LM4562 datasheet(23/33 Pages) NSC

23 / 33 page

Application Information

DISTORTION MEASUREMENTS

The vanishingly low residual distortion produced by LM4562

is below the capabilities of all commercially available equip-

ment. 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 ex-

tends the resolution of the distortion measurement equip-

ment.

The LM4562’s low residual distortion is an input referred

internal error. As shown in Figure 1, adding the 10

Ω resistor

connected between the amplifier’s inverting and non-

inverting inputs changes the amplifier’s noise gain. The re-

sult is that the error signal (distortion) is amplified by a factor

of 101. Although the amplifier’s closed-loop gain is unal-

tered, 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

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 gener-

ated using the above described circuit connected to an Audio

Precision System Two Cascade.

201572K4

FIGURE 1. THD+N and IMD Distortion Test Circuit

LM4562 Datasheet(PDF) 23 Page - National Semiconductor (TI)