[Neuroscience] Re: Series resistance and capacitance compensation in current clamp

Imre Vida via neur-sci%40net.bio.net (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
filter.

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.


Regards,

Imre



More information about the Neur-sci mailing list