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AD7921ARM-REEL Folha de dados(PDF) 11 Page - Analog Devices |
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AD7921ARM-REEL Folha de dados(HTML) 11 Page - Analog Devices |
11 / 28 page AD7911/AD7921 Rev. 0 | Page 11 of 28 TERMINOLOGY Integral Nonlinearity The maximum deviation from a straight line passing through the endpoints of the ADC transfer function. For the AD7911/ AD7921, 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 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 + 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. Total Unadjusted Error A comprehensive specification that includes gain error, linearity error, and offset error. Channel-to-Channel Isolation A measure of the level of crosstalk between channels. It is measured by applying a full-scale sine wave signal of 20 kHz to 500 kHz to the nonselected input channel and determining how much that signal is attenuated in the selected channel with a 10 kHz signal. The figure is given worst case across both channels for the AD7911/AD7921. Track-and-Hold Acquisition Time 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. The track-and-hold amplifier returns to track mode at the end of conversion. See the Serial Interface section for more details. Signal-to-Noise and Distortion Ratio (SINAD) The measured ratio of signal-to-noise and distortion at the output of the A/D converter. The signal is the rms value of the sine wave, and noise is the rms sum of all nonfundamental signals up to half the sampling frequency (fs/2), including harmonics but excluding dc. Signal-to-Noise Ratio (SNR) The measured ratio of signal to noise at the output to the A/D converter. The signal is the rms value of the sine wave input. Noise is the rms quantization error within the Nyquist bandwidth (fs/2). The rms value of a sine wave is one-half its peak-to-peak value divided by √2, and the rms value for the quantization noise is q/√12. 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 SNR is defined as dB 76 1 02 6 . N . SNR + = Therefore, for a 12-bit converter, SNR is 74 dB; for a 10-bit converter, SNR is 62 dB. However, various error sources in the ADC cause the measured SNR to be less than the theoretical value. These errors occur due to integral and differential nonlinearities, internal ac noise sources, and so on. Total Harmonic Distortion (THD) The ratio of the rms sum of harmonics to the fundamental, which is defined as 1 2 6 2 5 2 4 2 2 2 V V V V V V THD + + + + = 3 log 20 ) dB ( where: V1 is the rms amplitude of the fundamental. V2, V3, V4, V5, and V6 are the rms amplitudes of the second through the 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. Normally, the value of this specifica- tion 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. |
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