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AD7880 Datasheet(PDF) 8 Page - Analog Devices |
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AD7880 Datasheet(HTML) 8 Page - Analog Devices |
8 / 16 page AD7880 –8– REV. 0 12 11.5 11 10.5 10 INPUT FREQUENCY – kHz SAMPLE FREQUENCY = 61kHz T = 25 C A 15 30.5 Figure 13. Effective Number of Bits vs. Frequency Total Harmonic Distortion (THD) THD is the ratio of the rms sum of harmonics to the rms value of the fundamental. For the AD7880, THD is defined as: THD = 20 log V 2 2 +V 3 2 +V 4 2 +V 5 2 +V 6 2 V 1 (3) 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 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 are equal to zero. For example, the second or- der 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 inter- modulation 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 fundamental expressed in dBs. In this case, the input consists of two, equal amplitude, low distortion, sine waves. Figure 14 shows a typical IMD plot for the AD7880. Figure 14. IMD Plot 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 peak will be a noise peak. |
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