NPI SEC-05LX, Operating Instructions Manual

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Tuning Capacity Compensation
Version 1.11 page 2
Criteria for the selection of the switching frequency
Which are the most important criteria for the selection of the switching frequency? This
question was analyzed in detail by M. Weckstrom and colleagues (4)(5). They presented a
formula that describes the conditions for obtaining reliable results during a switching single
electrode clamp:
f
e
> 3f
sw
, f
sw
> 2f
s
, f
s
> 2f
f
>f
m
f
e
: upper cutoff frequency of the microelectrode
f
sw
: switching frequency of the dSEVC
f
s
: sampling frequency of the data acquisition system
f
f
: upper cutoff frequency of the lowpass filter for current recording,
f
m
: upper cutoff frequency of the membrane.
Example (6): With the time constant of 1-3 µs recorded for the electrodes used in this study,
f
e
is 80-160 kHz, the selected switching frequency of the dSEVC was 30 - 50kHz (calculated
range is 25-53 kHz), data were sampled at 10 kHz and the current signals have been filtered at
5 kHz. These settings are currently used for recordings in many labs.
The principle of operation in switched mode is shown below.
Figure 1: Microelectrode artifact settling.
Compensation of stray capacities with a SEC 05 amplifier. The upper trace shows the
comparison between the standard capacity compensation and the fast compensation of the
SEC systems. After full compensation the settling time of the microelectrode is reduced to a
few microseconds allowing very high switching frequencies (here: 40 kHz, middle and lower
trace). The microelectrode was immersed 3 mm deep in cerebrospinal fluid. Microelectrode
resistance: 45 M

, current: 1 nA, duty cycle 25%. SwF: switching frequency.
Original data kindly provided by Prof. Diethelm W. Richter, Goettingen. For details see (3).