MAX32600's internal circuitry as well as the uncommitted op-amp contribute noise in the system. What you see in the spectrum analyzer when probing the output of the voltage buffer is the sum of these two noise sources. Noise for an op-amp is specified with a graph of equivalent input noise versus frequency. Assuming an ideal DAC, the total harmonic distortion is directly related to the resolution of the DAC. The resolution also determines the dynamic range and hence the SNR. In addition to the error contributed just from the resolution of the DAC, there is the offset error, gain error and integral nonlinearity. There is also a list of AC errors that contribute to the noise at the output of the DAC. Poor power supply rejection can cause spurs and harmonic distortion.
Noise is usually measured with the input of the DAC set to zero.
If you are interested in measuring noise performance of signals at around a certain bias point (DC measurements), then only the p-channel input is recommended for better THD performance. Please see the datasheet to find out at what bias voltages is the p-channel recommended.
You can directly access the DAC output by configuring the AFE to output the signal directly to the available external pins. However, because of the output impedance of the DAC, you will still need to buffer the DAC output to interface with other circuitry (to provide proper signal amplitude, offset, etc.). The DAC output may also be filtered to remove unwanted image frequencies caused by the sampling process. Depending on the type of load (DC/AC) attached to the DAC output, certain filter topologies will be more suitable for your application.
The DC accuracy of the DAC with an internal reference can often be improved by overriding the internal reference with a more accurate and stable external one. As mentioned above, filtering of the DAC output may be implemented to obtain less noise for a frequency of interest.