As shown in the swept frequency harmonic distortion plots, the THS4541-Q1 provides extremely low distortion at lower frequencies. In general, FDA output harmonic distortion mainly relates to the open-loop linearity in the output stage corrected by the loop gain at the fundamental frequency. As the total load impedance decreases (including the effect of the feedback resistor elements in parallel for loading purposes), the output-stage, open-loop linearity degrades, increasing the harmonic distortion, as illustrated in Figure 6-16 and Figure 6-34. As the output voltage swings increase, very fine-scale, open-loop, output-stage nonlinearities increase, also degrading the harmonic distortion, as illustrated in Figure 6-14 and Figure 6-32. Conversely, decreasing the target output voltage swings drops the distortion terms rapidly. For harmonic-distortion testing, 2 V_PP is used as a nominal swing because this value represents a typical ADC, full-scale, differential input range.

Increasing the gain acts to decrease the loop gain, resulting in the increasing harmonic distortion terms, as illustrated in Figure 6-18 and Figure 6-36. One advantage to the capacitive compensation for the attenuator design (described in the Section 9.2.1 typical application example) is that the noise gain is shaped up with frequency to achieve a crossover at an acceptable phase margin at higher frequencies. This compensation holds the loop gain high at frequencies lower than the noise-gain zero, improving distortion in these lower bands.

Anything that moves the output pin voltage swings close to clipping into the supplies rapidly degrades harmonic distortion. Output clipping can occur from either absolute differential swing, or the swing can be moved closer to the supplies with the common-mode control. This effect is illustrated in Figure 6-17 and Figure 6-35.

The THS4541-Q1 does an exceptional job of converting from single-ended inputs to differential outputs with very low harmonic distortions. External resistors of 1% tolerance are used in characterization with good results. Imbalancing the feedback divider ratios does not degrade distortion directly. Imbalanced feedback ratios convert common-mode inputs to differential mode at the outputs with the gain described in the Section 7.4 section.