THS4561 Datasheet, PDF (28/35 Pages) Texas Instruments – Low-Power, High Supply Range, 70-MHz, Fully Differential Amplifier

THS4561
SBOS874 – AUGUST 2017
www.ti.com
10.1.4 Factors Influencing Harmonic Distortion
As illustrated in the swept frequency harmonic distortion plots ( and ), 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 and . When the output
voltage swings increase, very fine scale open-loop output stage nonlinearities increase that also degrade the
harmonic distortion; see and . 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 illustrates the effect of going up to
an TBD-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 and . 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 the section) holds the loop gain high at frequencies lower than the noise gain zero, thus
improving distortion at lower frequencies.
The THS4561 holds nearly constant distortion when the V_OCM operating point is moved in the allowed range; see
and . Clipping into the supplies with any combination of V_OCM and V_OPP rapidly degrades distortion performance.
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. Imbalanced feedback ratios convert
common-mode inputs to a differential mode at the outputs with the gain described in the section.
10.1.5 Driving Capacitive Loads
The capacitive load of an ADC or some other next-stage device is commonly required to be driven. Directly
connecting a capacitive load to the output pins of a closed-loop amplifier such as the THS4561 can lead to an
unstable response; see the step response plots into a capacitive load (, , , and ). One typical remedy to this
instability is to add two small series resistors (R_O) at the outputs of the THS4561 before the capacitive load. and
illustrate parametric plots of recommended R_O values versus differential capacitor load values and gains.
Operating at higher noise gains requires lower R_O values to obtain a ±0.5-dB flat response for the same
capacitive load. Some direct parasitic loading is acceptable without a series R_O that increases with gain setting
(see , , , and where the R_O value is 0 Ω). Even when these plots suggest that a series R_O is not required, good
practice is to leave a place for the R_O elements in a board layout (a 0-Ω value initially) for later adjustment in
case the response appears unacceptable.
The rail-to-rail output stage of the THS4561 has an inductive characteristic in the open-loop output impedance at
higher frequencies; see Figure 9. This inductive open-loop output impedance introduces added phase shift at the
output pins for direct capacitive loads and feedback capacitors. Larger values of feedback capacitors (greater
than 100 pF) can risk a low phase margin. Including a 10-Ω to 15-Ω series resistor with a feedback capacitor can
be used to reduce this effect.
The TINA-TI™ simulation model does a good job of predicting these issues and illustrating the effect for different
choices of capacitive load isolating resistors (R_O) and different feedback capacitor configurations.
10.1.6 Input Overdrive Performance
illustrates a 2X overdrive triangle waveform for the THS4561. The input resistor is driven with a TBD-V swing for
the gain of 2-V/V configuration in the test circuit of Figure 1 using a single 10-V supply. When the output
maximum swing is reached at approximately the supply values, the increasing input voltage 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.
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