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AD7769JP Datenblatt(PDF) 11 Page - Analog Devices |
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AD7769JP Datenblatt(HTML) 11 Page - Analog Devices |
11 / 16 page AD7769 –11– REV. A where A is the peak amplitude of the sine wave and p (V) the probability of occurrence at a voltage V. The histogram plot of Figure 17 corresponds very well with this shape. Figure 17. ADC Histogram Plot In digital signal processing applications, where the AD7769 is used to sample ac signals, it is essential that the signal sampling occurs at exactly equal intervals. This minimizes errors due to sampling uncertainty or jitter. A precise timer or clock source, to start the conversion process, is the best method of generating equidistant sampling intervals. MICROPROCESSOR/MICROCOMPUTER INTERFACING The AD7769 is designed for easy interfacing to microprocessors and microcomputers as a memory mapped peripheral or an I/O device. In addition, the AD7769 high speed bus timing allows direct interfacing to many DSP processors such as the TMS320C10 and ADSP-2101. AD7769–TMS320C10 Interface A typical interface to the TMS320C10 is shown in Figure 18. The AD7769 is mapped at a port address, and the interface is designed for the maximum TMS320C10 clock frequency of 20 MHz. Figure 18. AD7769 to TMS320C10 Interface This is for an ideal part with no differential or integral linearity errors. These errors will cause a degradation in SNR. By work- ing backwards from the above equation, it is possible to get a measure of ADC performance expressed in effective number of bits (N). The effective number of bits is plotted versus fre- quency in Figure 15. The effective number of bits typically falls between 7.7 and 7.9, corresponding to SNR Figures 48.1 and 49.7 dB. Figure 15. Effective Number of Bits vs. Frequency Figure 16 shows a spectrum analyzer plot of the output spec- trum from one of the DACs with an ideal sine wave table loaded to the data inputs of the DAC. In this case, the SNR is 47 dB. Figure 16. DAC Output Spectrum Histogram Plot When a sine wave of specified frequency is applied to the VINA or VINB input of the AD7769 and several thousand samples are taken, it is possible to plot a histogram showing the frequency of occurrence of each of the 256 ADC codes. If a particular step is wider than the ideal 1 LSB width, then the code associated with that step will accumulate more counts than for the code for an ideal step. Likewise, a step narrower than ideal width will have fewer counts. Missing codes are easily seen because a missing code means zero counts for a particular code. The absence of large spikes in the plot indicates small differential nonlinearity. Figure 17 shows a histogram plot for the ADC indicating very small differential nonlinearity and no missing codes for an input frequency of 204 kHz. For a sine wave input, a perfect ADC would produce a probability density function described by the equation: p (V) = 1 π( A2 –V 2)1/2 |
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Ähnliche Beschreibung - AD7769JP |
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