Current clamp operation modes?

jonesmat jonesmat at physiology.wisc.edu
Thu Dec 16 14:51:52 EST 2004


tomte wrote:
> Hi!
> I am working with the Axoclamp 2B Amplifier from Axon Instr.
> Since I have little experience with this amplifier i am still engaged
> with some questions:
> There are two different modes to do current clamp recordings
> (1) Bridge mode
> (2) Discontinuous Current Clamp (DCC) mode
> Even though i spend quiet a while reading the manual, i am still not
> sure in some points. For example: What are the
> advantages/disadvantages of these two modes (when compared with each
> other)? When (in which experimantal situation) shall one prefer
> working in DCC mode compared to Bridge mode and vice versa?
>
> Any explanations or comments are appreciated (does not apply to
> Kenneth Collins!).
> Cheers,
> Tom


These modes are two methods for addressing the same problem, namely
that if you are passing current (I) through an electrode with a
resistance (Rs), the voltage drop across the pipette resistance
produces adds to the apparent voltage of the cell itself.

Bridge mode is a basic voltage follower mode, similar to what you would
be doing if using sharp microelectrodes. In this mode, you dial a knob
while looking at the voltage response to a current step. This response
may have an initial "instantaneous" component before the usual
exponential charging phase. You dial the knob until this instantaneous
component goes to zero. This is called "balancing the bridge" because
(I think) it uses a Wheatstone Bridge circuit (e.g.,
http://www.efunda.com/designstandards/sensors/methods/wheatstone_bridge.cfm)
to measure the unknown series resistance (Rs), and subtract the
corresponding voltage drop from the measured voltage.

Discontinuous mode (also called "switching" mode) approaches the
problem by splitting the task of passing current and measuring voltage
into two steps, so that they're not both happening in the pipette at
the same time. You turn a knob that determines what fraction of time
will be used for passing current and what for measuring voltage, or
alternatively, that determines how fast to switch back and forth
between these two tasks.

Each mode has it's own advantages & disadvantages. For example,
switching mode introduces more noise. I think both also interact with
capacitance compensation circuitry, which can lead to unhealthy
feedback and oscillations. But for more depth I recommend reading about
these in The Axon Guide (http://www.axon.com/mr_Axon_Guide.html). This
is a more practical "cookbook" guide than the amplifiers manual would
be.


Matt




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