118 • Reference Section
MultiClamp 700A Theory and Operation, Copyright 2000, 2001 Axon Instruments, Inc.
This time constant is 330 µs for the model cell provided with the MultiClamp
700A (R
s
= 10 M, C
m
= 33 pF). This means that the actual membrane potential
response to a step voltage command will have a 10-90% risetime of more than
0.7 ms and will not settle to within 1% of its final value until about 1.5 ms after the
start of a step command. If you are interested in fast membrane currents, like
sodium currents, this slow relaxation of the voltage clamp is unacceptable.
3. Bandwidth errors. The R
s
appears in parallel with the membrane capacitance, C
m
,
of the cell. Together they form a one-pole RC filter with a –3 dB cutoff frequency
given by 1/2πR
s
C
m
. This filter will distort currents regardless of their amplitude.
For the parameters of the model cell, this filter restricts true measurement
bandwidth to 480 Hz without R
s
compensation.
Fortunately, electronic techniques have been developed to partially correct for the
errors caused by series resistance. In V-Clamp mode, the techniques are generally
referred to as R
s
Compensation.
Series resistance errors can also occur in I-Clamp mode. These errors are generally
corrected using the techniques of Bridge Balance and Capacitance Neutralization. (See
these entries in Chapter 5.)
Is R
s
Compensation Necessary?
Before embarking on R
s
compensation, it is worth examining whether it is really
necessary in your application. The size of R
s
can be estimated by selecting the Whole
Cell checkbox in the MultiClamp Commander and pressing the Auto button to
compensate the whole-cell capacitance. (See Chapter 5,
CAPACITANCE
COMPENSATION
.) The estimated R
s
is the M value displayed to the right of the
manual adjust button under Whole Cell. If R
s
= 10 M and the maximum membrane
current you anticipate is 100 pA, the steady-state voltage error will be at most 10 M x
100 pA = 1 mV which is probably insignificant. In this case you might think that R
s
compensation is not necessary.
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