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ADE7761A
ANALOG-TO-DIGITAL CONVERSION
ANTIALIAS FILTER (RC)
DIGITAL FILTER SAMPLING FREQUENCY
The analog-to-digital conversion in the ADE7761A is carried
out using second-order, Σ-Δ ADCs. Figure 19 shows a first-
order, Σ-Δ ADC (for simplicity). The converter is made up of
two parts: the Σ-Δ modulator and the digital low-pass filter.
SIGNAL
NOISE
SHAPED NOISE
MCLK
0
1
225
450
ANALOG
FREQUENCY (kHz)
LOW-PASS FILTER
R
C
INTEGRATOR
V REF
LATCHED
COMPAR-
ATOR
DIGITAL
LOW-PASS FILTER
1
24
SIGNAL
NOISE
HIGH RESOLUTION
OUTPUT FROM
DIGITAL LFP
....10100101....
1-BIT DAC
0
1
225
450
Figure 19. First-Order, Σ-Δ ADC
A Σ-Δ modulator converts the input signal into a continuous
serial stream of 1s and 0s at a rate determined by the sampling
clock. In the ADE7761A, the sampling clock is equal to CLKIN.
The 1-bit DAC in the feedback loop is driven by the serial data
stream. The DAC output is subtracted from the input signal. If
the loop gain is high enough, the average value of the DAC
output (and, therefore, the bit stream) approaches that of the
input signal level. For any given input value in a single sampling
interval, the data from the 1-bit ADC is virtually meaningless.
Only when a large number of samples are averaged is a meaningful
result obtained. This averaging is carried out in the second part
of the ADC, the digital low-pass filter. By averaging a large
number of bits from the modulator, the low-pass filter can
produce 24-bit data-words that are proportional to the input
signal level.
The Σ-Δ converter uses two techniques to achieve high
resolution from what is essentially a 1-bit conversion technique.
The first is oversampling, which 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 ADE7761A is
CLKIN (450 kHz) and the band of interest is 40 Hz to 1 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 20).
FREQUENCY (kHz)
Figure 20. Noise Reduction due to Oversampling and
Noise Shaping in the Analog Modulator
Antialias Filter
Figure 20 also shows an analog low-pass filter (RC) on input to
the modulator. This filter is present to prevent aliasing. Aliasing
is an artifact of all sampled systems, which means that frequency
components in the input signal to the ADC that are higher than
half the sampling rate of the ADC appear in the sampled signal
frequency below half the sampling rate. Figure 21 illustrates
the effect.
In Figure 21, frequency components (arrows shown in black)
above half the sampling frequency (also known as the Nyquist
frequency), that is, 225 kHz, are imaged or folded back down
below 225 kHz (arrows shown in gray). This happens with all
ADCs no matter what the architecture. In the example shown,
only frequencies near the sampling frequency (450 kHz) move
into the band of interest for metering (40 Hz to 1 kHz). This
fact allows the use of a very simple low-pass filter to attenuate
these frequencies (near 250 kHz) and thereby prevent distortion
in the band of interest. A simple RC filter (single pole) with a
corner frequency of 10 kHz produces an attenuation of
approximately 33 dB at 450 kHz (see Figure 21). This is
sufficient to eliminate the effects of aliasing.
ANTIALIASING EFFECTS
SAMPLING
However, oversampling alone is not an efficient enough method
to improve the signal-to-noise ratio (SNR) in the band of interest.
For example, an oversampling ratio of 4 is required just to
IMAGE
FREQUENCIES
FREQUENCY
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
0
1
225
FREQUENCY (kHz)
450
noise so that the majority of the noise lies at the higher frequencies.
This is what happens in the Σ-Δ modulator; the noise is shaped
Figure 21. ADC and Signal Processing in Current Channel or Voltage Channel
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. This noise shaping is also shown in Figure 20.
Rev. 0 | Page 13 of 24
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