NPI SEC-05LX, Operating Instructions Manual

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Version 1.11 page 1
APPENDIX
TUNING CAPACITY COMPENSATION IN
SEC AMPLIFIER SYSTEMS
VERSION 1.11
For accurate measurements in switched mode, it is essential that the capacity of the electrode
is fully compensated.
Important : Wrong compensation of electrode capacity leads to errors in measurements done
in switched mode of the amplifier (see Figure 2).
Microelectrode selection : Electrodes must be tested before use. This is done by applying
positive and negative current pulses. Electrodes that show significant changes in resistance
(rectification) cannot be used for intracellular recordings. By increasing the current amplitude
the capability of the electrode to carry current can be estimated. The test current must cover
the full range of currents used in the experiment. For details see (3).
Switching frequency is a key parameter of discontinuous single electrode clamp (dSEVC)
systems. The switching frequency determines the accuracy, speed of response, and signal-to
noise ratio of the dSEVC system (3)(6). Since its launch in 1984, one of the outstanding
features of the SEC series of single electrode voltage / current clamp systems has been the
ability to record routinely with high switching frequencies in the range of tens of kilohertz,
regardless of the microelectrode resistance (1). Principles of the dSEVC technique are found
in (1)(2).
Looking back: In the early eighties, when the design of the SEC 1L system was started, single
electrode clamping began to gain importance beside the two classical intracellular methods:
bridge recording or whole cell patch clamp recording. The great advantage compared to the
whole cell recording method using a patch amplifier was the elimination of series resistance
due to the time sharing protocol. No current flow during voltage recording means no
interference from the series resistance regardless of its value. Voltage clamp recordings
became possible with sharp microelectrodes in deep cell layers. The historical weak point of
this method was the low switching frequency due to the fact that stray capacities around the
microelectrode could not be compensated sufficiently.
The SEC systems provided a solution for this problem. With their improvements on capacity
compensation electronics, they could be used with switching frequencies of tens of kHz even
with high resistance microelectrodes. What are the technical principles that make possible
such high switching frequencies?
In SEC systems a special protocol is used to rapidly compensate the microelectrode. Figure 1
shows the compensation scheme of a sharp microelectrode immersed 3 mm in cerebrospinal
fluid. Here the increase in speed can be seen clearly. Recordings under such conditions and
possible applications have been presented in several papers (e.g. (3)).