However, if we zoom in on the 440Hz sine wave we can see that it's not a smooth curve. Rather it's quite jaggy and noisy.
If the frequency is increased to 8440 the sine curve has degenerated into something very unlike a sine wave. This is because the sample rate is fixed, so as the frequency increases there are fewer samples available for a full cycle. That means fewer points on the signal, and that means a chunkier curve.
The easy answer to this is just to increase the sample rate. That, however, is limited by the speed of the underlying hardware. A more significant problem is the sample array. A higher sample rate means a bigger sample array at lower frequencies.
This is where the Feather M4 has an advantage over a Feather M0, it's faster and has more memory for more samples.