PRINCIPLES OF OPERATION
•
81
Chapter 8
Whole-Cell Capacitance Compensation
The SERIES RESISTANCE and WHOLE-CELL CAP. controls are used to charge
the membrane capacitance (C
m
). Figure 19 is a simplified circuit of these controls.
Figure 19.
Whole-cell capacitance compensation circuit.
Assume that the fast and slow electrode compensation controls have already been
set to compensate for C
p
. By appropriately adjusting the SERIES RESISTANCE
and WHOLE CELL CAP. controls, the current injected through C2 will supply the
transient membrane current (I
m
). These adjustments do not alter the time constant
for charging the membrane. Their function is to offload the burden of this task
from the feedback resistor, R
f
. In many cells, even a small command voltage (V
c
)
of a few tens of millivolts can require such a large current to charge the membrane
that it cannot be supplied by R
f
. The headstage output saturates for a few hundred
microseconds or a few milliseconds, thus extending the total time necessary to
charge the membrane. This saturation problem is eliminated by the appropriate
adjustment of the SERIES RESISTANCE and WHOLE CELL CAP. controls. This
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