128 • Reference Section
MultiClamp 700A Theory and Operation, Copyright 2000, 2001 Axon Instruments, Inc.
emerges can be completely removed by small readjustments of the setting of
the Cp Fast and Whole Cell controls. (See “Adjusting R
s
Compensation”,
above.)
Setting ‘Prediction’ and ‘Correction’ Values
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% to
avoid saturation. (See “Saturation Effects”, above.) 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 for ionic currents; this is not true in the
MultiClamp 700A. On the other hand, in some cases it might be impossible to
advance the Correction percentage beyond about 70% without causing
instability. Nevertheless, Prediction, which is inherently stable up to 98% or
more, can be set to a value substantially higher than 70% (about 95%), thereby
ensuring that the true transmembrane potential changes rapidly in response to
the command potential even though a substantial series resistance remains
uncompensated in terms of ionic currents.
Oscillations
One of the practical problems when using the % Correction function of R
s
Compensation is that there is a great risk of oscillations because the Correction
circuitry is a form of positive feedback. The main cause of oscillations is the
inability of the circuitry to distinguish between current that flows down the
electrode and into the cell from current that flows through the stray
capacitance of the electrode into the bath. The current that flows through the
electrode resistance into the cell is the current that is intended to be
compensated. The Correction circuitry also tries to compensate for the current
into the electrode capacitance. However, in this case there is no significant
series resistance component to compensate, and the Correction circuit will
oscillate as soon as the Correction control is advanced.
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