THS4521 datasheet(32/51 Pages) TI

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10.1.4 Factors Influencing Harmonic Distortion

As illustrated in the swept frequency harmonic distortion plots (Figure 7-7 and Figure 7-21), the THS4561

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 7-9 and Figure 7-23. When the output voltage swings increase, very fine scale open-loop output stage

nonlinearities increase that also degrade the harmonic distortion; see Figure 7-8 and Figure 7-22. Conversely,

decreasing the target output voltage swings drops the distortion terms rapidly. Figure 7-8 and Figure 7-22

illustrate the effect of going up to a 10-VPP and 8-VPP differential output, respectively, 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 7-10 and Figure 7-24. One advantage of capacitive compensation that is typical in 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 holds the loop gain high at frequencies lower than the noise gain zero,

thus improving distortion at lower frequencies.

The THS4561 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. However, imbalanced feedback

ratios convert common-mode inputs to a differential mode at the outputs that can result in increased output

errors.

10.1.5 Input Overdrive Performance

Figure 10-5 and Figure 10-6 show a 2-V and a 2X output overdrive triangular waveform, respectively, for the

THS4561. When the output maximum swing is reached at approximately the supply values, increasing input

voltage beyond this condition turns on the internal protection diodes across the two input pins. The internal

protection diodes are two diodes in series in both polarities. This feature clamps the maximum differential

voltage across the inputs to approximately 1.5 V when the output is limited at the supplies but the input exceeds

the available range. The input resistors on both sides limit the current flow in the internal diodes under these

conditions.

Time, 500 ns per division

-15

-12

-9

-6

-3

D016

Input

Output

VS = 12 V, Single-ended to differential gain of 2, 2-V output

overdrive,

overdrive recovery = 250 ns

Figure 10-5. Overdrive Recovery Performance

Time, 500 ns per division

-10

-7.5

-5

-2.5

2.5

7.5

D034

Input

Output

VS = 5 V, Single-ended to differential gain of 2, 2x input

overdrive,

overdrive recovery = 210 ns

Figure 10-6. Overdrive Recovery Performance

THS4561

SBOS874C – AUGUST 2017 – REVISED DECEMBER 2020

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THS4521 Datasheet(PDF) 32 Page - Texas Instruments

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