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AD1674 Datasheet(PDF) 8 Page  Analog Devices 

AD1674 Datasheet(HTML) 8 Page  Analog Devices 
8 / 12 page AD1674 REV. C –8– DEFINITION OF SPECIFICATIONS INTEGRAL NONLINEARITY (INL) The ideal transfer function for an ADC is a straight line drawn between “zero” and “full scale.” The point used as “zero” occurs 1/2 LSB before the first code transition. “Full scale” is defined as a level 1 1/2 LSB beyond the last code transition. Integral nonlinearity is the worstcase deviation of a code from the straight line. The deviation of each code is measured from the middle of that code. DIFFERENTIAL NONLINEARITY (DNL) A specification which guarantees no missing codes requires that every code combination appear in a monotonic increasing sequence as the analog input level is increased. Thus every code must have a finite width. The AD1674 guarantees no missing codes to 12bit resolution; all 4096 codes are present over the entire operating range. UNIPOLAR OFFSET The first transition should occur at a level 1/2 LSB above ana log common. Unipolar offset is defined as the deviation of the actual transition from that point at 25 °C. This offset can be adjusted as shown in Figure 11. BIPOLAR OFFSET In the bipolar mode the major carry transition (0111 1111 1111 to 1000 0000 0000) should occur for an analog value 1/2 LSB below analog common. The bipolar offset error specifies the deviation of the actual transition from that point at 25 °C. This offset can be adjusted as shown in Figure 12. FULLSCALE ERROR The last transition (from 1111 1111 1110 to 1111 1111 1111) should occur for an analog value 1 1/2 LSB below the nominal full scale (9.9963 volts for 10 volts full scale). The fullscale error is the deviation of the actual level of the last transition from the ideal level at 25 °C. The fullscale error can be adjusted to zero as shown in Figures 11 and 12. TEMPERATURE DRIFT The temperature drifts for fullscale error, unipolar offset and bipolar offset specify the maximum change from the initial (25 °C) value to the value at T MIN or TMAX. POWER SUPPLY REJECTION The effect of power supply error on the performance of the device will be a small change in full scale. The specifications show the maximum fullscale change from the initial value with the supplies at various limits. FREQUENCYDOMAIN TESTING The AD1674 is tested dynamically using a sine wave input and a 2048 point Fast Fourier Transform (FFT) to analyze the resulting output. Coherent sampling is used, wherein the ADC sampling frequency and the analog input frequency are related to each other by a ratio of integers. This ensures that an integral multiple of input cycles is captured, allowing direct FFT pro cessing without windowing or digital filtering which could mask some of the dynamic characteristics of the device. In addition, the frequencies are chosen to he “relatively prime” (no common factors) to maximize the number of different ADC codes that are present in a sample sequence. The result, called Prime Coherent Sampling, is a highly accurate and repeatable measure of the actual frequencydomain response of the converter. NYQUIST FREQUENCY An implication of the Nyquist sampling theorem, the “Nyquist Frequency” of a converter is that input frequency which is one half the sampling frequency of the converter. SIGNALTONOISE AND DISTORTION (S/N+D) RATIO S/(N+D) is the ratio of the rms value of the measured input sig nal to the rms sum of all other spectral components below the Nyquist frequency, including harmonics but excluding dc. The value for S/(N+D) is expressed in decibels. TOTAL HARMONIC DISTORTION (THD) THD is the ratio of the rms sum of the first six harmonic com ponents to the rms value of a fullscale input signal and is ex pressed as a percentage or in decibels. For input signals or harmonics that are above the Nyquist frequency, the aliased component is used. INTERMODULATION DISTORTION (IMD) With inputs consisting of sine waves at two frequencies, fa and fb, any 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 are (fa + fb) and (fa – fb) and the third order terms are (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb). The IMD products are expressed as the decibel ratio of the rms sum of the measured input signals to the rms sum of the distortion terms. The two signals are of equal amplitude and the peak value of their sums is –0.5 dB from full scale. The IMD products are normalized to a 0 dB input signal. FULLPOWER BANDWIDTH The fullpower bandwidth is that input frequency at which the amplitude of the reconstructed fundamental is reduced by 3 dB for a fullscale input. FULLLINEAR BANDWIDTH The fulllinear bandwidth is the input frequency at which the slew rate limit of the sampleholdamplifier (SHA) is reached. At this point, the amplitude of the reconstructed fundamental has degraded by less than –0.1 dB. Beyond this frequency, dis tortion of the sampled input signal increases significantly. APERTURE DELAY Aperture delay is a measure of the SHA’s performance and is measured from the falling edge of Read/Convert (R/C) to when the input signal is held for conversion. APERTURE JITTER Aperture jitter is the variation in aperture delay for successive samples and is manifested as noise on the input to the A/D. 
