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ADE7754ARRL Scheda tecnica(PDF) 11 Page - Analog Devices |
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ADE7754ARRL Scheda tecnica(HTML) 11 Page - Analog Devices |
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11 / 44 page  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. VREF + - ....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 833kHz Sampling Frequency Frequency (Hz) Figure 7– ADC and signal processing in current channel or voltage channel |
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