ADE7953ACPZ datasheet(20/68 Pages) AD

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ADE7953

Data Sheet

Rev. A | Page 20 of 68

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

447.5

FREQUENCY (kHz)

HIGH RESOLUTION

OUTPUT FROM

DIGITAL LPF

895

447.5

FREQUENCY (kHz)

895

DIGITAL FILTER

SHAPED NOISE

ANTIALIASING FILTER

(RC)

SAMPLING

FREQUENCY

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 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.

ALIASING EFFECTS

SAMPLING

FREQUENCY

IMAGE

FREQUENCIES

1.23

447.5

FREQUENCY (kHz)

895

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)

VIN

IxN

LPF1

Figure 39. Current Channel ADC and Signal Path

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