Both measured voltages are visible as two spikes in the histograms. Figure 8 shows the histograms of the five tests: AnalogRead function, free running at ÷128, ÷64, ÷32 and ÷16. The measured values of the first voltage (ADC value 511) doesn't deviate from the previous noise test. So the measurement are still accurate. The surrounding bins are very small what means that the noise level didn't increase.