Axon Axopatch 200B, Theory Of Operation

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82 • PRINCIPLES OF OPERATION
Axopatch 200B, Copyright 1997-1999, Axon Instruments, Inc.
adjustment is particularly important during series resistance correction (see Series
Resistance section in
PRINCIPLES OF OPERATION
) since it increases the
current-passing demands on R
f
. By moving the pathway for charging the
membrane capacitance from R
f
to C2, the SERIES RESISTANCE circuitry can
operate without causing the headstage input to saturate. The value of C2 is 1 pF.
The effect of transferring the current-passing burden from R
f
to C2 is illustrated in
Figure 20.
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V
C
m
V
I
Rf
C2
I
Figure 20. Using the injection capacitor to charge the membrane capacitance.
After perfect whole-cell compensation is applied, the current to charge the
membrane capacitor is removed from the I
Rf
trace and only the steady state current
remains. All of the transient current appears in the I
C2
trace. (The I
C2
trace in the
figure was recorded using an oscilloscope probe connected to the internal
circuitry). The I and V
m
outputs on the Axopatch 200B show the I
Rf
and V
c
trace
illustrated in Figure 20. It is easy to mistakenly think that the time course for
charging the membrane is very fast but this is clearly not the case. Use of an
independent electrode in the cell would show that the cell charging rate is not
affected by these adjustments.