Datasheet AD7821 (Analog Devices) - 6

AD7821 TERMINOLOGY INTERMODULATION DISTORTION LEAST SIGNIFICANT BIT (LSB)
With inputs consisting of sine waves at two frequencies, fa and An ADC with 8-bit resolution can resolve one part in 28 (1/256 fb, any active device with nonlinearities will create distortion of full scale). For the AD7821 operating in either the unipolar products, of order (m+n), at sum and difference frequencies of or bipolar input range with 5 V full scale, one LSB is 19.53 mV. mfa+nfb, where m, n = 0, 1, 2, 3…. Intermodulation terms are those for which m or n is not equal to zero. For example, the
TOTAL UNADJUSTED ERROR
second order terms include (fa + fb) and (fa – fb), and the third This is a comprehensive specification which includes relative order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and accuracy, offset error, and full-scale error. (fa – 2fb). For the AD7821 intermodulation distortion is calcu- lated separately for both the second and third order terms.
SLEW RATE
Slew rate is the maximum allowable rate of change of input
SIGNAL-TO-NOISE RATIO (SNR)
signal such that the digital sample values are not in error. Signal-to-noise ratio is measured signal-to-noise at the output of the ADC. The signal is the rms magnitude of the fundamental.
TOTAL HARMONIC DISTORTION (THD)
Noise is the rms sum of all nonfundamental signals (excluding Total harmonic distortion is the ratio of the square root of the dc) up to half the sampling frequency. SNR is dependent on the sum of the squares of the rms value of the harmonics to the rms number of quantization levels used in the digitization process. value of the fundamental. For the AD7821, total harmonic dis- The theoretical SNR for a sine wave input is given by: tortion is defined as SNR = 6 0 . 2N ( + 1 7 . 6 dB )  2 2 2 2 2  V + V + V + V + V  ( ) 2 3 4 5 6  where N is the number of bits in the ADC. Thus, for an ideal 20 log dB  V  8-bit ADC, SNR = 50 dB. 1  
PEAK HARMONIC OR SPURIOUS NOISE
where V1 is the rms amplitude of the fundamental and V2, V3, V4, Peak harmonic or spurious noise is the rms value of the largest V5, and V6 are the rms amplitudes of the individual harmonics. nonfundamental frequency (excluding dc) up to half the sam- pling frequency to the rms value of the fundamental. REV. B –5– 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