HEKA EPC 9 Manual page 56

The main problem with current-clamp settings is stability, as the feedback loop will
oscillate when C-fast is set to its true value; the loop expects at least a small amount
of uncompensated C-fast for a phase lead at high frequencies. The EPC9 software will
assume that the user has compensated C-fast correctly, and in the process of
switching to current-clamp mode, it will set its value slightly lower (0.5 pF) to
optimize the speed of recording. The Current Monitor output can be used to monitor
the currents being passed, with the Gain control determining the sensitivity.
The new version "C" of the EPC9 hardware (and later verisons):
In the new EPC9 versions the current-clamp circuitry has been improved to better
follow rapid changes in membrane potential, such as in neuronal action potentials. In
the current-clamp mode a feedback circuit causes the effective input resistance of the
EPC9 headstage to be greater than 10e11 Ω. The speed of the feedback loop
determines the effective input capacitance of the amplifier; a high capacitance can
cause the time course of the action potential to be slowed. The capacitance of the
electrode, and to some extent the amplifier, can be neutralized by the C-fast setting,
which acts as a capacitance neutralization adjustment in the current-clamp mode.
However, like capacitance neutralization settings on conventional microelectrode
amplifiers, excessive capacitance neutralization can result in oscillation and
potentially the destruction of the cell membrane. The best way to use the C-fast
control is to first adjust it in the voltage-clamp mode, e.g., by using the Auto button; it
is then automatically adjusted to neutralize all but the amplifier input capacitance
when you switch to current-clamp mode.
The current-clamp circuitry now has two speed settings (which can be toggled by a
button that is normally hidden behind the oscilloscope window; it can be seen when
closing the oscilloscope). At the fast setting, the effective input capacitance of the
headstage is 2 pF. This amount of capacitance will cause little loading of most cells. If
desired you can neutralize a fraction of this capacitance as well by increasing the C-
fast setting by up to 2 pF from its initial value. Overshoot or ringing of the response
will occur as you approach complete compensation.
The fast current-clamp setting must be used with some caution because if C-fast is
grossly misadjusted, either by being too high (i.e., overcompensation) or too low,
oscillation may occur. The fast current-clamp mode may also oscillate in situations
where the impedance of the electrode and the cell are low, below about 5 MΩ at a
frequency of 10 kHz. Thus oscillation may occur when the pipette has a resistance
below 5 MΩ and is open to the bath solution, or in recording from a cell with
substantial membrane capacitance when the series resistance is less than 5 MΩ.
In the slow setting, the current-clamp system is more stable, but an effective input
capacitance of 24 pF is produced. Because this value is larger than the range of the C-
fast controls, misadjustment of C-fast will not cause oscillation; the amplifier is also
Operating Modes EPC9 Manual 56