Molecular Devices Axoclamp 900A, Руководство пользователя

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Guide to Electrophysiological Recording
Bessel Filter
This is the analog filter used for most signals for which minimum
distortion in the time domain is required. The Bessel filter does not provide
as sharp a roll
‐
off as the Butterworth filter, but it is well behaved at sharp
transitions in the signal, such as might occur at capacitance transients or
single
‐
channel current steps.
Butterworth Filter
This is the filter of choice when analyzing signals in the frequency domain,
e
.g., when making power spectra for noise analysis. The Butterworth filter
has a sharp, smooth roll
‐
off in the frequency domain, but introduces an
overshoot and “ringing” appearance to step signals in the time domain.
Choosing the Cutoff Frequency
In practice, there are two important considerations when selecting the filter
cutoff frequency.
Aliasing
If the digitizing interface samples at 2 kHz, for example, any noise in the
sampled signal that has a frequency greater than 1 kHz will appear in the
digitized trace as extra noise in the range 0 to 1 kHz. In other words,
higher
‐
frequency noise (> 1 kHz) will appear under the alias of lower
‐
frequency noise (< 1 kHz). This error is called aliasing. A fundamental
principle of signal analysis, called the Nyquist Principle, therefore states
that, in order to avoid aliasing, the digitizing frequency (f
d
) should be at
least twice the filter cutoff frequency (f
c
):
f
d
≥
2f
c
The minimum permissible digitizing frequency (exactly twice f
c
) is called
the Nyquist frequency. In practice, it is better to sample at two or more
times the Nyquist frequency. Thus, f
d
= 5f
c
is commonly used. This means
that, if the Axoclamp 900A filter is set at 5 kHz, your interface should be
capable of digitizing at 25 kHz.
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