Multichannel Systems Portable_ME-System Manual - Manual (Page 24)

Portable ME-System Manual
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6.2 Integrated Data Acquisition
Analog input signals are acquired from the data source and digitized by the 16- or 32-channel analog-
digital converter that is integrated into the main unit. Recorded signals are converted in real-time into
digital data streams at sampling rates of up to 50 kHz per channel. You will not miss even the fastest
biological signals. Data is transferred to the computer via High Speed USB 2.0 port.
A 16-bit digital (TTL) input / output channel is available. You can use the digital TTL inputs, for example,
for synchronizing stimulation and recording, or for synchronizing the Potable ME-System with other
systems, video tracking, for example. The digital TTL outputs can be used for triggering other systems
and instruments, for example, for applying a feedback.
The voltage input range of the DAQ is +/- 4 V. with the standard gain of 1000, this means that the signals
can be up to +/- 4 mV in size before getting clipped. Please refer to the MC_Rack help or manual for more
information.
DC Offset Correction
An offset correction is generally not necessary, because the intrinsic DC offsets of the Portable ME-System
is very low in comparison to the signals of interest. You can use the MC_Rack offset correction feature to
remove even this low offset and reset all channels to zero. Please refer to the MC_Rack help or manual for
more information.
Note: If you observe a large offset on any channel(s), you should contact your local retailer for
troubleshooting. The offset correction is not intended for removing large offsets, because the offset
correction will decrease the input voltage range.
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. For example, this means approximately 15 kHz for spike recording, or 10 kHz
for cardiac signals (which have fast and slow waveform components).
Please note that the sampling rate also depends on the amplifier’s bandwidth. According to the Nyquist-
Shannon sampling theorem, the sampling rate should be at least twice the bandwidth of the analog
(hardware) low pass filter, that is 10 kHz sampling rate when using a filter amplifier with a cutoff frequency
of 5 kHz. The 1/2 bandwidth frequency is also called Nyquist frequency. You should not lower the sampling
rate when using a low-pass digital filter. This is the case because the whole amplifier bandwidth is recorded
and then high frequency noise is removed with the 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. The resulting low-frequency noise passes the
digital low pass filter and increases your noise level. You may ignore these recommendations if saving hard
disk space is more important for your application than the noise level.
Note: The sampling frequency should be at least five times the highest signal frequency and at least twice
the bandwidth of the filter amplifier.