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Figure 16.20: The LM4562 in voltage-follower mode, showing CM distortion versus signal voltage­ with varying extra source resistance. The frequency is 10 kHz. A non-linearity is kicking in at about 4 Vrms. 1 to 10 Vrms out, ±18 V supply rails.

resistor rather than a direct connection. This is of course not ideal if you are seeking the best possible noise performance.

The LME49990 Opamp

The LME49990 from National Semiconductor is a single opamp available only as an 8-lead narrow body SOIC surface-mount package. It was released in early 2010. It is part of their “Overture” series, which the data sheet describes as an “ultra-low distortion, low noise, high slew rate operational amplifier series optimized and fully specified for high performance, high fidelity applications”, and from my measurements on the LME49990 I’ll go along with that. The Overture series also includes the LME49880, which is a dual JFET-input opamp. The LM49710 is another BJT opamp with very low noise and distortion specs, but for unknowable reasons it does not appear to be part of the Overture series.

Figure 16.21 shows the distortion performance in the shunt-feedback configuration, which prevents any common-mode distortion issues. The input and feedback resistors are 1 kΩ and 2k2, giving a gain

490  Opamps for Active Crossovers

Figure 16.21: The LME49990 in shunt-feedback mode, with a 1 kΩ input resistor and a 2k2 feedback resistor giving a gain of 2.2x. Shown for no load (NL) and 1 kΩ, 500 Ω loads. The generator output is also plotted. Note the top of the vertical scale is at only 0.001%. The output level is 9 Vrms, with ±17 V supply rails.

of 2.2 times (and a noise gain of 3.2 times, as for the series version of this test). The traces are for no load (apart from the feedback resistor) and 1 kΩ, and 500 Ω loads at an output of 9 Vrms, and also the AP 2722 output for reference. As you can see, these traces are pretty much piled up on top of each other, with no distortion visible on the residual except for a small amount between 10 kHz and 20 kHz with the 500 Ω load; clearly the LME49990 is very good at driving 500 Ω loads. The step at 20 kHz is an artefact of the Audio Precision SYS-2702 measuring system.

If we compare this plot with Figure 16.5, we can see that the LME49990 is somewhat superior to the 5532, but since we are down in the noise floor most of the time, the differences are not great.

In the series configuration, with 1 kΩ and 2k2 feedback resistors giving a gain of 3.2 times and a significant common-mode mode voltage of 3 Vrms, things are not quite so linear; there is now clearly detectable distortion at high frequencies, as shown in Figure 16.22. Even so, the distortion is less than half that of a 5532 in the same situation—compare Figure 16.6.

Opamps for Active Crossovers  491

Figure 16.22: The LM4562 in series-feedback mode, with 1 kΩ, 2k2 feedback resistors giving a gain of 3.2x. No load, 1 kΩ, and 500 Ω loads. 9 Vrms output; ±17 V supply rails.

Common-Mode Distortion in the LME49990

It looks as though common-mode distortion may be more of an issue with the LME49990 than it was with the LM4562.As we saw earlier, BJT input opamps do not show common-mode distortion unless the configuration has both a significant common-mode voltage and a significant source impedance.

If we repeat the series-feedback test with no external load, but increasing source resistance, we get Figure 16.23, where as usual more source resistance means more high-frequency distortion. The exception is for a 1 kΩ source resistance, where the distortion actually decreases; this is because the 1 kΩ is partially cancelling the 688 Ω source resistance of the feedback network. We saw exactly the same effect with the 5532; see Figure 16.10. The horizontal low-frequency parts of the traces are raised by the Johnson noise from the added source resistances and also by the opamp current noise flowing in those resistances. There is no distortion visible in this region.

The voltage-follower configuration has the worst working conditions for CM distortion because there is no amplification, and so the CM voltage on the inputs is as large as the output voltage. Figure 16.24 shows that in this case the CM distortion is much worse. With a 2 kΩ source resistance, the THD at 10 kHz has increased from 0.0015 % to 0.0042 %, and all the other figures show a similar increase. The conclusion has to be that if you are working with a large CM voltage and a significant source