DISTORTION MEASUREMENTS
the error signal (distortion) is amplified by a factor of 101. Al-
though the amplifier’s closed-loop gain is unaltered, the feed-
back 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 8.
The vanishingly low residual distortion produced by
LME49990 is below the capabilities of all commercially avail-
able equipment. This makes distortion measurements just
slightly more difficult than simply connecting a distortion me-
ter to the amplifier’s inputs and outputs. The solution, how-
ever, is quite simple: an additional resistor. Adding this
resistor extends the resolution of the distortion measurement
equipment.
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 compo-
nents that are within the measurement equipment’s capabili-
ties. This datasheet’s THD+N and IMD values were generat-
ed using the above described circuit connected to an Audio
Precision System Two Cascade.
The LME49990’s low residual distortion is an input referred
internal error. As shown in Figure 8, adding the 10Ω resistor
connected between the amplifier’s inverting and non-inverting
inputs changes the amplifier’s noise gain. The result is that
30059707
FIGURE 8: THD+N and IMD Distortion Test Circuit
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