Axon Axopatch 200B, Theory Of Operation

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22 • USE OF THE PATCH CLAMP – A TUTORIAL
Axopatch 200B, Copyright 1997-1999, Axon Instruments, Inc.
Detailed Method For Setting Series Resistance Compensation
New patch clamp users who are not yet skilled in setting all of the capacity compensation
controls may find it impossible to achieve the exceptional series resistance compensation
shown above. We, therefore, present here a detailed description of a method for setting the
controls that ensures outstanding series resistance compensation. In this approach,
PREDICTION and CORRECTION are set sequentially rather than concurrently as in the
above section. Minor readjustments are then done to achieve the final result. We also
provide the user with a detailed explanation of the series-resistance circuit operation.
If you are satisfied with your ability to set all of the capacity compensation controls, you
can skip the following indented sections and resume at the
Current Clamp (Model Cell)
section.
Note that PREDICTION is an open loop process, i.e. , it does not involve feedback, and
instability is only possible if the internal circuitry that develops the prediction signals is pushed
too far. Generally, the circuit is stable up to values of about 98%, but it can become non-linear,
depending on the magnitude of V
STEP
(Figure 4). If the PREDICTION potentiometer has been
advanced too far it may not be noticeable in 5 kHz bandwidth until the current begins to
oscillate. However, in 100 kHz bandwidth you will observe ringing developing in the transient
as the PREDICTION percentage becomes too large. For best results you should start with
whole-cell transients canceled as in Figure 6a (same as Figure 3d). To begin, advance the
PREDICTION potentiometer gradually up to a final value of 95 or 96%. A brief transient will
emerge at the leading and trailing edge of the command potential step. The amplitude of this
transient is typically about 500 pA
p-p
1
(in 5 kHz bandwidth) for a 40 mV step using the cell
model (Figure 6b). The residual transient at this stage is typically biphasic, with a total duration
of about 200 µs in a bandwidth of 5 kHz, but it can be eliminated.
To eliminate the residual transient that has resulted from the use of PREDICTION, small
readjustments in the settings of the PIPETTE CAPACITANCE COMPENSATION FAST MAG
control, the WHOLE CELL CAP., and SERIES RESISTANCE settings are needed. To begin,
slightly reduce the setting of the FAST MAG control in PIPETTE CAPACITANCE
COMPENSATION. The amplitude of the initial negative-going component of the residual
transient will decrease. Continue to change the control setting until the waveform of the residual
transient changes to a monophasic positive going response. At the point where the negative
1
pA
p-p
- pA peak-to-peak