Reference Section • 73
Chapter 5
Capacitance Neutralization
• Used to partially cancel the effects of microelectrode capacitance in I-Clamp
mode.
• This control is adjusted in the
field in the I-
Clamp pane.
• See also Bridge Balance.
Input Capacitance
The capacitance (C
in
) at the input of the headstage amplifier is due to the capacitance
of the amplifier input itself (C
in1
) plus the capacitance to ground of the microelectrode
and any connecting lead (C
in2
). C
in
combined with the microelectrode resistance (R
e
)
acts as a low-pass filter for signals recorded at the tip of the microelectrode. For
optimal performance at high frequencies this RC time constant must be made as small
as possible.
Two techniques may be used to increase the recording bandwidth.
• Use microelectrodes with the lowest possible resistance compatible with stable
recording, and take steps to minimize the contribution to C
in
by the capacitance of
the microelectrode. In practice, this means using patch electrodes where possible,
or using sharp microelectrodes with minimal capacitance. (See Chapter 4,
SHARP
MICROELECTRODE RECORDING
).
• Electronically neutralize C
in
.
The second approach has been implemented in the MultiClamp 700A in two ways.
Primary Method for Neutralizing C
in
A special technique is used in the CV-7A headstage to keep the contribution to
C
in
from the input amplifier as small as possible. The technique is known as
“bootstrapping”. Unity gain feedback is used to reduce the component of stray
capacitance that exists between the amplifier input and its power supplies and
case. Sophisticated circuitry is used to superimpose the unity-gain output of
the buffer amplifier back onto its own power supplies and the headstage case,
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