LME49724 Datasheet, PDF (18/30 Pages) National Semiconductor (TI) – High Performance, High Fidelity, Fully-Differential Audio Operational Amplifier

LME49724
SNAS438A – NOVEMBER 2008 – REVISED APRIL 2013
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* Value is application and converted dependent.
Figure 53. Typical Analog-to-Digital Converter Circuit
DISTORTION MEASUREMENTS
The vanishing low residual distortion produced by the LME49724 is below the capabilities of commercially
available equipment. This makes distortion measurements more difficult than simply connecting a distortion
meter to the amplifier’s inputs and outputs. The solution, however, is quite simple: an additional resistor. Adding
this resistor extends the resolution of the distortion measurement equipment.
The LME49724’s low residual distortion is an input referred internal error. As shown in Figure 54, adding a
resistor connected between the amplifier’s inputs changes the amplifier’s noise gain. The result is that the error
signal (distortion) is increased. Although the amplifier’s closed-loop gain is unaltered, the feedback available to
correct distortion errors is reduced, which means that measurement resolution increases. To ensure minimum
effects on distortion measurements, keep the value of R₅ low. The distortion reading on the audio analyzer must
be divided by a factor of (R₃ + R₄)/R₅, where R₁ = R₂ and R₃ = R₄, to get the actual measured distortion of the
device under test. The values used for the LME49724 measurements were R₁, R₂, R₃, R₄ = 1kΩ and R₅ = 20Ω.
This technique is verified by duplicating the measurements with high closed-loop gain and/or making the
measurements at high frequencies. Doing so produces distortion components that are within the measurement
equipment’s capabilities.
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