usually about 100kHz. The measurement is swept over the frequency range for various conditions THD+N results for the OP-275 are plotted in Figure 8.2 as a function of frequency The signal level is 3V rms, and the amplifier is connected as a unity-gain follower The data is shown for three load conditions: 600ohm. 2kohm. and 10kohm. Notice that a THD+N value of.0008% corresponds to 8ppm, or-102dBc. The input voltage noise of the OP-275 is typically 6n V/rtHz@ 1kHz, and integrated over an 80kHz noise bandwidth, yields an rms noise level of 1.7uV rms For a 3V rms signal level the corresponding signal-to-noise ratio is 125dB. Because the THd is considerably greater than the noise level, the THD component is the primary contributor. Multiple plots with variable bandwidths can be used to help separate noise and distortion THD N FOR THE OP-275 OVER 100kHz BANDWIdTH IS DOMINATED BY DISTORTION RL=600.2K,10k FREQUENCY-Hz Figure 8.2 Now, consider the AD797, a low noise amplifier(Inv/rtHz) where measurement equipment distortion, and not the amplifier distortion, limits the measurement The ID specification for the AD797 is 120dBc@ 20kHz, and a plot is shown in Figure 8.3. The distortion is at the limits of measurement of available equipment, and the actual amplifier noise is even lower by 20dB. The measurement was made with a spectrum analyzer by first filtering out the fundamental sinewave frequency ahead first five harmonics were then measured and combined in an RSS fashion to get the of the analyzer. This is to prevent overdrive distortion in the spectrum analyzer. The THD figure. The legend on the graph indicates that the measurement equipment floor"is about 120dBe; hence at frequencies below 10kHz, the THD may be even4 usually about 100kHz. The measurement is swept over the frequency range for various conditions. THD+N results for the OP-275 are plotted in Figure 8.2 as a function of frequency. The signal level is 3V rms, and the amplifier is connected as a unity-gain follower. The data is shown for three load conditions: 600ohm, 2kohm, and 10kohm. Notice that a THD+N value of 0.0008% corresponds to 8ppm, or –102dBc. The input voltage noise of the OP-275 is typically 6nV/rtHz @ 1kHz, and integrated over an 80kHz noise bandwidth, yields an rms noise level of 1.7µV rms. For a 3V rms signal level, the corresponding signal-to-noise ratio is 125dB. Because the THD is considerably greater than the noise level, the THD component is the primary contributor. Multiple plots with variable bandwidths can be used to help separate noise and distortion. THD + N FOR THE OP -275 OVER 100kHz BANDWIDTH IS DOMINATED BY DISTORTION Figure 8.2 Now, consider the AD797, a low noise amplifier (1nV/rtHz) where measurement equipment distortion, and not the amplifier distortion, limits the measurement. The THD specification for the AD797 is 120dBc @ 20kHz, and a plot is shown in Figure 8.3. The distortion is at the limits of measurement of available equipment, and the actual amplifier noise is even lower by 20dB. The measurement was made with a spectrum analyzer by first filtering out the fundamental sinewave frequency ahead of the analyzer. This is to prevent overdrive distortion in the spectrum analyzer. The first five harmonics were then measured and combined in an RSS fashion to get the THD figure. The legend on the graph indicates that the measurement equipment "floor" is about 120dBc; hence at frequencies below 10kHz, the THD may be even less