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AD7878BQ 데이터시트(PDF) 9 Page - Analog Devices |
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AD7878BQ 데이터시트(HTML) 9 Page - Analog Devices |
9 / 16 page AD7878 –9– REV. A Figure 11 shows a typical plot of effective number of bits versus frequency for an AD7878KN with a sampling frequency of 100 kHz. The effective number of bits typically falls between 11.7 and 11.85 corresponding to SNR figures of 72.2 and 73.1 dB. Figure 11. Effective Number of Bits vs. Frequency Harmonic Distortion Harmonic Distortion is the ratio of the rms sum of harmonics to the fundamental. For the AD7878, Total Harmonic Distortion (THD) is defined as: THD = 20 log (V 2 2 +V 3 2 +V 4 2 +V 5 2 +V 6 2 ) V1 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 harmonic. The THD is also derived from the FFT plot of the ADC output spectrum. Intermodulation Distortion 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 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) while the third order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb). Using the CCIF standard, where two input frequencies near the top end of the input bandwidth are used, the second and third order terms are of different significance. 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 am- plitude of the fundamental expressed in dBs. Intermodulation distortion is calculated using an FFT algorithm but, in this case, the input consists of two equal amplitude, low distortion sine waves. Figure 12 shows a typical IMD plot for the AD7878. 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 will be determined by the largest harmonic in the spectrum, but for parts where the harmonics are buried in the noise floor the largest peak will be a noise peak. Figure 12. AD7878 IMD Plot Histogram Plot When a sine wave of a specified frequency is applied to the VIN input of the AD7878 and several million samples are taken, it is possible to plot a histogram showing the frequency of occur- rence of each of the 4096 ADC codes. If a particular step is wider than the ideal 1 LSB width, then the code associated with that step will accumulate more counts than for the code for an ideal step. Likewise, a step narrower than ideal will have fewer counts. Missing codes are easily seen in the histogram plot because a missing code means zero counts for a particular code. Large spikes in the plot indicate large differential nonlinearity. Figure 13 shows a histogram plot for the AD7878KN with a sampling frequency of 100 kHz and an input frequency of 25 kHz. For a sine-wave input, a perfect ADC would produce a cusp probability density function described by the equation: p (V ) = 1 π ( A2 –V 2 ) where A is the peak amplitude of the sine wave and p (V) is the probability of occurrence at a voltage V. The histogram plot of Figure 13 corresponds very well with this cusp shape. The ab- sence of large spikes in this plot indicates small dynamic differ- ential nonlinearity (the largest spike in the plot represents less than 1/4 LSB of DNL error). The AD7878 has no missing codes under these conditions since no code records zero counts. Figure 13. AD7878 Histogram Plot |
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