Compensation Procedures
EPC9 Manual
62
The table gives maximum
α
values (i.e.,
%-comp
settings) and the resulting
τ
c
values in the 2 µs
setting for some values of the uncompensated time
constant
τ
u
. At the 10 µs setting, full 90% com-
pensation may be used without overshoot for time
constants
τ
u
greater than about 1 ms; the 100 µs
setting is appropriate for
τ
u
values on the order of
10 ms or longer. In practice, you can estimate
τ
u
from the ratio of the settings of
C-slow
and
R-series
.
For example, if
C-slow
is 10 pF and R
s
is 10 M
Ω
, the
time constant is 10 pF · 10 M
Ω
= 100 µs.
The use of the R
s
-compensation circuitry can be summarized as follows: When you
set the capacitance transient cancellation (
C-slow
,
R-series
,
C-fast
,
τ
-fast
) to minimize
the size of the transients when voltage pulses are applied, you have also properly set
them for series resistance compensation. Then you enable
R-series
and turn up the
%-
comp
control to the desired value. Any misadjustment of the transient cancellation
will be apparent and can be compensated. The EPC9 makes the procedure very easy:
C-slow
and
R-series
values can be obtained automatically by clicking on
Auto C-slow
.
Beyond that, the pulse generator provides an option where
Auto C-slow
is performed
before each command pulse, achieving an accurate update of
R-series
. The procedure
will be described in more detail later (
Chapter
10. Using the Patch Clamp
on page 75
).
Capacitance Compensation
The EPC9 provides automatic procedures for both fast and slow capacitance
subtraction. In both cases, the ongoing pulse protocols are suspended and short
trains of square-wave pulses are applied (number and amplitude of these pulses are
specified by
C-slow Num. Cycles
and
C-slow Peak Amplitude
). The resulting capacitive
transients are averaged, leak-subtracted, and then used to calculate the required
corrections to the components of the compensation network. A detailed description
of the procedures to estimate capacitance is found in Sigworth
et. al
(1995). The
procedure for
C-fast
compensation (pipette and stray capacitance) is concerned
mainly with the very initial portion of the transient (the first 10-30 µs), whereas the
procedure for
C-slow
compensation (whole-cell capacitance) regards a somewhat
later time window. The width of this window is based on an initial guess of what the
time constant of the slow transient might be. Therefore, it is a good idea to set typical
values for
C-fast
,
τ
-fast
,
C-slow
and
R-series
in advance (they may be saved in the
macro for whole-cell recording, for example). This will provide the fitting routine
with reasonable starting values. Alternatively, the
Auto
procedures can be performed
u (µs)
c (µs)
90
0.85
13
50
0.80
10
30
0.75
8
22
0.70
7
13
0.60
5
8
0.50
4
