Aliasing due to unwanted, spurious out-of-band signals is a problem in many applications that use A/D converters. These signals, if not filtered properly, can seriously impact the performance of data acquisition systems.
If a signal is sampled at a sampling rate smaller than twice the Nyquist frequency, false lower frequency component(s) appear in the sampled data. This phenomenon is called Aliasing. Hence if the signal is not sampled often enough, information is irretrievably lost.
If a signal is sampled once per period, a DC signal, with arbitrary amplitude, is generated.
If it is sampled exactly twice per period, a triangular wave is generated (assuming that ADC is not sampling exactly at zero crossing). The frequency of this triangular wave is same as that of the original input signal.
If a 5 MHz sine wave digitized by a 6 MS/s ADC, the dotted line indicates the aliased signal recorded by the ADC. The 5 MHz frequency aliases back in the pass-band, falsely appearing as a 1 MHz sine wave.
Alias frequency is the absolute value of the difference between the frequency of the input signal and the closest integer multiple of the sampling rate.
Alias Freq. = ABS Closest Integer Multiple of Sampling Freq. – Input Freq
(where ABS = absolute value)
Real-world signals often contain frequency components that lie above the Nyquist frequency. These frequency components can be Noise, Harmonics etc. These frequencies are erroneously aliased and added to the components of the signal that are sampled accurately, producing distorted sampled data.
As it is theoretically impossible to remove the aliasing error after digitization, an analog low pass filter will be used before the ADC stage to filter out any unwanted signal frequencies. This filter is called
Anti Aliasing Filter. However the cutoff frequency of this filter has to be set in accordance to the ADC sampling rate. Advanced data acquisition boards will automatically adjust this cutoff frequency, to the Nyquist frequency of the selected sampling speed. In systems where you want to perform accurate measurements using sampled data, the sampling rate must be set high enough (about 5 to 10 times the highest frequency component in the signal). This oversampling will also compensate for the practical roll-off characteristics of anti-aliasing low pass filter.
All data acquisition products designed by VTI Instruments deliver superior anti-aliasing performance. Specifically, devices are designed with analog trifilar filters at their front end as well as with flexible, programmable digital filters built into their firmware. For examples of products that deliver this level of performance in data acquisition applications see the EX1629, EX10XXA, EMX-xxxx, and the VT1432B family product pages.