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AD7278BRMZ Datasheet(PDF) 16 Page - Analog Devices |
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AD7278BRMZ Datasheet(HTML) 16 Page - Analog Devices |
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16 / 29 page 
AD7276/AD7277/AD7278 Rev. C | Page 15 of 28 TERMINOLOGY Integral Nonlinearity The maximum deviation from a straight line passing through the endpoints of the ADC transfer function. For the AD7276/ AD7277/AD7278, the endpoints of the transfer function are zero scale at 0.5 LSB below the first code transition and full scale at 0.5 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, AGND + 0.5 LSB. Gain Error The deviation of the last code transition (111 . . . 110) to (111 . . . 111) from the ideal after adjusting for the offset error, that is, VREF − 1.5 LSB. Total Unadjusted Error A comprehensive specification that includes gain, linearity, and offset errors. Track-and-Hold Acquisition Time The time required after the conversion for the output of the track-and-hold amplifier to reach its final value within ±0.5 LSB. See the Serial Interface section for more details. Signal-to-Noise + Distortion Ratio (SINAD) The measured ratio of signal to noise plus distortion at the output of the ADC. The signal is the rms amplitude of the fundamental, and noise is the rms sum of all nonfundamental signals up to half the sampling frequency (fS/2), including harmonics but excluding dc. The ratio is dependent on the number of quantization levels in the digitization process: the more levels, the smaller the quantization noise. For an ideal N-bit converter, the SINAD is defined as dB 76 . 1 02 . 6 + = N SINAD According to this equation, the SINAD is 74 dB for a 12-bit converter and 62 dB for a 10-bit converter. However, various error sources in the ADC, including integral and differential nonlinearities and internal ac noise sources, cause the measured SINAD to be less than its theoretical value. Total Harmonic Distortion (THD) The ratio of the rms sum of harmonics to the fundamental. 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, excluding dc) to the rms value of the fundamental. Normally, the value of this specification is determined by the largest harmonic in the spectrum; however, for ADCs with harmonics buried in the noise floor, it is determined by 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 and n = 0, 1, 2, 3, …. Intermodulation distortion terms are those for which neither m nor n are 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). The AD7276/AD7277/AD7278 are tested using the CCIF standard in which two input frequencies are used (see fa and fb in the specifications). In this case, the second-order terms are usually distanced in frequency from the original sine waves, and 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 intermodulation distortion is calculated in a similar manner to the THD specification, that is, the ratio of the rms sum of the individual distortion products to the rms amplitude of the sum of the fundamentals expressed in decibels. Aperture Delay The measured interval between the leading edge of the sampling clock and the point at which the ADC takes the sample. Aperture Jitter The sample-to-sample variation when the sample is taken. |
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