[Neuroscience] Re: Series resistance and capacitance compensation
in current clamp
(by I.Vida from bio.gla.ac.uk)
Fri Feb 23 15:02:28 EST 2007
Concerning bridge-balance/series resistance compensation in
current-clamp recordings: this is a simple procedure that
"calculates" the voltage error due to the electrode resistance
("series resistance") when a current is applied. The error,
product of Rs (as set by the Rs knob on the amplifier) and
the applied current according to Ohm's law, is simply subtracted
from the measured voltage. (("Bridge balance" name comes from the
Wheatstone-bridge circuit that was originally used in amplifiers
to do this job.)
So, you could easily do the same off-line, by software as long as
you know these two values.
The major problem with the EPC7, as Christian indicated, is that this
amplifier was designed to do voltage-clamp rather than current-clamp
recordings. The head-stage has a current-feedback (or current-to-voltage)
circuitry that keeps the voltage constant at the input, and not a voltage
follower circuit as in conventional CC amplifiers. Current clamp is
implemented by an add-on circuit that tries to keep the current
flowing through the electrode constant in a feed-back manner.
This mechanism has limitations, affecting/distorting primarily
fast signals (for a detailed discussion see the review of
Magistretti et al., 1996 TINS).
An additional problem is, if i remember correctly is that the
EPC-7 has no pipette compensation in current clamp mode either.
Thus pipette resistance and capacitance will act as a low-pass
Despite these problems, i believe that if there was a sag/rectification
in your recordings, you should be able to see it even with the EPC-7.
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