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AD7452 Datasheet(PDF) 9 Page - Analog Devices |
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AD7452 Datasheet(HTML) 9 Page - Analog Devices |
9 / 25 page AD7452 Data Sheet Rev. C | Page 8 of 24 TERMINOLOGY Signal-to-(Noise + Distortion) Ratio The measured ratio of signal to (noise + distortion) at the output of the ADC. The signal is the rms amplitude of the fun- damental. 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 digitiza- tion process; the more levels, the smaller the quantization 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. Total Harmonic Distortion (THD) Total harmonic distortion is the ratio of the rms sum of harmonics to the fundamental. For the AD7452, it is defined as 1 6 5 4 3 2 V V V V V V THD 2 2 2 2 2 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 to 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 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 pro- ducts at the 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 are 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 AD7452 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 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. Aperture Delay The amount of time from the leading edge of the sampling clock until the ADC actually takes the sample. Aperture Jitter The sample-to-sample variation in the effective point in time at which the actual sample is taken. Full Power Bandwidth The full power bandwidth of an ADC is the input frequency at which the amplitude of the reconstructed fundamental is reduced by 0.1 dB or 3 dB for a full-scale input. Common-Mode Rejection Ratio (CMRR) This is the ratio of the power in the ADC output at full-scale frequency, f, to the power of a 100 mV p-p sine wave applied to the common-mode voltage of VIN+ and VIN– of frequency fS CMRR(dB) = 10 log(Pf/PfS) Pf is the power at the frequency f in the ADC output; PfS is the power at frequency fS in the ADC output. Integral Nonlinearity (INL) The maximum deviation from a straight line passing through the endpoints of the ADC transfer function. Differential Nonlinearity (DNL) The difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC. Zero Code Error The deviation of the midscale code transition (111…111 to 000...000) from the ideal VIN+ – VIN– (that is, 0 LSB) Positive Gain Error This is the deviation of the last code transition (011...110 to 011...111) from the ideal VIN+ – VIN– (that is, VREF – 1 LSB), after the zero code error has been adjusted out. |
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