4.5.5.3 AC Signal

Task: The graph will be plotted in Data Visualizer using an AC signal without noise and the impact of the LPF mode on a given input AC signal is observed. In other words, when the frequency of a given input AC is reached to roll-off frequency, the attenuation in an input signal is observed.
  • ADC2 computation mode: LPF mode
  • Input signal: AC 1V peak-to-peak, frequency 1 Hz, and offset at 1.25V
Test setup:
  • Verify the computation mode number in the Data Visualizer graph is shown as 5 and that LED D5 is illuminated
  • Configure the Signal & Noise Generator to generate an AC signal of amplitude ~1V, frequency 1 Hz, and offset at 1.25V
  • Disable the random noise if enabled earlier
  • Verify the input signal using an oscilloscope. The expected result is as shown below

The LPF removes unwanted high frequency signals from an AC input signal. The LPF hardware operates the same way as when it filters DC signals; each new sample is added to the accumulator, averaged, and divided by 2ADCRS to get the filtered output. The difference between AC and DC signal is how the ADCRS value is used. When filtering AC signals, the ADCRS value determines the -3 dB roll-off frequency of the single-pole filter.

Table 4-5 shows the -3 dB points in terms of radians per second for each valid ADCRS value.

Note: The table with -3 dB points for each ADCRS value is available in the data sheet in the ADC2 chapter of any device containing the ADC2 module.