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AD7274 Datasheet(PDF) 10 Page - Analog Devices |
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AD7274 Datasheet(HTML) 10 Page - Analog Devices |
10 / 20 page –10– REV. PrB PRELIMINARYTECHNICALDATA AD7273/AD7274 Preliminary Technical Data TERMINOLOGY Integral Nonlinearity (INL) This is the maximum deviation from a straight line pass- ing through the endpoints of the ADC transfer function. For the AD7273/AD7274, the endpoints of the transfer function are zero scale, a 1/2 LSB below the first code transition, and full scale, a point 1/2 LSB above the last code transition. Differential Nonlinearity (DNL) 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. Gain Error This is the deviation of the last code transition (111 . . . 110) to (111 . . . 111) from the ideal, i.e, VREF – 1.5LSB after the offset error has been adjusted out. Total Unadjusted Error (TUE) This is a comprehensive specification which includes gain, linearity and offset errors. Track/Hold Acquisition Time The Track/Hold acquisition time is the time required for the output of the track/hold amplifier to reach its final value, within ±0.5 LSB, after the end of conversion. See Serial Interface section for more details. Signal to Noise Ratio (SNR) This is 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 bandwitdh (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 dependant 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: SNR = 6.02 N + 1.76 dB Thus for a 12-bit converter this is 74 dB, for a 10-bit converter it is 62 dB. Practically, though, 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, etc. Signal-to- (Noise + Distortion) Ratio (SINAD) This is the measured ratio of signal to (noise + 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 nonfundamentals signals up to half the sampling frequency (fs/2), including harmonics but excluding dc. THD (dB ) = 20 log V2 2 +V 3 2 +V 4 2 +V 5 2 +V 6 2 V1 Total Harmonic Distortion (THD) Total harmonic distortion is the ratio of the rms sum of harmonics to the fundamental. It is defined as: 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 (SFDR) 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 determined 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 (IMD) 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, etc. Intermodulation distortion terms are those for which neither m nor n are equal to zero. For example, the second order terms in- clude (fa + fb) and (fa – fb), while the third order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb). The AD7273/AD7274 are tested using the CCIF standard where two input frequencies are used (see fa and fb in the specification page). 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 fre- quency 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 dBs. Power Supply Rejection Ratio (PSRR) The power supply rejection ratio is defined as the ratio of the power in the ADC output at full-scale frequency, f, to the power of a 200 mV p-p sine wave applied to the ADC VDD supply of frequency fs. PSRR (dB) = 10 log (Pf/ Pfs) Pf is the power at frequency f in the ADC output; Pfs is the power at frequency fs coupled onto the ADC VDD supply. Aperture Delay This is the measured interval between the leading edge of the sampling clock and the point at which the ADC actually takes the sample. Aperture Jitter This is the sample-to-sample variation in the effective point in time at which the sample is taken. |
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