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AD7450ABRT-REEL7 Datasheet(PDF) 10 Page - Analog Devices |
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AD7450ABRT-REEL7 Datasheet(HTML) 10 Page - Analog Devices |
10 / 28 page AD7440/AD7450A Rev. B | Page 10 of 28 TERMINOLOGY 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 the following: Signal-to-(Noise + Distortion) = (6.02N + 1.76)dB. Thus for a 12-bit converter, this is 74 dB; and for a 10-bit converter, this is 62 dB. Total Harmonic Distortion (THD) THD is the ratio of the rms sum of harmonics to the fundamental. For the AD7440/AD7450A, 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 to the sixth harmonics. Peak Harmonic or Spurious Noise Peak harmonic (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. 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 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 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 is equal to 0. 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 AD7440/AD7450A is tested using the CCIF standard of two input frequencies near the top end of the input bandwidth. In this case, the second-order terms are distanced in frequency from the original sine waves, while the third-order terms are 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 This is the amount of time from the leading edge of the sampling clock until the ADC actually takes the sample. Aperture Jitter This is 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) The common-mode rejection ratio 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 as follows: 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) This is the maximum deviation from a straight line passing through the endpoints of the ADC transfer function. Differential Nonlinearity (DNL) This is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC. Zero-Code Error This is the deviation of the midscale code transition (111...111 to 000...000) from the ideal VIN+ − VIN– (i.e., 0 LSB). |
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