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AD7821KR Datasheet(PDF) 6 Page - Analog Devices |
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AD7821KR Datasheet(HTML) 6 Page - Analog Devices |
6 / 17 page AD7821 REV. B –5– TERMINOLOGY LEAST SIGNIFICANT BIT (LSB) An ADC with 8-bit resolution can resolve one part in 2 8 (1/256 of full scale). For the AD7821 operating in either the unipolar or bipolar input range with 5 V full scale, one LSB is 19.53 mV. TOTAL UNADJUSTED ERROR This is a comprehensive specification which includes relative accuracy, offset error, and full-scale error. SLEW RATE Slew rate is the maximum allowable rate of change of input signal such that the digital sample values are not in error. TOTAL HARMONIC DISTORTION (THD) Total harmonic distortion is the ratio of the square root of the sum of the squares of the rms value of the harmonics to the rms value of the fundamental. For the AD7821, total harmonic dis- tortion is defined as 20 2 2 3 2 4 2 5 2 6 2 1 log VVV VV V dB ++ ++ () where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5, and V6 are the rms amplitudes of the individual harmonics. INTERMODULATION DISTORTION With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities will create distortion products, of order (m+n), at sum and difference frequencies of 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 second order terms include (fa + fb) and (fa – fb), and the third order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb). For the AD7821 intermodulation distortion is calcu- lated separately for both the second and third order terms. SIGNAL-TO-NOISE RATIO (SNR) Signal-to-noise ratio is measured signal-to-noise at the output of the ADC. The signal is the rms magnitude of the fundamental. Noise is the rms sum of all nonfundamental signals (excluding dc) up to half the sampling frequency. SNR is dependent on the number of quantization levels used in the digitization process. The theoretical SNR for a sine wave input is given by: SNR N dB =+ () 602 176 .. where N is the number of bits in the ADC. Thus, for an ideal 8-bit ADC, SNR = 50 dB. PEAK HARMONIC OR SPURIOUS NOISE Peak harmonic or spurious noise is the rms value of the largest nonfundamental frequency (excluding dc) up to half the sam- pling frequency to the rms value of the fundamental. |
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