Figure 9-12 shows the THD+N performance with 100-kΩ and 300-Ω loads, and with A-weighting and with no weighting. Both loads show similar performance. With no weighting, the THD+N performance is dominated by the noise for both loads. A-weighting provides filtering that improves the noise, revealing the increased distortion with RL = 300 Ω.

Figure 9-13 and Figure 9-14 show the FFT output with a 1-kHz tone and 100-kΩ and 300-Ω loads. To show relative performance of the device versus the test set, one channel has the OPA836 device in-line between the generator output and the analyzer. The other channel is in “Gen Mon” loopback mode, which internally connects the signal generator to the analyzer input. With 100-kΩ load, Figure 9-13, the curves are indistinguishable from each other except for noise, which means the OPA836 device cannot be directly measured. With 300-Ω load, as shown in Figure 9-14, the main difference between the curves is that the OPA836 device shows slightly higher even-order harmonics, but the performance of the test set masks the odd-order harmonics.

Figure 9-12 OPA836 1 V_RMS 20-Hz to 80-kHz THD+N

Figure 9-14 OPA836 and AP Gen Mon 10-kHz FFT Plot; V_OUT = 1 V_RMS, R_L = 300 Ω

Figure 9-13 OPA836 and AP Gen Mon 10-kHz FFT Plot; V_OUT = 1 V_RMS, R_L = 100 kΩ