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PRINCIPLES OF OPERATION
Axopatch 200B, Copyright 1997-1999, Axon Instruments, Inc.
In TRACK mode, V
p
is continuously adjusted to keep I = 0 (or near zero) even though the
pipette offset may be changing at a fairly rapid rate. TRACK mode is most often used
during seal formation to stop the I trace from jumping into saturation.
I is severely distorted during TRACK mode; the effect is similar to AC coupling.
Note:
The Axopatch 200B should
never
be left in TRACK mode once data is being
recorded.
The rate at which TRACK returns I to zero depends on the pipette resistance. If you are
applying a test pulse, you will find that for pipette resistance of 10 MΩ or lower you will
see an obvious droop.
After a seal is formed the TRACK circuit becomes very slow in its efforts to keep I = 0.
Leak Subtraction
The passive membrane response to a voltage step consists typically of a transient and a
steady-state component. It is often helpful to subtract these from the output so that only
active responses are observed.
The transient component is eliminated by using the CAPACITANCE COMPENSATION
controls as discussed in the Capacitance Compensation section above. The steady-state
component is eliminated by the LEAK SUBTRACTION control. This circuit simply
subtracts a scaled version of the command voltage from the current. The Adjustment of
Leak Subtraction section in the
TUTORIAL
describes the use of this control.
Since both the CAPACITANCE COMPENSATION and LEAK SUBTRACTION controls
are driven by the command voltage, the passive responses remain eliminated for all
polarities and magnitudes of command.
Please note that the setting of the LEAK SUBTRACTION control is to be multiplied by
β = 1 or 0.1.
LEAK SUBTRACTION is disabled in Current Clamp and Track modes.
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