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ADE7754 датащи(PDF) 10 Page - Analog Devices |
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ADE7754 датащи(HTML) 10 Page - Analog Devices |
10 / 44 page REV. 0 –10– ADE7754 Figure 6 shows how the gain settings in PGA 1 (current channel) and PGA 2 (voltage channel) are selected by various bits in the gain register. The no-load threshold and sum of the absolute value can also be selected in the gain register. See Table X. 0 0 0 0 0 0 0 0 7 6 5 4 3 2 1 0 RESERVED = 0 RESERVED = 0 PGA 2 GAIN SELECT 00 = 1 01 = 2 10 = 4 PGA 1 GAIN SELECT 00 = 1 01 = 2 10 = 4 NO LOAD ABS GAIN REGISTER* CURRENT AND VOLTAGE CHANNEL PGA CONTROL *REGISTER CONTENTS SHOW POWER-ON DEFAULTS ADDR: 18h Figure 6. Analog Gain Register ANALOG-TO-DIGITAL CONVERSION The ADE7754 carries out analog-to-digital conversion using second order Σ-∆ ADCs. The block diagram in Figure 7 shows a first order (for simplicity) Σ-∆ ADC. The converter is made up of two parts, the Σ-∆ modulator and the digital low-pass filter. VREF ....10100101...... DIGITAL LOW-PASS FILTER MCLK/12 INTEGRATOR 1-BIT DAC R C ANALOG LOW-PASS FILTER 1 24 LATCHED COMPARATOR + – Figure 7. First Order ( - ) ADC A Σ-∆ modulator converts the input signal into a continuous serial stream of 1s and 0s 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 subtracted 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. 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-∆ converter uses two techniques to achieve high resolu- tion from what is essentially a 1-bit conversion technique. The first is oversampling; the signal is sampled at a rate (frequency) many times higher than the bandwidth of interest. For example, the sampling rate in the ADE7754 is CLKIN/12 (833 kHz), and the band of interest is 40 Hz to 2 kHz. Oversampling spreads 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 8. 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 to increase the SNR by only 6 dB (1 bit). To keep the oversampling ratio at a reasonable level, the quantization noise can be shaped so that most of the noise lies at the higher frequencies. In the Σ-∆ modulator, the noise is shaped by the integrator, which has a high-pass type of 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 shown in Figure 8. FREQUENCY (kHz) 0 417 833 2 SAMPLING FREQUENCY SHAPED NOISE ANTIALIAS FILTER (RC) DIGITAL FILTER NOISE SIGNAL HIGH RESOLUTION OUTPUT FROM DIGITAL LPF NOISE SIGNAL FREQUENCY (kHz) 0 417 833 2 Figure 8. Noise Reduction Due to Oversampling and Noise Shaping in the Analog Modulator Antialias Filter Figure 7 shows an analog low-pass filter (RC) on the input to the modulator. This filter is used to prevent aliasing, an artifact of all sampled systems. 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 at a frequency below half the sampling rate. Figure 9 illustrates the effect; frequency com- ponents (arrows shown in black) above half the sampling frequency (also known as the Nyquist frequency), i.e., 417 kHz, get imaged or folded back down below 417 kHz (arrows shown in gray). This happens with all ADCs, regardless of the archi- tecture. In the example shown, only frequencies near the sampling frequency, i.e., 833 kHz, will move into the band of interest for metering, i.e., 40 Hz to 2 kHz. This allows use of a very simple LPF (low-pass filter) to attenuate these high frequencies (near 900 kHz) and thus 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 40 dBs at 833 kHz. See Figure 9. This is sufficient to eliminate the effects of aliasing. |
Аналогичный номер детали - ADE7754_15 |
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Аналогичное описание - ADE7754_15 |
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