Here's an experiment that I would find interesting, so I'm suggesting it.
- Put opamp in unity gain buffer configuration and ground the input, so it outputs 0V.
- Connect output of opamp to output of audio source via suitable resistor, for example 1k.
- Play signal and measure distortion at the output of the opamp.
This will show the distortion added by the opamp's output stage due to load current.
You can also measure intermodulation by using two outputs from your analyzer at different frequencies, and feeding them into the opamp's output via two resistors. Or you can send one signal in the input, and one in the output via a resistor.
Since the output is a virtual ground, there is very little actual signal there, so the distortion can't hide behind it, and it is much easier to measure. Likewise, the ADC will not contribute its own distortion, since it is only acquiring the very small voltage at the output of the opamp, not the full signal amplitude. In the case of intermodulation, the ADC will see the full amplitude of one frequency but not the other, so it does not contribute its own IMD.
This will not show distortion due to other mechanisms, like input stage nonlinearity due to common mode. Only output.
Another interesting experiment is to measure distortion in the same way, with a small DC current pulled from the output of the opamp. You can vary the current, and the distortion profile will change depending on where the offset current shifts the signal on the output stage crossover of the opamp.
- Put opamp in unity gain buffer configuration and ground the input, so it outputs 0V.
- Connect output of opamp to output of audio source via suitable resistor, for example 1k.
- Play signal and measure distortion at the output of the opamp.
This will show the distortion added by the opamp's output stage due to load current.
You can also measure intermodulation by using two outputs from your analyzer at different frequencies, and feeding them into the opamp's output via two resistors. Or you can send one signal in the input, and one in the output via a resistor.
Since the output is a virtual ground, there is very little actual signal there, so the distortion can't hide behind it, and it is much easier to measure. Likewise, the ADC will not contribute its own distortion, since it is only acquiring the very small voltage at the output of the opamp, not the full signal amplitude. In the case of intermodulation, the ADC will see the full amplitude of one frequency but not the other, so it does not contribute its own IMD.
This will not show distortion due to other mechanisms, like input stage nonlinearity due to common mode. Only output.
Another interesting experiment is to measure distortion in the same way, with a small DC current pulled from the output of the opamp. You can vary the current, and the distortion profile will change depending on where the offset current shifts the signal on the output stage crossover of the opamp.