I need to make a test source with THD ~ 0.0001% at the frequencies below
1kHz. Leaving many details aside, the biggest problem seems to be an opamp.The typical THD vs frequency plot for a good opamp looks like this:
ADTL082
It saturates at ~0.001% at the frequency ~2 kHz. Why is that? According to the GBV = 5MHz and G = 2e5, the corner should be at 25 Hz.
Perhaps the limit is set by Audio Precision; it can't measure THD below ~0.008% in direct way. But, even if 0.001% is the artifact of the measurement, there is still not enough of open loop gain to get to
0.0001% at 1kHz. Would it be possible to make a cascaded amp to boost the open loop gain and improve distortion?Vladimir Vassilevsky DSP and Mixed Signal Design Consultant