參數(shù)資料
型號(hào)： ADE7754AR
廠商： ANALOG DEVICES INC
元件分類(lèi)： 模擬信號(hào)調(diào)理
英文描述： ADE7754
中文描述： SPECIALTY ANALOG CIRCUIT, PDSO24
封裝： MS-013AD, SOIC-24
文件頁(yè)數(shù)： 11/44頁(yè)
文件大?。?/td> 630K
代理商： ADE7754AR
REV. PrG 01/03
PRELIMINARY TECHNICAL DATA
ADE7754
–
11
–
ADE7754 ANALOG TO DIGITAL CONVERSION
The analog-to-digital conversion in the ADE7754 is carried
out using second order sigma-delta ADCs. The block dia-
gram in Figure 5 shows a first order (for simplicity)
sigma-delta ADC. The converter is made up of two parts,
first the sigma-delta modulator and secondly the digital low
pass filter.
V
REF
+
-
....10100101......
Digital Low Pass Filter
Σ
MCLK/12
INTEGRATOR
1-Bit DAC
LATCHED
COMPARATOR
+
-
R
C
Analog Low Pass Filter
1
24
Figure 5 - First order Sigma-Delta (
Σ
) ADC
A sigma-delta modulator converts the input signal into a
continuous serial stream of 1's and 0's at a rate determined by
the sampling clock. In the ADE7754 the sampling clock is
equal to CLKIN/12. The 1-bit DAC in the feedback loop is
driven by the serial data stream. The DAC output is sub-
tracted from the input signal. If the loop gain is high enough
the average value of the DAC output (and therefore the bit
stream) will approach 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, will a meaningful result be
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 which are proportional to the input
signal level. The sigma-delta converter uses two techniques
to achieve high resolution from what is essentially a 1-bit
conversion technique. The first is oversampling. By over
sampling we mean that the signal is sampled at a rate
(frequency) which is many times higher than the bandwidth
of interest. For example the sampling rate in the ADE7754
is CLKIN/12 (833kHz) and the band of interest is 40Hz to
2kHz. Oversampling has the effect of spreading the quanti-
zation 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 6
.
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 6dB (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 sigma-delta 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 6.
Frequency (Hz)
0
417kHz
833kHz
2kHz
Sampling
Frequency
Shaped
Noise
Antialias filter (RC)
Digital filter
Noise
Signal
Frequency (Hz)
0
417kHz
833kHz
2kHz
Noise
Signal
High resolution
output from Digital
LPF
Figure 6
–
Noise reduction due to Oversampling & Noise
shaping in the analog modulator
Antialias Filter
Figure 5 also shows an analog low pass filter (RC) on the
input to the modulator. This filter is present to prevent
aliasing. Aliasing is an artifact of all sampled systems.
Basically it means that frequency components in the input
signal to the ADC which are higher than half the sampling
rate of the ADC will appear in the sampled signal at a
frequency below half the sampling rate. Figure 7 illustrates
the effect, frequency components (arrows shown in black)
above half the sampling frequency (also know as the Nyquist
frequency), i.e., 417kHz get imaged or folded back down
below 417kHz (arrows shown in grey). This will happen with
all ADCs no matter what the architecture. In the example
shown it can be seen that only frequencies near the sampling
frequency, i.e., 833kHz, will move into the band of interest
for metering, i.e, 40Hz - 2kHz. This fact allows us to use a
very simple LPF (Low Pass Filter) to attenuate these high
frequencies (near 900kHz) and so prevent distortion in the
band of interest. A simple RC filter (single pole) with a
corner frequency of 10kHz produces an attenuation of ap-
proximately 40dBs at 833kHz—see
Figure 7
. This is sufficient
to eliminate the effects of aliasing.
Aliasing Effects
Image
frequencies
0
2kHz
417kHz
Frequency (Hz)
833kHz
Sampling Frequency
Figure 7
–
ADC and signal processing
in current channel or voltage channel
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相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
ADE7754ARRL 制造商:Analog Devices 功能描述:Energy Measurement 24-Pin SOIC W T/R 制造商:Rochester Electronics LLC 功能描述:THREE PHASE ENERGY METER IC W/WATT & VA - Bulk
ADE7754ARZ 功能描述:IC ENERGY METERING 3PHASE 24SOIC RoHS:是 類(lèi)別:集成電路 (IC) >> PMIC - 能量測(cè)量 系列:- 產(chǎn)品培訓(xùn)模塊:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 標(biāo)準(zhǔn)包裝:2,500 系列:*
ADE7754ARZRL 功能描述:IC ENERGY METERING 3PHASE 24SOIC RoHS:是 類(lèi)別:集成電路 (IC) >> PMIC - 能量測(cè)量 系列:- 產(chǎn)品培訓(xùn)模塊:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 標(biāo)準(zhǔn)包裝:2,500 系列:*
ADE7755 制造商:AD 制造商全稱(chēng):Analog Devices 功能描述:Energy Metering IC with Pulse Output
ADE7755AARSRL 制造商:Analog Devices 功能描述:ENERGY METERING IC WITH P - Tape and Reel
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