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ADE7761AARS-REF 参数 Datasheet PDF下载

ADE7761AARS-REF图片预览
型号: ADE7761AARS-REF
PDF下载: 下载PDF文件 查看货源
内容描述: 电能计量IC ,带有片上故障和中性丢失检测 [Energy Metering IC with On-Chip Fault and Missing Neutral Detection]
分类和应用:
文件页数/大小: 24 页 / 527 K
品牌: ADI [ ADI ]
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ADE7761A  
ANTIALIAS FILTER (RC)  
DIGITAL FILTER  
ANALOG-TO-DIGITAL CONVERSION  
SAMPLING FREQUENCY  
SHAPED NOISE  
SIGNAL  
NOISE  
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.  
0
1
225  
FREQUENCY (kHz)  
450  
MCLK  
ANALOG  
LOW-PASS FILTER  
HIGH RESOLUTION  
OUTPUT FROM  
DIGITAL LFP  
LATCHED  
COMPAR-  
ATOR  
INTEGRATOR  
DIGITAL  
LOW-PASS FILTER  
SIGNAL  
NOISE  
R
V
REF  
1
24  
C
....10100101....  
0
1
225  
FREQUENCY (kHz)  
450  
1-BIT DAC  
Figure 19. First-Order, Σ-Δ ADC  
Figure 20. Noise Reduction due to Oversampling and  
Noise Shaping in the Analog Modulator  
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.  
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.  
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).  
ANTIALIASING EFFECTS  
SAMPLING  
FREQUENCY  
IMAGE  
FREQUENCIES  
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  
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.  
This is what happens in the Σ-Δ 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. This noise shaping is also shown in Figure 20.  
0
1
225  
450  
FREQUENCY (kHz)  
Figure 21. ADC and Signal Processing in Current Channel or Voltage Channel  
Rev. 0 | Page 13 of 24