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AD7476ARTZ-REEL Datasheet(PDF) 8 Page - Analog Devices |
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AD7476ARTZ-REEL Datasheet(HTML) 8 Page - Analog Devices |
8 / 20 page REV. D –8– AD7476/AD7477/AD7478 TERMINOLOGY Integral Nonlinearity This is the maximum deviation from a straight line passing through the endpoints of the ADC transfer function. For the AD7476/ AD7477, the endpoints of the transfer function are zero scale, a point 1/2 LSB below the first code transition, and full scale, a point 1/2 LSB above the last code transition. For the AD7478, the endpoints of the transfer function 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 This is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC. Offset Error This is the deviation of the first code transition (00 . . . 000) to (00 . . . 001) from the ideal (i.e., AGND + 0.5 LSB). For the AD7478, this is the deviation of the first code transition (00 . . . 000) to (00 . . . 001) from the ideal (i.e., AGND + 1 LSB). Gain Error For the AD7476/AD7477, this is the deviation of the last code transition (111 . . . 110) to (111 . . . 111) from the ideal (i.e., VREF – 1.5 LSB) after the offset error has been adjusted out. For the AD7478, this is the deviation of the last code transition (111 . . . 110) to (111 . . . 111) from the ideal (i.e., VREF – 1 LSB) after the offset error has been adjusted. Track-and-Hold Acquisition Time The track-and-hold amplifier returns into track mode after 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 ±0.5 LSB, after the end of conversion. See the Serial Interface Timing section for more detail. Signal-to-(Noise + Distortion) Ratio This is the measured ratio of signal-to-(noise + 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 quanti- zation noise. The theoretical signal-to-(noise + distortion) ratio for an ideal N-bit converter with a sine wave input is given by Signal-to-(Noise + Distortion) = (6.02N + 1.76) dB Thus for a 12-bit converter, this is 74 dB; for a 10-bit converter it is 62 dB; and for an 8-bit converter it is 50 dB. Total Unadjusted Error This is a comprehensive specification that includes gain error, linearity error, and offset error. Total Harmonic Distortion (THD) Total harmonic distortion is the ratio of the rms sum of harmon- ics to the fundamental. For the AD7476/AD7477/AD7478, it is defined as: THD VVVVV V 2 2 3 2 4 2 5 2 6 2 1 () log () dB = ++++ 20 where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5, and V6 are the rms amplitudes of the second through the sixth harmonics. Peak Harmonic or Spurious Noise Peak harmonic or spurious noise is defined as 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. Normally, the value of this specification is deter- mined by the largest harmonic in the spectrum, but for ADCs where the harmonics are buried in the noise floor, it will be a noise peak. Intermodulation Distortion With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities will create 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 is equal to zero. For example, the second-order terms include (fa + fb) and (fa – fb), while the third-order terms include (2fa + fb), (2fa – fb), (fa + 2fb), and (fa – 2fb). The AD7476/AD7477/AD7478 are tested using the CCIF standard where two input frequencies are used, fa = 498.7 kHz and fb = 508.7 kHz. 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 the intermodulation distortion is as per the THD specification where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the sum of the fundamentals, expressed in dB. |
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