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AD7484BSTZ Datasheet(PDF) 11 Page - Analog Devices |
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AD7484BSTZ Datasheet(HTML) 11 Page - Analog Devices |
11 / 20 page AD7484 Rev. C | Page 11 of 20 TERMINOLOGY Integral Nonlinearity The integral nonlinearity is the maximum deviation from a straight line passing through the endpoints of the ADC transfer function. 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. Differential Nonlinearity The differential nonlinearity is the difference between the measured and ideal 1 LSB change between any two adjacent codes in the ADC. Offset Error The offset error is the deviation of the first code transition (00…000) to (00…001) from the ideal, that is, AGND + 0.5 LSB. Gain Error The gain error is the deviation of the last code transition (111…110) to (111…111) from the ideal, that is, VREF − 1.5 LSB, after the offset error has been adjusted out. Track-and-Hold Acquisition Time The track-and-hold acquisition time is the time required for the output of the track-and-hold amplifier to reach its final value, within ±1/2 LSB, after the end of conversion (the point at which the track-and-hold returns to track mode). Signal-to-Noise + Distortion (SINAD) Ratio The SINAD ratio is the measured ratio of signal-to-noise + distortion at the output of the ADC. The signal is the rms ampli- tude 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 SINAD ratio for an ideal N-bit converter with a sine wave input is given by Signal-to-Noise + Distortion = (6.02N + 1.76)dB Therefore, this is 86.04 dB for a 14-bit converter. Total Harmonic Distortion (THD) The THD is the ratio of the rms sum of the harmonics to the fundamental. It is defined as ( ) 1 2 6 2 5 2 4 2 3 2 2 V V V V V V THD + + + + = log 20 dB 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 The peak harmonic or spurious noise is 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. The value of this specification is usually 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 and 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), whereas the third order terms include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb). The AD7484 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, whereas the third order terms are usually at a frequency close to the input frequencies. As a result, the second order 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 dBs. |
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