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AD7821 bảng dữ liệu(PDF) 10 Page - Analog Devices |
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AD7821 bảng dữ liệu(HTML) 10 Page - Analog Devices |
10 / 17 page AD7821 REV. B –9– SIGNAL-TO-NOISE RATIO AND DISTORTION The dynamic performance of the AD7821 is evaluated by apply- ing a very low distortion sine wave signal to the analog input (VIN) which is then sampled at a 512 kHz sampling rate. A Fast Fourier Transform (FFT) plot is then generated from which Signal-to-Noise Ratio (SNR) and harmonic distortion data are obtained. Figure 8 shows a 2048 point FFT plot of the AD7821 with an input signal of 100.25 kHz. The SNR is 49.1 dB. It should be noted that the harmonics are taken into account when calculat- ing the SNR. The theoretical relationship between SNR and resolution (N) is expressed by the following equation: SNR N dB =+ () 602 176 .. (1) Figure 8. FFT Plot EFFECTIVE NUMBER OF BITS By working backwards from Equation (1) it is possible to get a measure of ADC performance expressed in effective number of bits (N). A plot of the effective number of bits versus input frequency is given in the Typical Performance Characteristics section. The effective number of bits typically falls between 7.7 and 7.9, corresponding to SNR figures of 48.1 dB and 49.7 dB. INTERMODULATION DISTORTION For intermodulation distortion (IMD), an FFT plot consisting of very low distortion sine waves at two frequencies is generated by sampling an analog input applied to the ADC. Figure 9 shows a 2048 point plot for IMD. Figure 9. FFT Plot for IMD HISTOGRAM PLOT When a sine wave of specified frequency is applied to the VIN input of the AD7821 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. A perfect ADC produces a probability density function described by the equation: P(V ) = 1 π( A2 −V 2)1/2 where A is the peak amplitude of the sine wave and P(V) is the probability of occurrence at a voltage V. 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 the 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 differ- ential nonlinearity. Figure 10 shows a histogram plot for the AD7821, which corre- sponds very well with the ideal shape. The plot indicates very small differential nonlinearity and no missing codes for an input frequency of 100.25 kHz. Figure 10. Histogram Plot In digital signal processing applications, where the AD7821 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 ADC conversion process, is the best method of gen- erating equidistant sampling intervals. The two modes of operation given in the data sheet are suitable for DSP applications because the sampling instant of the AD7821 is well defined. VIN is sampled on the falling edge of WR or RD in the WR-RD or RD modes, respectively. DIGITAL INTERFACE The AD7821 has two basic interface modes which are determined by the status of the MODE pin. When this pin is low, the converter is in the RD mode, with this pin high, the AD7821 is set up for the WR-RD mode. The RD mode is designed for microprocessors that can be driven into a WAIT state. A READ operation (i.e., CS and RD are taken low) starts a conversion and data is read when the conversion is complete. The WR-RD mode does not require microprocessor WAIT states. A WRITE operation (i.e., CS and WR are taken low) initiates a conversion, and a READ operation reads the result when the conversion is complete. |
Số phần tương tự - AD7821_17 |
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Mô tả tương tự - AD7821_17 |
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