1.7 Aliasing

When sampling a signal, there is a limit to the visualized input frequency. The Nyquist-Shannon Sampling Theorem states that at the sampling frequency, the highest frequency seen in the input is at half of the sampling frequency. After this point, higher frequency components or signals will appear as lower-frequency aliases of the original. Implement an analog low-pass filter before the ADC to prevent aliasing, which usually appears as noise in the measurements.

There are many ways to implement a low-pass filter – the simplest way is to use an RC network, as shown in Figure 1-10.

Figure 1-10. Simple RC Low-Pass Filter

The RC network has a -3 dB point set by the following formula (excluding RLIM):

f − 3 d B ⁢ =   1 2 π ( Σ R ) C
Note: Resistor RLIM will only be used if the capacitor (C1) is very large. In this case, a fully charged capacitor can discharge current into the microcontroller on power-down through the ESD diodes. Adding RLIM will increase the source impedance. Using an integrated op amp to buffer the input allows RLIM to be sizable, but some noise will be added from the op amp into the signal.

When selecting R1 and C1 for the filter, it is of importance to keep the following in mind:

  • Small resistances reduce source impedance for faster signal acquisitions
  • Capacitors must be much larger than parasitic capacitance but not excessively large