PRINCIPLES OF OPERATION
•
89
Chapter 8
signal at 32 kHz; 10 µs corresponds to 16 kHz, 20 µs corresponds to 8 kHz, etc. The LAG
control is used to ensure stability when large amounts of CORRECTION are used. It is
generally good practice to begin using CORRECTION with the LAG control set at
10-20 µs or more. However, once the desired level of CORRECTION has been achieved, it
is usually possible (if desired) to significantly reduce the LAG setting; 5 µs is usually quite
adequate for 90% CORRECTION.
Continuing with the example considered above, i.e., a cell with R
s
= 10 MΩ and 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" moves 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 LAG should not be forgotten).
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. Procedures for eliminating this transient by
minor readjustments of SERIES RESISTANCE, WHOLE CELL CAP., FAST MAG and
FAST τ controls are described in detail in the
TUTORIAL
.
There are many situations in which it will be desirable to have the % PREDICTION and
the % CORRECTION controls set at different values. For example, for a 200 mV step
command PREDICTION should be limited to about 80% (see Figure 4; however, somewhat
higher values can often be used) to avoid saturation. However, it is usually possible to
compensate series resistance up to 90 to 95% or more by use of the CORRECTION control.
In other patch clamps the issue of saturation would limit the amount of compensation used
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