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AD7291BCPZ-RL7 Datasheet(PDF) 11 Page - Analog Devices |
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AD7291BCPZ-RL7 Datasheet(HTML) 11 Page - Analog Devices |
11 / 28 page AD7291 Rev. 0 | Page 11 of 28 TERMINOLOGY Signal-to-Noise and Distortion Ratio (SINAD) The measured ratio of signal-to-noise and distortion at the output of the ADC. The signal is the rms amplitude of the fundamental. Noise is the sum of all nonfundamental signals up to half the sampling frequency (fS/2), excluding dc. The ratio is dependent on the number of quantization levels in the digitization process; the more levels, the smaller the quantization noise. The theoretical signal-to-noise and distortion ratio for an ideal N-bit converter with a sine wave input is given by Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB Thus, the SINAD is 74 dB for an ideal 12-bit converter. Total Harmonic Distortion (THD) The ratio of the rms sum of harmonics to the fundamental. For the AD7291, it is defined as 1 2 6 2 5 2 4 2 3 2 2 log 20 ) dB ( V V V V V V THD + + + + = where: V1 is the rms amplitude of the fundamental. V2, V3, V4, V5, and V6 are the rms amplitudes of the second through sixth harmonics. Peak Harmonic or Spurious Noise The ratio of the rms value of the next largest component in the ADC output spectrum (up to fS/2 and excluding dc) to the rms value of the fundamental. Typically, the value of this specification is determined by the largest harmonic in the spectrum, but for ADCs where the harmonics are buried in the noise floor, it is a noise peak. Intermodulation Distortion With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities creates distortion products at sum and difference frequencies of mfa ± nfb, where m, n = 0, 1, 2, 3, and so on. Intermodulation distortion terms are those for which neither m nor n equals zero. For example, second-order terms include (fa + fb) and (fa − fb), while third- order terms include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb). The AD7291 is tested using the CCIF standard where two input frequencies near the top end of the input bandwidth are used. In this case, the second-order terms are usually distanced in frequency from the original sine waves while the third-order terms are usually at a frequency close to the input frequencies. As a result, the second- and third-order terms are specified separately. The calculation of intermodulation distortion is, like the THD specification, the ratio of the rms sum of the individual distortion products to the rms amplitude of the sum of the fundamentals, expressed in dB. Aperture Delay The measured interval between the sampling clock’s leading edge and the point at which the ADC takes the sample. Aperture Jitter This is the sample-to-sample variation in the effective point in time at which the sample is taken. Full-Power Bandwidth The input frequency at which the amplitude of the recon- structed fundamental is reduced by 0.1 dB or 3 dB for a full-scale input. Power Supply Rejection Ratio (PSRR) PSRR is defined as the ratio of the power in the ADC output at full-scale frequency, f, to the power of a 100 mV p-p sine wave applied to the ADC VDD supply of frequency, fS. The frequency of the input varies from 5 kHz to 25 MHz. PSRR (dB) = 10 log(Pf/PfS) where: Pf is the power at frequency, f, in the ADC output. PfS is the power at frequency, fS, in the ADC output. Integral Nonlinearity The maximum deviation from a straight line passing through the endpoints of the ADC transfer function. The endpoints are zero scale, a point 1 LSB below the first code transition, and full scale, a point 1 LSB above the last code transition. Differential Nonlinearity The difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC. Offset Error The deviation of the first code transition (00…000) to (00…001) from the ideal—that is, GND1 + 1 LSB. Offset Error Match The difference in offset error between any two channels. Gain Error The deviation of the last code transition (111…110) to (111…111) from the ideal (that is, VREF − 1 LSB) after the offset error has been adjusted out. Gain Error Match The difference in gain error between any two channels. Track-and-Hold Acquisition Time The track-and-hold amplifier returns to track mode at the end of conversion. Track-and-hold acquisition time is the time required for the output of the track-and-hold amplifier to reach its final value, within ±1 LSB, after the end of conversion. |
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