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Reference Section
MultiClamp 700A Theory and Operation, Copyright 2000, 2001 Axon Instruments, Inc.
instantly jump to its maximum value following a step voltage command. In fact, due
to limitations in the speed of the Prediction circuitry, this over-estimates the maximum
value of V
cp
, particularly when % Prediction is large. In actual practice, Prediction can
typically be set to about 94% for a 100 mV command step.
Readjustment of Whole Cell Compensation with ‘Prediction’
As the Prediction potentiometer is advanced, the signal applied to the
headstage capacitor C2 is modified appropriately so that it will continue to
cancel the whole-cell capacity transient despite the fact that the speed of this
transient has increased. This is simply accomplished by reducing the time
constant of this signal as % Prediction is increased. If the circuitry worked
perfectly, and if the whole-cell capacity transient had been perfectly canceled
with 0% Prediction, no transient would appear as % Prediction is increased up
to the maximum allowable values. However, due to the complexity of this
circuitry and a variety of non-ideal characteristics, cancellation of whole-cell
capacity transients does not remain perfect as % Prediction is increased. The
small residual transient that emerges can, however, be completely removed by
small readjustments of the setting of the Cp Fast and Whole Cell controls.
(See “Adjusting R
s
Compensation”, above.)
It should be noted that Prediction would work for any command waveform,
not just steps. This may be useful for capacitance measurements using phase
sensitive techniques or lock-in amplifiers.
The ‘Correction’ Control
Although Prediction can greatly speed the response time of the true membrane
potential with respect to the command potential and, thus, overcome one
important effect of series resistance, it does not correct for the effects of series
resistance associated with the flow of membrane ionic current (
i.e.
IR drops
and filtering effects described above). This is the role of the % Correction
value. Correction feeds back a portion of the measured membrane current; this
signal is added to the command potential. The percentage set by the
Correction potentiometer refers to the R
s
(M
Ω
) value under Whole Cell. For
example, if this value is 10 M
Ω
, a 90% setting of the Correction control means
