86
•
Reference Section
MultiClamp 700A Theory and Operation, Copyright 2000, 2001 Axon Instruments, Inc.
Risetime
The risetime is typically given as the time taken for a signal to increase from
10% to 90% of its peak value. The more heavily a step response is filtered, the
greater the 10-90% risetime. For the 4-pole Bessel filter in the MultiClamp
700A, the filtered 10-90% risetime (T
r,
in ms) of a step input depends on f
c
(in
kHz) approximately as:
T
r
≈
0.35/fc
(This can be measured by applying Seal Test to the model BATH in V-Clamp
mode and looking at “Scaled Output: Membrane Current” while changing the
filter setting.) Suppose you are interested in measuring action potentials, for
which you expect the 10-90% risetime to be about 0.4 ms. You would then
choose the filter cutoff frequency to be high enough that the filter risetime is
about ten times faster than 0.4 ms so the action potentials are minimally
distorted by the filter. According to the above equation, then, the appropriate
filter setting would be 10 kHz. In practice, you may need to make other
compromises. For example, if the signal is very noisy you may wish to filter
more heavily and accept that the action potential risetime is artifactually
slowed.
High-pass Filter
The Scaled Output and Scope signals can be high-pass filtered by setting the AC value
in the Output Gains and Filters section of the main MultiClamp Commander panel.
This is typically done in order to remove a DC component of the signal. When the
filter cutoff is set to DC this high-pass filter is bypassed.
Command Filter Frequency
Command stimuli applied in V-Clamp or I-Clamp can be filtered at different cutoff
frequencies, selectable in the General tab under the Options button. You might wish to
do this in order to smooth out sharp transitions in the command signal that, if
unfiltered, might produce very large capacitance transients that saturate the headstage
circuitry, even after capacitance compensation. (See Chapter 5,
EXTERNAL
COMMAND INPUTS
.)
