The input driver and the ADC both introduce harmonic distortion in the data acquisition block that generates undesired signals in the output harmonically related to the input signal. Total harmonic distortion (THD) can be very important in applications measuring ac signals. However, there are also ADC dc-measurement applications that are only concerned with SNR and linearity. To make sure that the total system distortion performance is not dominated by the front-end stage, the distortion of the driver circuitry must be at least 10 dB less than the distortion of the ADC, as shown in Equation 6:

The harmonic distortion of an FDA mainly relates to the open-loop linearity in the output stage corrected by the loop gain at the fundamental frequency. When the total load impedance decreases, including the effect of the feedback resistors loadings, the output stage open-loop linearity degrades, and thus worsens the harmonic distortion, as seen in Figure 6-10.

Another effect that results from the RC filter is that the load impedance changes over frequency, which also influences the THD.

An additional dependency is given by the output voltage swing. Increasing the output voltage swing increases the nonlinearities of the open-loop output stage, thus degrading the harmonic distortion.

In summary, the harmonic distortion is negatively affected not only with decreasing load impedance and increasing output voltage swing, but also with increasing noise gain.

Section 9.2.2 provides an measurement results of the THD performance using the THP210 and the ADS891x ADC series.