ade7953 Analog Devices, Inc., ade7953 Datasheet - Page 19

• Manufacturer Part Number: ade7953
• Description: Single Phase, Multifunction Metering Ic With Neutral Current Measurement Ade7953
• Manufacturer: Analog Devices, Inc.

The Σ -Δ converter uses two techniques—oversampling
and noise shaping—to achieve high resolution from what
is essentially a 1-bit conversion technique.
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).
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 Noise Shaping section).
SIGNAL
SIGNAL
NOISE
NOISE
Figure 37. Noise Reduction due to Oversampling and
0
0
Noise Shaping in the Analog Modulator
HIGH RESOLUTION
OUTPUT FROM
DIGITAL LPF
3
3
DIGITAL FILTER
FREQUENCY (kHz)
FREQUENCY (kHz)
447.5
447.5
ANTIALIASING FILTER
(RC)
SHAPED NOISE
895
895
SAMPLING
FREQUENCY
Rev. 0 | Page 19 of 68
Noise Shaping
Noise shaping is the second technique used to achieve high
resolution. In the Σ -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.
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.
0
FREQUENCIES
1.23
IMAGE
3
ALIASING EFFECTS
FREQUENCY (kHz)
Figure 38. Aliasing Effect
447.5
FREQUENCY
SAMPLING
ADE7953
895