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ADE7880
The Σ-Δ converter uses two techniques to achieve high resolu-
tion from what is essentially a 1-bit conversion technique. The
first is oversampling. Oversampling means that the signal is
sampled at a rate (frequency) that is many times higher than
the bandwidth of interest. For example, the sampling rate in
the ADE7880 is 1.024 MHz, and the bandwidth of interest is
40 Hz to 3.3 kHz. Oversampling has the effect of spreading the
quantization noise (noise due to sampling) over a wider
bandwidth. With the noise spread more thinly over a wider
bandwidth, the quantization noise in the band of interest is
lowered, as shown in Figure 39. However, oversampling alone is
not efficient enough to improve the signal-to-noise ratio (SNR)
in the band of interest. For example, an oversampling factor of 4 is
required just to increase the SNR by a mere 6 dB (1 bit). To keep
the oversampling ratio at a reasonable level, it is possible to
Data Sheet
sampling frequency, that is, 1.024 MHz, move into the band of
interest for metering, that is, 40 Hz to 3.3 kHz. To attenuate the
high frequency (near 1.024 MHz) noise and prevent the distortion
of the band of interest, a low-pass filter (LPF) must be introduced.
For conventional current sensors, it is recommended to use one
RC filter with a corner frequency of 5 kHz for the attenuation to
be sufficiently high at the sampling frequency of 1.024 MHz.
The 20 dB per decade attenuation of this filter is usually
sufficient to eliminate the effects of aliasing for conventional
current sensors. However, for a di/dt sensor such as a Rogowski
coil, the sensor has a 20 dB per decade gain. This neutralizes the
20 dB per decade attenuation produced by the LPF. Therefore,
when using a di/dt sensor, take care to offset the 20 dB per
decade gain. One simple approach is to cascade one additional
RC filter, thereby producing a ?40 dB per decade attenuation.
shape the quantization noise so that the majority of the noise
lies at the higher frequencies. In the Σ-Δ modulator, the noise is
shaped by the integrator, which has a high-pass-type response
for the quantization noise. This is the second technique used to
achieve high resolution. The result is that most of the noise is at
the higher frequencies where it can be removed by the digital
ALIASING EFFECTS
SAMPLING
FREQUENCY
SIGNAL
ANTIALIAS FILTER
(RC)
DIGITAL FILTER
SHAPED NOISE
0
3.3 4 512
FREQUENCY (kHz)
IMAGE
FREQUENCIES
Figure 40. Aliasing Effects
1024
NOISE
SAMPLING
FREQUENCY
ADC Transfer Function
All ADCs in the ADE7880 are designed to produce the same
0
3.3 4
512
FREQUENCY (kHz)
1024
24-bit signed output code for the same input signal level. With a
full-scale input signal of 0.5 V and an internal reference of 1.2 V,
SIGNAL
NOISE
HIGH RESOLUTION
OUTPUT FROM
DIGITAL LPF
the ADC output code is nominally 5,326,737 (0x514791) and
usually varies for each ADE7880 around this value. The code
from the ADC can vary between 0x800000 (?8,388,608) and
0x7FFFFF (+8,388,607); this is equivalent to an input signal
level of ±0.787 V. However, for specified performance, do not
exceed the nominal range of ±0.5 V; ADC performance is
guaranteed only for input signals lower than ±0.5 V.
0
3.3 4
512
FREQUENCY (kHz)
1024
CURRENT CHANNEL ADC
Figure 39. Noise Reduction Due to Oversampling and
Noise Shaping in the Analog Modulator
Antialiasing Filter
to the ADC. This filter is placed outside the ADE7880 , and its role
is to prevent aliasing. Aliasing is an artifact of all sampled systems
as shown in Figure 40. Aliasing means that frequency components
in the input signal to the ADC, which are higher than half the
sampling rate of the ADC, appear in the sampled signal at a
frequency below half the sampling rate. Frequency components
above half the sampling frequency (also known as the Nyquist
frequency, that is, 512 kHz) are imaged or folded back down
below 512 kHz. This happens with all ADCs regardless of the
architecture. In the example shown, only frequencies near the
IA of the current channels (it is the same for IB and IC). The
ADC outputs are signed twos complement 24-bit data-words
and are available at a rate of 8 kSPS (thousand samples per
second). With the specified full-scale analog input signal
of ±0.5V, the ADC produces its maximum output code value.
ential inputs (IAP and IAN). The ADC output swings between
?5,326,737 (0xAEB86F) and +5,326,737 (0x514791). Note that
these are nominal values and every ADE7880 varies around
these values. The input, IN, corresponds to the neutral current
of a 3-phase system. If no neutral line is present, connect this
input to AGND. The datapath of the neutral current is similar
Rev. A | Page 26 of 104
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