BC-535 Preliminary, Rev. 060126
7
Offset
The
OFFSET
block contains the
INPUT OFFSET
and the
AUTO
-
ZERO
controls.
The
INPUT OFFSET
section is comprised of a rotary potentiometer with
low/high LED’s, the
AUTO
-
ZERO
pushbutton, the
UNLOCK
toggle, and an
ACTIVE
LED. This section is used to compensate for junction potentials
produced by dissimilar solutions or other electrode potential differences.
The
OFFSET
circuit (
AUTO
-
ZERO
and
OFFSET CONTROL
) must be armed
prior to use.
This is achieved by use of the
UNLOCK TOGGLE
. This toggle is
of the momentary-on style and is operated by an upward movement.
When the circuit is armed the
ACTIVE
LED will be lit. Offset adjustments
can then be easily achieved by use of the
AUTO
-
ZERO
control
or
ROTARY
POTENTIOMETER
. The circuit can be disarmed by a second movement of the
UNLOCK TOGGLE
.
The
AUTO
-
ZERO
control provides the most direct means for setting the junction potential. When
armed, pressing the pushbutton initiates a cycle wherein the amplifier searches for and sets the offset
potential. The offset circuit is automatically disarmed at the completion of the cycle. Cycle time is
approximately 1 s.
The
ROTARY POTENTIOMETER
is used to provide manual adjustment of up to
±
120 mV at the
headstage input. Manual adjustment is only available when the offset circuit is armed. Fine adjustment
of the rotary control can be achieved by pressing the control in while turning. Low/high LED’s are
provided to indicate which direction the manual offset control should be adjusted to achieve a null
junction potential setting. The offset circuit must be manually disarmed when using this control.
In both cases, the applied
OFFSET
potential can be monitored on the
METER
by selecting
OFFSET
in
the
METER
block.
Meter
The
METER
block contains a 3.5 digit LED
METER
and a four
position switch for selecting
OFFSET
,
CAP TEST
, current output (I
m
), or
voltage command (
Σ
V
c
).
Selection of
OFFSET
displays the potential applied to the
headstage via the
MANUAL
or
AUTO
-
ZERO
control located in the
OFFSET
block. Offset potential is displayed in units of pA. Alternatively, this
display indicates the potential required to bring I
m
to zero when the
command input is set to zero.
Selection of
CAP TEST
places the instrument into capacitance test
mode. This useful mode dynamically tests and reports the membrane
capacitance. Capacitance values are reported on the meter in units of
pF. A rear panel BNC also reports the calculated membrane capacitance whenever
CAP TEST
is
selected. Reported units are 1 mV/pF.
Selection of I
m
displays the value of the DC current presented at the I
m
OUTPUT
BNC. The meter is
capable of displaying currents up to
±
1999 pA.
Warner
Instruments
A Harvard Apparatus Company .