Datasheet

ADE7953 Data Sheet
Rev. B | Page 20 of 72
Oversampling
Oversampling is the first technique used to achieve high
resolution. 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 ADE7953 is
895 kHz, and the bandwidth of interest is 40 Hz to 1.23 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 37).
NOISE
SIGNAL
NOISE
SIGNAL
0 3 447.5
FREQUENCY (kHz)
HIGH RESOLUTION
OUTPUT FROM
DIGITAL LPF
895
0 3 447.5
FREQUENCY (kHz)
895
DIGITAL FILTER
SHAPED NOISE
ANTIALIASING FILTER
(RC)
SAMPLING
FREQUENCY
09320-014
Figure 37. Noise Reduction due to Oversampling and
Noise Shaping in the Analog Modulator
However, oversampling alone is not sufficient to improve the
signal-to-noise ratio (SNR) in the bandwidth of interest. For
example, an oversampling ratio of 4 is required 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 (see
the following section.
Noise Shaping
Noise shaping is the second technique used to achieve high
resolution. In the -Δ modulator, the noise is shaped by the
integrator, which has a high-pass-type response for the quanti-
zation noise due to feedback. 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 37.
Antialiasing Filter
As shown in Figure 36, an external low-pass RC filter is required
on the input to each modulator. The role of this filter is to prevent
aliasing. Aliasing refers to the frequency components in the input
signal that are folded back and appear in the sampled signal. This
effect occurs with signals that are higher than half the sampling
rate of the ADC (also known as the Nyquist frequency) appear-
ing in the sampled signal at a frequency below half the sampling
rate. This concept is depicted in Figure 38.
A
LIASING EFFECTS
SAMPLING
FREQUENCY
IMAGE
FREQUENCIES
0 1.23 3 447.5
FREQUENCY (kHz)
895
09320-015
Figure 38. Aliasing Effect
The arrows shown in Figure 38 depict the frequency compo-
nents above the Nyquist frequency (447.5 kHz in the case of
the ADE7953) being folded back down. Aliasing occurs with
all ADCs, regardless of the architecture.
xIGAIN
DSP
REFERENCE
HPFEN BIT
CONFIG[2]
DIGITAL
INTEGRATOR
INTENx BIT
CONFIG[1:0]
ACTIVE AND REACTIVE
POWER CALCULATION
CURRENT PEAK,
OVERCURRENT
DETECTION
Ix WAVEFORM
SAMPLING REGISTER
CURRENT RMS (IRMS)
CALCULATION
HPF
ADC
PGA
IxP
ZX_I DETECTION
PGA_x BITS
×1, ×2, ×4, ×8, ×16,
×22 (FOR IA ONLY)
V
IN
IxN
0
9320-019
LPF1
Figure 39. Current Channel ADC and Signal Path