As illustrated in the swept frequency harmonic distortion plots (Figure 7-13 and Figure 7-31), the THS4551 provides extremely low distortion at lower frequencies. In general, an FDA output harmonic distortion 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 resistor elements in parallel for loading purposes, the output stage open-loop linearity degrades, thus increasing the harmonic distortion; see Figure 7-16 and Figure 7-34. When the output voltage swings increase, very fine scale open-loop output stage nonlinearities increase that also degrade the harmonic distortion; see Figure 7-14 and Figure 7-32. Conversely, decreasing the target output voltage swings drops the distortion terms rapidly. A nominal swing of 2 V_PP is used for harmonic distortion testing where Figure 7-14 illustrates the effect of going up to an 8-V_PP differential input that is more common with SAR converters.

Increasing the noise gain functions to decrease the loop gain resulting in the increasing harmonic distortion terms; see Figure 7-18 and Figure 7-36. One advantage to the capacitive compensation for the attenuator designs is that the noise gain is shaped up with frequency to achieve a crossover at an acceptable phase margin at higher frequencies. This technique (see Section 10.1.6) holds the loop gain high at frequencies lower than the noise gain zero, thus improving distortion at lower frequencies.

The THS4551 holds nearly constant distortion when the V_OCM operating point is moved in the allowed range; see Figure 7-17 and Figure 7-35. Clipping into the supplies with any combination of V_OCM and V_OPP rapidly degrades distortion performance.

The THS4551 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. Unbalancing the feedback divider ratios does not degrade distortion directly. Imbalanced feedback ratios convert common-mode inputs to a differential mode at the outputs with the gain described in Section 9.3.4.