108 • Reference Section
MultiClamp 700A Theory and Operation, Copyright 2000, 2001 Axon Instruments, Inc.
Sources of Noise
Cell and Seal
V-Clamp: The higher the resistance (R) and the smaller the capacitance (C)
between the interior of the electrode and ground, the smaller the current noise.
Thus, minimum noise is achieved for an isolated patch (large R, small C) with
a high seal resistance (large R). In whole-cell recordings from larger cells
(smaller R, larger C) the noise of the cell usually dominates, meaning that
subsequent noise sources (listed below) become less important. (See
Chapter 4,
PATCH CLAMPING.)
I-Clamp: The voltage noise is dominated by the load resistance but is also
affected by the stray capacitance. For a purely resistive load the noise is given
approximately by 12R µVrms (10 kHz bandwidth), where R is the parallel
combination of the feedback resistor (R
f
) and the load resistance (i.e. the
electrode resistance plus input resistance of the cell). Thus, a low resistance
electrode/cell combination is preferred. A large stray capacitance will reduce
the noise by acting like an RC filter, but this will also reduce the measurement
bandwidth. Increasing the Capacitance Neutralizaton setting will improve the
bandwidth but increase the noise.
Electrode and Holder
V-Clamp: Current noise increases markedly with electrode capacitance. This
can be minimized by coating the electrode and other strategies. (See
Chapter 4,
PATCH CLAMPING.) Increasing electrode resistance apparently
decreases the current noise, but this is due to the RC filtering effect of the
electrode resistance in parallel with the electrode capacitance. In fact, it is
desirable to decrease the electrode resistance in order to maximize the
bandwidth of the clamp, even if this apparently increases the noise of the
recording.
I-Clamp: Voltage noise increases markedly with electrode capacitance and
resistance. Thus, both should be minimized as much as possible. (See
Chapter 4,
SHARP MICROELECTRODE RECORDING.)
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