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BA-01X User Manual
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version 1.9
page 22
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Accuracy: Now both DISPLAYs (potential / current) the should read 000. Due to the
limited resolution of the display, unbalanced offsets and thermal drifts an offset of ±001 to
±002 can occur on the display. This small deviation can be trimmed internally, but this
procedure is necessary only if very small currents in the pA range should be accurately
applied.
7.2.
Offset Compensation
If an electrode is immersed into the bath solution an offset voltage will appear, even if no
current is passed. This offset potential is the sum of various effects at the tip of the electrode
filled with electrolyte (“tip potential”, junction potential etc.). This offset voltage must be
compensated, i.e. set to zero carefully with the OFFSET control (#16, Figure 1) before
recording from a cell. When adjusting the OFFSET make sure that no current flows through
the electrode. Thus, it is recommended to disconnect CURRENT INPUT and to disable
GATE (TTL) STEP SIZE and the HOLDING CURRENT unit (see chapter 3.2).
If a cell model is connected the OFFSET control should read a value around 5, otherwise it is
likely that the headstage or the amplifier is damaged.
7.3.
Capacitance Compensation
High resistances of electrodes and stray capacitances (C
stray
) form a low-pass filter which
deteriorates the shape of recorded intracellular signals (see also Figure 11). The frequency
response (bandwidth) of the amplifier is improved considerably by using the capacitance
compensation function. This function is based on positive feedback (“negative capacitance”)
circuit. The tuning of the capacitance compensation control is performed using pulses applied
to the CURRENT INPUT or pulses provided by the electrode resistance test circuit.
With the cell model connected or the electrode in the bath the CAPACITY COMP. control is
turned clockwise until there is no artifact on the POTENTIAL OUTPUT (see Figure 9).
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Make the basic settings at the amplifier (see chapter 0).
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Connect a cell model or immerse the electrode into the bath as deep as necessary during
the experiment.
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Tune the OFFSET to zero (see chapter 7.2)
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Push the ELECTRODE RESISTANCE TEST button (#31, Figure 1) or apply pulses to
the CURRENT STIMULUS INPUT and watch the POTENTIAL OUTPUT.
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Compensate the input capacitance as shown in Figure 9 using the CAPACITY COMP.
potentiometer (#17, Figure 1).
Figure 9 illustrates the capacitance compensation procedure using a 100 M
Ω
resistor that
represents the electrode. The pulses were generated using the automated electrode resistance
test circuit of the BA-01X. The upper diagram shows an undercompensated capacitance. In
the diagram in the middle the capacitance is slightly overcompensated and in the lower
diagram it is well compensated.