 Hz to

1 kHz). This fact allows the use of a very simple low-pass filter

to attenuate these frequencies (near 250 kHz) and thereby

prevent distortion in the band of interest. A simple RC filter

(single pole) with a corner frequency of 10 kHz produces an

attenuation of approximately 33 dB at 450 kHz (see Figure 16).

This is sufficient to eliminate the effects of aliasing.

0

1kHz

225kHz

450kHz

FREQUENCY (Hz)

IMAGE

FREQUENCIES

SAMPLING

FREQUENCY

ANTIALIASING EFFECTS

Figure 16. ADC and Signal Processing in Current Channel or Voltage Channel

ACTIVE POWER CALCULATION

The ADCs digitize the voltage signals from the current and

voltage transducers. A high-pass filter in the current channel

removes any dc component from the current signal. This

eliminates any inaccuracies in the active power calculation due

to offsets in the voltage or current signals (see the HPF and

Offset Effects section).

The active 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 active power component (dc component), the

instantaneous power signal is low-pass filtered. Figure 17

illustrates the instantaneous active power signal and shows how

the active power information can be extracted by low-pass

filtering the instantaneous power signal. This scheme correctly

calculates active 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.

The low frequency output of the ADE7760 is generated by

accumulating this active power information. This low frequency

inherently means a long accumulation time between output

pulses. The output frequency is, therefore, proportional to the

average active power. This average active power information can

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

active energy information. Because of its high output frequency

and therefore shorter integration time, the CF output is propor-

tional to the instantaneous active power. This is useful for

system calibration purposes that would take place under steady

load conditions.

F2

CF

F1

DIGITAL-TO-

FREQUENCY

DIGITAL-TO-

FREQUENCY

HPF

MULTIPLIER

LPF

ADC

ADC

CH1

CH2

INSTANTANEOUS

POWER SIGNAL –p(t)

INSTANTANEOUS

ACTIVE POWER SIGNAL

V× I

V× I

2

TIME

p(t) = i(t).v(t)

WHERE:

v(t) = V × cos(

ϖt)

i(t) = I × cos(

ϖt)

p(t) =

V× I

{1 + cos (2

ϖt)}

2

Figure 17. Signal Processing Block Diagram