Reference Section • 127
Chapter 5
that 9 M of series resistance is compensated; the residual (uncompensated)
series resistance in terms of Correction, R
src
, is 1 M.
The Bandwidth setting under R
s
Compensation gives the –3 dB cutoff
frequency of a one-pole RC filter through which the Correction signal is
passed prior to being summed with V
c
. The Bandwidth is used to ensure
stability when large amounts of Correction are used. It is generally good
practice to begin using Correction with the Bandwidth set at 10 kHz or less.
However, once the desired level of Correction has been achieved, it is usually
possible (if desired) to significantly increase the Bandwidth setting; 30 kHz is
usually quite achievable for 90% Correction.
Continuing with the example considered above (R
s
= 10 M, C
m
= 50 pF), a
90% Correction setting will reduce voltage errors in the true membrane
potential resulting from the flow of ionic current to 10% of the error present
with 0% Correction. For example, a 2 nA ionic current would produce a
20 mV error in V
m
with 0% Correction, whereas 90% Correction will reduce
this error to only 2 mV. At the same time, the use of Correction will reduce
the filtering effect of R
s
and C
m
on the measured current. With 0% Correction
the actual bandwidth of current measurement prior to any output filtering is
limited to 1/2πR
s
C
m
, which will be about 320 Hz in this example. As %
Correction is increased this “filter” changes to 1/2πR
src
C
m
, so that for 90%
Correction the possible bandwidth for current measurement is increased to
3.2 kHz in this example. With 95% Correction the possible bandwidth is
increased to 6.4 kHz and with 98% it is further increased to 16 kHz (although
the effects of the Bandwidth value should not be forgotten).
Readjustment of Whole Cell Compensation with ‘Correction’
If the capacity transient has been canceled prior to the use of Correction (and
for now assume that Prediction has already been set at 95%) then, in principle,
there is no capacity current to feed back when Correction is utilized. Note that
the discussion here of capacity current should be distinguished from the
discussions of the ionic current. Therefore, no transient should develop as
Correction is advanced. In practice, however, a small transient will emerge as
% Correction is increased. Again, this is due to non-ideal characteristics of the
circuitry. As in the case of ‘Prediction’, the small residual transient that
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