BC-535 Preliminary, Rev. 060126
8
Selection of
Σ
V
c
displays the sum of all command voltages (V
m
HOLD
and
COMMAND INPUT
) applied
to the headstage. The meter is capable of displaying command voltages up to
±
1999 mV. The meter
displays DC values and will average AC signals or pulses.
Outputs
The
OUTPUTS
block contains controls for
selecting the I
m
GAIN
of the amplifier and signal
filtering using the built-in 4-pole Bessel filter. This
block also contains the audio output controls.
Amplifier gain is selected via an 11 position
selector switch. Gain settings are from 0.5 to
1000 mV/pA in 1-2-5 steps. For transmission to
external devices, the selected gain setting appears
as a defined voltage at the
GAIN TELEGRAPH
BNC
on the instrument rear panel.
Internal filtering of the I
m
signal is selected
via a 9 position selector switch. Filter settings are
from 50 Hz to 20 kHz in 1-2-5 steps. A
BYPASS
TOGGLE
switch bypasses the 4-pole Bessel filter and presents the full bandwidth (75 kHz) of the
amplifier at the I
m
OUTPUT
. Filtering is applied post-gain. For transmission to external devices, the
selected filter setting appears as a defined voltage at the
FILTER TELEGRAPH
BNC on the instrument
rear panel.
The
AUDIO
section
is comprised of an on/off toggle and volume control. Audio output is useful
during membrane formation to monitor the successful application of lipids. An open hole prior to
membrane formation produces a characteristic low frequency sound while the same aperture with
membrane produces a different characteristically higher frequency sound. The pitch of the signal is
keyed to the membrane capacitance and will increase as the capacitance increases allowing non-visual
monitoring of membrane ‘thinning’.
Capacitance compensation
The capacitance compensation circuit allows for cancellation
of large currents (capacity currents) generated when a step
potential is applied to the bilayer membrane.
The
CAP COMP
block contains controls for the adjustment of
AMPLITUDE
and
TIME CONSTANT
for both
FAST
(0-10
µ
s) and
SLOW
(0-10 ms) components of the current. The adjustment is made
in pairs, that is, the
FAST
pair (
AMPLITUDE
and
TIME CONSTANT
) is
first adjusted to minimize the transient, followed by adjustment
of the
SLOW
pair. Each pair is adjusted in turn as many times as
required to completely minimize the transient.
The
AMPLITUDE
control for the
FAST
component is a ten turn
potentiometer with a counting dial and can be used to provide a
Warner
Instruments
A Harvard Apparatus Company .