ade7854 Analog Devices, Inc., ade7854 Datasheet - Page 20

• Manufacturer Part Number: ade7854
• Description: Poly Phase Multifunction Energy Metering Ic With Neutral Current Measurement
• Manufacturer: Analog Devices, Inc.

ADE7854
ADE7854 is 1.024MHz and the band of interest is 40 Hz to 4
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 — see
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 ratio of 4 is required just
to increase the SNR by only 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 ADE7854 and its
role is to prevent aliasing. Aliasing is an artifact of all sampled
systems and is illustrated 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 (arrows shown in black) above half the
sampling frequency (also know as the Nyquist frequency, i.e.,
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 sampling frequency,
i.e., 1.024MHz, move into the band of interest for metering, i.e.,
40 Hz to 4 kHz. To attenuate the high frequency (near
1.024MHz) noise and prevent the distortion of the band of
interest, a LPF (low-pass filter) has to be introduced. For
SIGNAL
SIGNAL
NOISE
NOISE
Figure 14. Noise Reduction Due to Oversampling and
0
0
Noise Shaping in the Analog Modulator
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
Rev. PrC| Page 20 of 71
conventional current sensors, it is recommended to use one RC
filter with a corner frequency of 5KHz in order for the
attenuation to be sufficiently high at the sampling frequency of
1.024MHz. 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 20 dB per decade attenuation produced by the LPF.
Therefore, when using a di/dt sensor, care should be taken to
offset the 20 dB per decade gain. One simple approach is to
cascade one more RC filter, so a –40 dB per decade attenuation
is produced.
ADC Transfer Function
All ADCs in the ADE7854 are designed to produce the same
24-bit signed output code for the same input signal level. With a
full-scale input signal of 0.5 V and an internal reference of 1.2 V,
the ADC output code is nominally 5,928,256 (0x5A7540). The
code from the ADC may vary between 0x800000 (-8,388,608)
and 0x7FFFFF (+8,388,607); this is equivalent to an input signal
level of ±0.707V. However, for specified performance, it is
recommended not to exceed the nominal range of ±0.5V. The
ADC performance is guaranteed only for input signals lower
than ±0.5V.
CURRENT CHANNEL ADC
Figure 19 shows the ADC and signal processing path for the
input IA of the current channels (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. This
diagram shows a full-scale voltage signal being applied to the
differential inputs IAP and IAN. The ADC output swings
between −5,928,256 (0xA58AC0 ) and+5,928,256 (0x5A7540).
Current Waveform Gain Registers
There is a multiplier in the signal path of each phase current.
The current waveform can be changed by ±100% by writing a
correspondent twos complement number to the 24-bit signed
current waveform gain registers (AIGAIN[23:0], BIGAIN[23:0]
and CIGAIN[23:0]). For example, if 0x400000 is written to
those registers, the ADC output is scaled up by 50%. To scale
the input by -50%, write 0xC00000 to the registers. Equation (1)
0
FREQUENCIES
IMAGE
4
Figure 15. Aliasing effects at ADE7854
Preliminary Technical Data
ALIASING EFFECTS
Frequency [KHz]
512
Frequency
Sampling
1024