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AD7870A Datasheet(PDF) 10 Page - Analog Devices |
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AD7870A Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page AD7870A –10– REV. 0 Effective Number of Bits The formula given in (1) relates the SNR to the number of bits. Rewriting the formula, as in (2), it is possible to obtain a mea- sure of performance expressed in effective number of bits (N). N = SNR –1.76 6.02 (2) The effective number of bits for a device can be calculated di- rectly from its measured SNR. Figure 12 shows a typical plot of effective number of bits versus frequency for an AD7870AJN, 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 dB and 73.1 dB. Figure 12. Effective Number of Bits vs. Frequency Harmonic Distortion Harmonic distortion is the ratio of the rms sum of harmonics to the fundamental. For the AD7870A, 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 V 1 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 sec- ond 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 in- termodulation distortion is as per the THD specification where it is the ratio of the rms sum of the individual distortion prod- ucts to the rms amplitude of the fundamental expressed in dBs. In this case, the input consists of two, equal amplitude, low dis- tortion sine waves. Figure 13 shows a typical IMD plot for the AD7870A. Figure 13. 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. AC Linearity Plot When a sine wave of specified frequency is applied to the VIN input of the AD7870A, and several million samples are taken, a histogram showing the frequency of occurrence of each of the 4096 ADC codes can be generated. From this histogram data it is possible to generate an ac integral linearity plot as shown in Figure 14. This shows very good integral linearity performance from the AD7870A at an input frequency of 25 kHz. The ab- sence of large spikes in the plot shows good differential linearity. Simplified versions of the formulae used are outlined below. INL(i ) = V (i )–V (o) V ( fs)–V (o) × 4096 – i where INL(i) is the integral linearity at code i. V(fs) and V(o) are the estimated full-scale and offset transitions and V(i) is the estimated transition for the ith code. |
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