ADE7880 datasheet(26/104 Pages) AD

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ADE7880

Data Sheet

Rev. A | Page 26 of 104

The Σ-Δ converter uses two techniques to achieve high resolu-

tion from what is essentially a 1-bit conversion technique. The

first is oversampling. 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 ADE7880 is 1.024 MHz, and the bandwidth 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, as shown in Figure 39. However, oversampling alone is

not efficient enough to improve the signal-to-noise ratio (SNR)

in the band of interest. For example, an oversampling factor of 4 is

required just to increase the SNR by a mere 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. In the Σ-Δ modulator, the noise is

shaped by the integrator, which has a high-pass-type response

for the quantization noise. This is the second technique used to

achieve high resolution. 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 39.

NOISE

SIGNAL

NOISE

SIGNAL

03.3 4

512

FREQUENCY (kHz)

HIGH RESOLUTION

OUTPUT FROM

DIGITAL LPF

1024

03.3 4

512

FREQUENCY (kHz)

1024

DIGITAL FILTER

SHAPED NOISE

ANTIALIAS FILTER

(RC)

SAMPLING

FREQUENCY

Figure 39. Noise Reduction Due to Oversampling and

Noise Shaping in the Analog Modulator

Antialiasing Filter

Figure 38 also shows an analog low-pass filter (RC) on the input

to the ADC. This filter is placed outside the ADE7880, and its role

is to prevent aliasing. Aliasing is an artifact of all sampled systems

as shown in Figure 40. Aliasing means that frequency components

in the input signal to the ADC, which are higher than half the

sampling rate of the ADC, appear in the sampled signal at a

frequency below half the sampling rate. Frequency components

above half the sampling frequency (also known as the Nyquist

frequency, that is, 512 kHz) are imaged or folded back down

below 512 kHz. This happens with all ADCs regardless of the

architecture. In the example shown, only frequencies near the

sampling frequency, that is, 1.024 MHz, move into the band of

interest for metering, that is, 40 Hz to 3.3 kHz. To attenuate the

high frequency (near 1.024 MHz) noise and prevent the distortion

of the band of interest, a low-pass filter (LPF) must be introduced.

For conventional current sensors, it is recommended to use one

RC filter with a corner frequency of 5 kHz 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. However, for a di/dt sensor such as a Rogowski

coil, the sensor has a 20 dB per decade gain. This neutralizes the

20 dB per decade attenuation produced by the LPF. Therefore,

when using a di/dt sensor, take care to offset the 20 dB per

decade gain. One simple approach is to cascade one additional

RC filter, thereby producing a −40 dB per decade attenuation.

ALIASING EFFECTS

SAMPLING

FREQUENCY

IMAGE

FREQUENCIES

3.3

512

FREQUENCY (kHz)

1024

Figure 40. Aliasing Effects

ADC Transfer Function

All ADCs in the ADE7880 are designed to produce the same

24-bit signed output code for the same input signal level. With a

full-scale input signal of 0.5 V and an internal reference of 1.2 V,

the ADC output code is nominally 5,326,737 (0x514791) and

usually varies for each ADE7880 around this value. The code

from the ADC can vary between 0x800000 (−8,388,608) and

0x7FFFFF (+8,388,607); this is equivalent to an input signal

level of ±0.787 V. However, for specified performance, do not

exceed the nominal range of ±0.5 V; ADC performance is

guaranteed only for input signals lower than ±0.5 V.

CURRENT CHANNEL ADC

Figure 41 shows the ADC and signal processing path for Input

IA of the current channels (it is the same for IB and IC). The

ADC outputs are signed twos complement 24-bit data-words

and are available at a rate of 8 kSPS (thousand samples per

second). With the specified full-scale analog input signal

of ±0.5V, the ADC produces its maximum output code value.

Figure 41 shows a full-scale voltage signal applied to the differ-

ential inputs (IAP and IAN). The ADC output swings between

−5,326,737 (0xAEB86F) and +5,326,737 (0x514791). Note that

these are nominal values and every ADE7880 varies around

these values. The input, IN, corresponds to the neutral current

of a 3-phase system. If no neutral line is present, connect this

input to AGND. The datapath of the neutral current is similar

to the path of the phase currents as shown in Figure 42.

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