Multichannel Systems ME-System Manual - Manual (Page 19)

Welcome to (USB-) ME-Systems
19
2.7.4 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 an 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
a 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 filter amplifier.
Example:
You have a broadband filter 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 had an 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.