ADE7933 datasheet(36/125 Pages) AD

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ADE7978/ADE7933/ADE7932/ADE7923

Data Sheet

Rev. D | Page 36 of 125

Oversampling

Oversampling is the first technique used to achieve high reso-

lution. Oversampling means that the signal is sampled at a rate

(frequency) that is many times higher than the bandwidth of

interest. For example, when XTAL1 = 4.096 MHz, the sampling

rate in the ADE7933/ADE7932 and ADE7923 is 1.024 MHz,

whereas the bandwidth of interest is 40 Hz to 3.3 kHz. Over-

sampling 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 48).

NOISE

SIGNAL

NOISE

SIGNAL

3.3 4

512

FREQUENCY (kHz)

HIGH RESOLUTION

OUTPUT FROM

ADE7978 DIGITAL LPF

1024

3.3 4

512

FREQUENCY (kHz)

1024

ADE7978

DIGITAL FILTER

SHAPED NOISE

ANTIALIASING FILTER

(RC)

SAMPLING

FREQUENCY

Figure 48. 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 Noise Shaping section).

Noise Shaping

Noise shaping is the second technique used to achieve high

resolution. In the Σ-∆ modulator, the noise is shaped by the

integrator, which has a high-pass type response for the quantization

noise. The result is that most of the noise is at the higher

frequencies where it can be removed by the digital low-pass

filter in the ADE7978. This noise shaping is shown in Figure 48.

Antialiasing Filter

As shown in Figure 47, an external low-pass analog RC filter is

required on the input to the ADE7933/ADE7932 and ADE7923

ADC. The role of this filter is to prevent aliasing. Aliasing is an

artifact of all sampled systems, as shown in Figure 49. Aliasing

refers to the frequency components in the input signal to the

ADC that are imaged or folded back and appear in the sampled

signal at a frequency below half the sampling rate. This effect

occurs with signals that are higher than half the sampling rate of

the ADC (also known as the Nyquist frequency, that is, 512 kHz).

ALIASING EFFECTS

SAMPLING

FREQUENCY

IMAGE

FREQUENCIES

3.3

512

FREQUENCY (kHz)

1024

Figure 49. Aliasing Effects

In Figure 49, only frequencies near the sampling frequency of

1.024 MHz move into the band of interest for metering, that is,

40 Hz to 3.3 kHz. To attenuate high frequency (near 1.024 MHz)

noise and prevent the distortion of the band of interest, a low-pass

filter (LPF) must be introduced. It is recommended that one RC

filter with a corner frequency of 5 kHz be used for the attenuation

to be sufficiently high at the sampling frequency of 1.024 MHz.

The 20 dB per decade attenuation of this filter is usually sufficient

to eliminate the effects of aliasing for conventional current sensors.

ADC Transfer Function

The ADE7933/ADE7932 and ADE7923 provide a stream of bits

at the DATA pin based on the SYNC clock signal provided by

the ADE7978 (see the Bit Stream Communication Between the

ADE7978 and the ADE7933/ADE7932 section). The ADE7978

digital filter processes the bit streams coming from all ADE7933,

ADE7932, and ADE7923 devices in the system and produces

the 24-bit signed output codes of the ADCs.

With a full-scale input signal of ±31.25 mV on the current

channel and ±0.5 V on the voltage channels and with an internal

reference of 1.2 V, the ADC output code is nominally 5,320,000

and usually varies for each ADE7933/ADE7932 and ADE7923

around this value. The code obtained by the ADE7978 from the

ADE7933/ADE7932 and ADE7923 ADCs can vary from

0x800000 (−8,388,608) to 0x7FFFFF (+8,388,607); this code is

equivalent to an input signal level of ±49.27 mV on the current

channel and ±0.788 V on the voltage channels. However, for

specified performance, do not exceed the nominal range of

±31.25 mV for the current channel and ±0.5 V for the voltage

channels; ADC performance is guaranteed only for input

signals within these limits. For input signals outside these

limits, the digital low pass filter of the ADCs (see Figure 47)

overflows.

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