Datasheet AD7821 (Analog Devices) - 10

AD7821 SIGNAL-TO-NOISE RATIO AND DISTORTION
possible to plot a histogram showing the frequency of occurrence The dynamic performance of the AD7821 is evaluated by apply- of each of the 256 ADC codes. A perfect ADC produces a ing a very low distortion sine wave signal to the analog input probability density function described by the equation: (VIN) which is then sampled at a 512 kHz sampling rate. A Fast Fourier Transform (FFT) plot is then generated from which P(V ) = 1 π( A2 −V 2)1/2 Signal-to-Noise Ratio (SNR) and harmonic distortion data are obtained. where A is the peak amplitude of the sine wave and P(V) is the Figure 8 shows a 2048 point FFT plot of the AD7821 with an probability of occurrence at a voltage V. input signal of 100.25 kHz. The SNR is 49.1 dB. It should be If a particular step is wider than the ideal 1 LSB width, then the noted that the harmonics are taken into account when calculat- code associated with that step will accumulate more counts than ing the SNR. The theoretical relationship between SNR and for the code for an ideal step. Likewise, a step narrower than the resolution (N) is expressed by the following equation: ideal width will have fewer counts. Missing codes are easily seen because a missing code means zero counts for a particular code. SNR = 6 0 . 2N ( + 1 7 . 6 dB ) (1) 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 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
Figure 10. Histogram Plot For intermodulation distortion (IMD), an FFT plot consisting In digital signal processing applications, where the AD7821 is of very low distortion sine waves at two frequencies is generated used to sample ac signals, it is essential that the signal sampling by sampling an analog input applied to the ADC. Figure 9 occurs at exactly equal intervals. This minimizes errors due to shows a 2048 point plot for IMD. 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 Figure 9. FFT Plot for IMD complete. The WR-RD mode does not require microprocessor WAIT states. A WRITE operation (i.e., CS and WR are taken
HISTOGRAM PLOT
low) initiates a conversion, and a READ operation reads the When a sine wave of specified frequency is applied to the VIN input result when the conversion is complete. of the AD7821 and several thousand samples are taken, it is REV. B –9– Document Outline FEATURES GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS SPECIFICATIONS Test Circuits ORDERING GUIDE ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATIONS PIN FUNCTION DESCRIPTIONS TERMINOLOGY LEAST SIGNIFICANT BIT (LSB) TOTAL UNADJUSTED ERROR SLEW RATE TOTAL HARMONIC DISTORTION INTERMODULATION DISTORTION SIGNAL-TO-NOISE RATIO PEAK HARMONIC OR SPURIOUS NOISE Typical Performance Characteristics CIRCUIT INFORMATION BASIC DESCRIPTION OPERATING SEQUENCE REFERENCE AND INPUT INPUT CURRENT INPUT TRANSIENTS INHERENT TRACK-AND-HOLD SINUSOIDAL INPUTS DIGITAL SIGNAL PROCESSING APPLICATIONS SIGNAL-TO-NOISE RATIO AND DISTORTION EFFECTIVE NUMBER OF BITS INTERMODULATION DISTORTION HISTOGRAM PLOT DIGITAL INTERFACE RD Mode (MODE = 0) WR-RD Mode (MODE = 1) MICROPROCESSOR INTERFACING AD7821 – 68008 INTERFACE AD7821 – 8088 INTERFACE AD7821 – TMS32010 INTERFACE AD7821 – 8051 INTERFACE APPLYING THE AD7821 UNIPOLAR OPERATION BIPOLAR OPERATION 16-CHANNEL TELECOM A/D CONVERTER SIMULTANEOUS SAMPLING ADCS OUTLINE DIMENSIONS Revision History