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

Bill via neur-sci%40net.bio.net (by connelly.bill from gmail.com)
Sat Feb 24 22:02:03 EST 2007


Its worth noting that the Axopatch 200B has the same problem. I.e.
doesn't do true current clamp, hence the two settings, current clamp
normal and current clamp fast (for Rs above 10MOhm).

On Feb 24, 9:02 am, Imre Vida <I.V... from bio.gla.ac.uk> wrote:
> 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




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