Multichannel Systems Portable_ME-System Manual - Manual (Page 23)

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6 System Components
6.1 Integrated Filter Amplifiers (FA)
Raw data from the preamplifiers is amplified by a 16- or 32-channel filter amplifier with fixed gain and
bandwidth.
A differential amplifier with input type I includes a common ground and a common reference electrode
input. The reference electrode is ideally identical to the recording electrodes and placed into a comparable
but inactive area or tissue. Background or noise signals that are picked up by both the reference electrode
and the recording electrodes are removed.
The reference electrode input also compensates the potential difference between the miniature preamplifier
(MPA8I) output and the filter amplifier (FA) input that results from the supply voltage drop in the line from
the MPA8I to the FA. If two or more MPA8Is are connected to the FA, the amplifier offsets are averaged for
compensation.
Important: It is very important that the reference electrode inputs are always connected. Connect it
to ground when you are not using a reference electrode.
Signal Amplification and Filters
The filter amplifier combines a band pass filter and the signal amplification in one instrument.
The broad bandwidth of 1 to 5000 Hz is suitable for a broad range of applications, such as spike and field
potential recording from neurons or recording of cardiac signals. The digital filter of the MC_Rack program
can be used to adjust the pass band and filter the raw data. Please see the MC_Rack help or manual for
more information. This way, you are very flexible in designing your experiments. Please note that you may
need a higher sampling rate to avoid aliasing. See also the chapter "Integrated Data Acquisition" for
more information. For slow signals like field potentials, a bandwidth of 1 to 300 Hz is appropriate. If you
like to record fast signals like spikes, a pass band of 300 Hz to 3 kHz is suitable. Cardiac signals have fast
and slow components; therefore, you usually need a wider bandwidth of 1 Hz to 3 kHz.
Please note that the gain of the filter amplifier is a fixed hardware property; and that you cannot change
the gain of the amplifier by software controls. Please also note that the ratio of the output signal to the
input signal, that is, the gain, is not a fixed parameter for the complete bandwidth. The gain that was
specified for the amplifier, for example, 100 (1000 in total with the preamplifier), is not fully reached at the
borders of the amplifier's pass band. The general rule is, that at the lower and upper limit of the frequency
band, the gain is, that is approximately 70 %, of the full gain. Therefore, you should use a bandwidth that
is at a safe distance of the signals of interest. Outside the pass band, the gain decreases with the frequency
and finally approaches zero.
For more information on gain and filters in general, please refer to standard literature or contact your local
retailer.