SLOS375B August 2014 – February 2024 THS4541
PRODUCTION DATA
Refer to the PDF data sheet for device specific package drawings
As shown in the swept frequency harmonic distortion plots, the THS4541 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. Figure 6-16 and Figure 6-34 shows that 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. Figure 6-14 and Figure 6-32 shows that as the output voltage swings increase, very fine-scale, open-loop, output-stage nonlinearities increase, also degrading the harmonic distortion. Conversely, decreasing the target output voltage swings drops the distortion terms rapidly. For harmonic-distortion testing, 2 VPP is used as a nominal swing because this value represents a typical ADC, full-scale, differential input range.
Figure 6-18 and Figure 6-36 shows how increasing the gain acts to decrease the loop gain, resulting in the increasing harmonic distortion terms. One advantage to the capacitive compensation for the attenuator design (described in the typical application example of Section 9.2.1) 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. Figure 6-17 and Figure 6-35 shows this effect.
The THS4541 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 Section 7.4.