ade7880 Analog Devices, Inc., ade7880 Datasheet - Page 24

• Manufacturer Part Number: ade7880
• Description: Polyphase Multifunction Energy Metering Ic With Harmonic Monitoring
• Manufacturer: Analog Devices, Inc.

ADE7880
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 14. 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
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
low-pass filter. This noise shaping is shown in Figure 14.
Antialiasing Filter
Figure 13 also shows an analog low-pass filter (RC) on the input
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 15. 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, 512kHz) are imaged or folded back
down below 512kHz. This happens with all ADCs regardless of
the architecture. In the example shown, only frequencies near the
sampling frequency, that is, 1.024MHz, move into the band of
interest for metering, that is, 40Hz to 3.3kHz. To attenuate the
high frequency (near 1.024MHz) noise and prevent the
distortion of the band of interest, a low-pass filer (LPF) must be
SIGNAL
SIGNAL
NOISE
NOISE
Figure 14. Noise Reduction Due to Oversampling and
0
0
Noise Shaping in the Analog Modulator
3.3
3.3
High Resolution
4
4
Output From
Digital LPF
Digital Filter
Frequency [KHz]
Frequency [KHz]
512
512
Antialias Filter
(RC)
Shaped Noise
1024
1024
Frequency
Sampling
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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 1.024MHz. The 20dB 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 20dB per decade gain. This
neutralizes the 20dB per decade attenuation produced by the
LPF. Therefore, when using a di/dt sensor, take care to offset the
20dB per decade gain. One simple approach is to cascade one
additional RC filter, thereby producing a −40 dB per decade
attenuation.
ADC Transfer Function
All ADCs in the ADE7880 are designed to produce the same
24-bit signed output code for the same input signal level. With a
full-scale input signal of 0.5V and an internal reference of 1.2V,
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.
CURRENT CHANNEL ADC
Figure 16 shows the ADC and signal processing path for
InputIA 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.
Figure 16 shows a full-scale voltage signal applied to the differ-
ential inputs (IAP and IAN). The ADC output swings between
−5,326,737 (0xAEB86F) and +5,326,737 (0x514791). Note 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 to the
path of the phase currents as shown in Figure 17.
0
FREQUENCIES
3.3
IMAGE
4
Preliminary Technical Data
ALIASING EFFECTS
Figure 15. Aliasing Effects
Frequency [KHz]
512
Frequency
Sampling
1024