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5962-8951801RA 参数 Datasheet PDF下载

5962-8951801RA图片预览
型号: 5962-8951801RA
PDF下载: 下载PDF文件 查看货源
内容描述: [High Speed, µP-Compatible, CMOS, 8-Bit Sampling ADC]
分类和应用: 转换器
文件页数/大小: 16 页 / 290 K
品牌: ADI [ ADI ]
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AD7821  
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.  
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:  
1
P(V ) =  
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.  
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:  
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.  
SNR = 6.02N + 1.76 dB  
(1)  
(
)
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 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.  
Figure 10. Histogram Plot  
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.  
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.  
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  
REV. B  
–9–