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