74 • Reference Section
MultiClamp 700A Theory and Operation, Copyright 2000, 2001 Axon Instruments, Inc.
fixing the voltage drop across C
in1
to a constant value, thereby preventing
current flow through C
in1
. The effective value of C
in1
is thus reduced to well
below its real value. This eliminates the high-frequency current loss through
the power supply capacitance, thereby increasing the bandwidth. Since the
power supply capacitance is present whether or not the power supply is
bootstrapped, there is no noise penalty due to implementing this technique.
Secondary Method for Neutralizing Cin
In some cases the steps discussed above may not be sufficient to decrease the
RC time constant of the voltage-recording microelectrode, particularly in
situations where high resistance microelectrodes must be used. For this reason
an effective, though less desirable, technique is provided that can electrically
reduce the effective magnitude of C
in2
. The technique is known as
“capacitance compensation”, “negative capacitance” or “capacitance
neutralization”. A compensation amplifier at the output of the unity gain
buffer drives a current injection capacitor connected to the input. At the ideal
setting of the compensation-amplifier gain, the current injected by the injection
capacitor is exactly equal to the current that passes through C
in2
to ground.
Adjusting Capacitance Neutralization
Check the Tuning checkbox and choose amplitude (nA) and frequency (Hz) parameters
that result in a sawtooth pattern of about 10 mV amplitude on “Scaled Output:
Membrane Potential”. Carefully increase the Pipette Capacitance Neutralization value
until overshoot just starts to appear in the step response. This is easiest to see if you
have already adjusted Bridge Balance. (See Chapter 5,
BRIDGE BALANCE.) If you
go too far the overshoot may become a damped oscillation, which may escalate into a
continuous oscillation, killing the cell.
Sometimes the overshoot is difficult to see. In this case, you may prefer to look at the
“Scaled Output: Membrane Potential” trace at high gain on an oscilloscope, advancing
the Pipette Capacitance Neutralization value until the trace becomes noisy and
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