ADE7932 datasheet(31/120 Pages) AD

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Data Sheet

ADE7978/ADE7933/ADE7932

Rev. 0 | Page 31 of 120

The Σ-∆ 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 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 CLKIN = 4.096 MHz, the sampling

rate in the ADE7933/ADE7932 is 1.024 MHz, whereas the band-

width of interest is 40 Hz to 3.3 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 40).

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 40. 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 quanti-

zation 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 40.

Antialiasing Filter

As shown in Figure 39, an external low-pass analog RC filter is

required on the input to the ADE7933/ADE7932 ADC. The role

of this filter is to prevent aliasing. Aliasing is an artifact of all

sampled systems, as shown in Figure 41. 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 41. Aliasing Effects

In Figure 41, 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 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

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 around this value.

The code obtained by the ADE7978 from the ADE7933/ADE7932

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.

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