Multichannel Systems MEA-System Manual - Manual (Page 17)

Appendix
17
Sampling rate
It is recommended to adjust the sampling rate according to your signals, because the higher
the sampling rate, the bigger the file size will be.
As a rule of thumb, the sampling rate should equal five times the highest signal frequency for
a good digitized representation of the continuous analog signals. If the sampling rate is too low,
you will miss signals and / or see artifacts. Considering the preceding statements about filtering
data, you would for example use a 5 kHz sampling rate when using a MEA amplifier with a cutoff
frequency of 1 kHz.
Please note that if you use a broadband amplifier and a digital filter, you may have to use an even
higher sampling rate.
This is the case because the whole amplifier bandwidth is recorded and then high frequency noise
is removed with a digital low pass filter after recording. Frequencies (noise) that are above half
the sampling rate (for example above 2.5 kHz at a 5 kHz sampling rate) will be transformed into
lower frequencies. This is called aliasing. This low frequency noise passes the digital “Low Pass”
filter and increases your noise level.
According to the Nyquist-Shannon sampling theorem, the sampling rate should equal twice the
bandwidth of the analog (hardware) low pass filter. The 1/2 bandwidth frequency is also called
Nyquist frequency. You may ignore this if saving hard disk space is more important for your
application than the noise level.
Aliasing
Note: The sampling frequency should be at least five times the highest signal frequency and
at least twice the bandwidth of the MEA amplifier.
Example:
You have a broadband MEA1060 amplifier with a bandwidth of 0.1 Hz to 10 kHz. The expected
signals have a maximum frequency of 1 kHz. Therefore, you want to filter the data with a digital
Low Pass filter and a cutoff frequency of 1 kHz. A sampling rate of 5 kHz (five times the highest
signal frequency) would be required for faithfully reproducing the signals, but you should use a
sampling rate of at least 20 kHz because the sampling rate should equal twice the bandwidth of
the analog filter, regardless of the digital filter properties.
If you have a MEA1060 amplifier with a cutoff frequency of 1 kHz instead, no digital filter would
be required, and a sampling rate of 5 kHz would be enough.