Datasheet AD7874 (Analog Devices) - 10

AD7874 Total Harmonic Distortion (THD) Peak Harmonic or Spurious Noise
Total Harmonic Distortion (THD) is the ratio of the rms sum Harmonic or Spurious Noise is defined as the ratio of the rms of harmonics to the rms value of the fundamental. For the value of the next largest component in the ADC output spec- AD7874, THD is defined as trum (up to fs/2 and excluding dc) to the rms value of the fun- damental. Normally, the value of this specification will be 2 2 2 2 2 V +V +V +V +V determined by the largest harmonic in the spectrum, but for THD = 20 log 2 3 4 5 6 parts where the harmonics are buried in the noise floor the peak V1 will be a noise peak. where V1 is the rms amplitude of the fundamental and V2, V3, V
AC Linearity Plot
4, V5 and V6 are the rms amplitudes of the second through the sixth harmonic. The THD is also derived from the FFT plot of When a sine wave of specified frequency is applied to the VIN in- the ADC output spectrum. put of the AD7874 and several million samples are taken, a his- togram showing the frequency of occurrence of each of the 4096
Intermodulation Distortion
ADC codes can be generated. From this histogram data it is With inputs consisting of sine waves at two frequencies, fa and possible to generate an ac integral linearity plot as shown in Fig- fb, any active device with nonlinearities will create distortion ure 11. This shows very good integral linearity performance products at sum and difference frequencies of mfa ± nfb where from the AD7874 at an input frequency of 10 kHz. The absence m, n = 0, 1, 2, 3 . ., etc. Intermodulation terms are those for of large spikes in the plot shows good differential linearity. Sim- which neither m or n are equal to zero. For example, the second plified versions of the formulae used are outlined below. order terms include (fa + fb) and (fa – fb) while the third order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb).   INL(i ) = (V (i ) − V (o)) ⋅ 4096 Using the CCIF standard where two input frequencies near the V ( fs)  −V(o)  − i top end of the input bandwidth are used, the second and third where INL(i) is the integral linearity at code i. V(fs) and V(o) are order terms are of different significance. The second order terms the estimated full-scale and offset transitions, and V(i) is the es- are usually distanced in frequency from the original sine waves timated transition for the ith code. while the third order terms are usually at a frequency close to V(i), the estimated code transition point is derived as follows: the input frequencies. As a result, the second and third order terms are specified separately. The calculation of the intermodu- π ⋅ cum(i) [ ] lation distortion is as per the THD specification where it is the V (i ) = − A ⋅ Cos N ratio of the rms sum of the individual distortion products to the rms amplitude of the fundamental expressed in dBs. In this case, where A is the peak signal amplitude, N is the number of histo- the input consists of two, equal amplitude, low distortion sine gram samples waves. Figure 10 shows a typical IMD plot for the AD7874. i and cum(i ) = ∑ V(n)occurrences n =o Figure 11. AD7874 AC INL Plot Figure 10. AD7874 IMD Plot REV. C –9–