ade7760 Analog Devices, Inc., ade7760 Datasheet - Page 13

• Manufacturer Part Number: ade7760
• Description: Energy Metering Ic With On-chip Fault Detection
• Manufacturer: Analog Devices, Inc.

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.
Figure 16. ADC and Signal Processing in Current Channel or Voltage Channel
SIGNAL
SIGNAL
NOISE
NOISE
0
0
0
1kHz
FREQUENCIES
Figure 15. Noise Reduction Due to Oversampling and
IMAGE
1kHz
1kHz
HIGH RESOLUTION
OUTPUT FROM
DIGITAL FILTER
DIGITAL LFP
Noise Shaping in the Analog Modulator
ANTIALIASING EFFECTS
FREQUENCY (Hz)
ANTIALIAS FILTER (RC)
225kHz
FREQUENCY (Hz)
FREQUENCY (Hz)
225kHz
225kHz
SHAPED NOISE
SAMPLING FREQUENCY
450kHz
450kHz
450kHz
FREQUENCY
SAMPLING
Rev. 0 | Page 13 of 24
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.
CH1
CH2
V × I
TIME
Figure 17. Signal Processing Block Diagram
POWER SIGNAL –p(t)
ADC
ADC
INSTANTANEOUS
p(t) = i(t).v(t)
WHERE:
MULTIPLIER
v(t) = V × cos( t)
i(t) = I × cos( t)
p(t) = V × I {1 + cos (2 t)}
HPF
2
ACTIVE POWER SIGNAL
INSTANTANEOUS
LPF
V × I
2
FREQUENCY
FREQUENCY
DIGITAL-TO-
DIGITAL-TO-
ADE7760
CF
F1
F2