INSTRUMENT OPERATION
•
67
Chapter 6
In I-CLAMP mode, if the current is zero, 10 V
m
is ten times the actual membrane
potential. If current is flowing in the pipette, 10 V
m
includes the voltage drop
across the pipette. The actual membrane potential is given by the following
equation:
10 V
m
Actual Membrane Potential (mV) =
_______ _
I R
s
10
where I = pipette current in nA
R
s
= resistance in MΩ set on SERIES RESISTANCE control
Panel Meter
The main panel meter displays one of five signals. Selection is made by a rotary knob. The
five signals are:
1) V
HOLD
/I
HOLD
: The value set on the HOLDING COMMAND potentiometer.
When V
HOLD
/I
HOLD
is selected, it will show the absolute value
of the HOLDING COMMAND even if the HOLDING
COMMAND control is OFF.
2) I: The pipette current. The reading is automatically scaled to
suit the headstage gain. The operation is autoranging; that is,
the decimal point and the units indicators automatically shift
so that large DC currents can be displayed.
3) I
RMS
: The rms current noise. The noise is measured in a 5 kHz
bandwidth using a Butterworth filter that is independent of the
front-panel lowpass Bessel filter. The front-panel gain does
not affect this reading.
4) V
m
: The membrane potential.
5) V
TRACK
: The output of the automatic offset nulling circuit.
6) TEMP: Headstage circuitry temperature in °C (range ± 20 °C).
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