BA-01X User Manual
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version 1.9
page 24
7.4.
Bridge Balance
If current is passed through an electrode the occurring voltage deflection (potential drop at
R
EL
) affects the recording of membrane potential. Therefore, this deflection must be
compensated carefully by means of the BRIDGE BALANCE control. This control is
calibrated in M
Ω
and has two ranges selected by a RANGE switch (#21, Figure 1).
With the cell model connected or the electrode in the bath the BRIDGE BALANCE control is
turned on clockwise until there is no artifact on the POTENTIAL OUTPUT (see Figure 10).
o
Make the basic settings at the amplifier (see chapter 0).
o
Connect a cell model or immerse the electrode into the bath as deep as necessary during
the experiment.
o
Tune the OFFSET to zero (see chapter 7.2) and compensate the input capacitance (see
chapter 7.3). This is very important since a badly compensated input capacitance prevents
setting the BRIDGE BALANCE to correct values.
o
Determine the electrode resistance using the ELECTRODE RESISTANCE switch and set
the BRIDGE BALANCE RANGE switch (#21, Figure 1) accordingly.
o
Apply current pulses to the electrode either using an external stimulator (via the
CURRENT INPUT connector (#24, #26, Figure 1) or by using the gated stimulus unit.
o
Watch the POTENTIAL OUTPUT at the oscilloscope and adjust the BRIDGE
BALANCE as shown in Figure 10 using the BRIDGE BALANCE potentiometer (#22,
Figure 1). After adjustment you should see a straight voltage trace without artifacts caused
by the potential drop at R
EL
.
Figure 10 illustrates the BRIDGE BALANCE procedure using a 100 M
Ω
resistor that
represents the electrode. The current stimuli were generated using the gated stimulus unit
gated by two TTL pulses. The amplitude was set to 0.5 nA. In the upper diagram the bridge is
slightly undercompensated and in the diagram in the middle it is slightly overcompensated.
The lower diagram shows a well balanced bridge (compensated).
Important: BRIDGE BALANCE and CAP. COMP must be tuned several times during an
experiment since most parameters change during a recording session. Figure 13 shows
artifacts caused by uncompensated stray capacitance and bridge during recording from a cell.
It also shows how to cancel these artifacts by tuning with CAP. COMP and BRIDGE
BALANCE.
OFFSET deviations can be detected by comparing the readout on the potential display before
and after an experiment (with the electrode in the tissue, but not in a cell).
7.5.
Electrode Selection
Electrodes must be tested before use. This is done by applying positive and negative current
pulses and by compensating with the BRIDGE BALANCE control. Electrodes which show
significant changes in resistance (rectification) cannot be used for intracellular recordings. By
increasing the current amplitude the capability of the electrode to apply current can be
estimated. The test current must cover the full range of currents used in the experiment.