LME49720
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SNAS393C –MARCH 2007–REVISED APRIL 2013
APPLICATION INFORMATION
DISTORTION MEASUREMENTS
The vanishingly low residual distortion produced by LME49720 is below the capabilities of all commercially
available equipment. This makes distortion measurements just slightly 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 LME49720’s low residual distortion is an input referred internal error. As shown in Figure 110, adding the
10Ω resistor connected between the amplifier’s inverting and non-inverting inputs changes the amplifier’s noise
gain. The result is that the error signal (distortion) is amplified by a factor of 101. Although the amplifier’s closed-
loop gain is unaltered, the feedback available to correct distortion errors is reduced by 101, which means that
measurement resolution increases by 101. To ensure minimum effects on distortion measurements, keep the
value of R1 low as shown in Figure 110.
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. This datasheet’s THD+N and IMD values were generated using the above described
circuit connected to an Audio Precision System Two Cascade.
R2
1000W
-
LME49720
R1
10W
Distortion Signal Gain = 1+(R2/R1)
+
Analyzer Input
Generator Output
Audio Precision
System Two
Cascade
Actual Distortion = AP Value/100
Figure 110. THD+N and IMD Distortion Test Circuit
The LME49720 is a high speed op amp with excellent phase margin and stability. Capacitive loads up to 100pF
will cause little change in the phase characteristics of the amplifiers and are therefore allowable.
Capacitive loads greater than 100pF must be isolated from the output. The most straightforward way to do this is
to put a resistor in series with the output. This resistor will also prevent excess power dissipation if the output is
accidentally shorted.
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