Compensation Procedures
EPC9 Manual
60
The EPC9 incorporates additional circuitry to allow capacitance transient
cancellation to occur while R
s
-compensation is in use (see Sigworth, Chapter 4 in
Single Channel Recording
). This is shown as the prediction pathway in the figure
below, and it accelerates the charging of the membrane capacitance by imposing
large, transient voltages on the pipette when step changes are commanded (this is
sometimes called “supercharging”). These voltages would occur due to the action of
the correction pathway alone as the large capacitive charging currents elicit pipette
voltage changes; however, when these currents are cancelled by the transient
cancellation, their effect must be predicted by the cancellation circuitry: hence the
prediction pathway.
Ref. Pot.
V
0
-Offset
Σ
Pipette
Current
Monitor
Stim. In
Bath Electrode
IV Converter
C-slow
Transient Gen.
Σ
Σ
V-hold
Corr. Loop
Pred. Loop
Together, the two parts of the EPC9 R
s
-compensation circuitry cancel the effects of a
fraction of the series resistance. This means that the charging of the membrane
capacitance is accelerated, with a time constant under compensation of
c
=
1
−
(
)
u
where
τ
u
is the uncompensated time constant. Similarly, the voltage errors due to
membrane currents are also reduced by the factor (1-
α
). The fractional compensation
α
is determined by the setting of the
%-comp
control on the EPC9 software. For
proper compensation, however, the circuitry needs to have an estimate of the total
series resistance (for the correction pathway), and both the series resistance and
membrane capacitance must be known for the capacitance transient cancellation (
C-
slow
) circuitry. In the EPC9, the estimation of series resistance has been combined
with the transient cancellation, in that the R
s
control has a dual effect. Its setting
affects both the kinetics of the transient cancellation and the scaling of the correction
feedback signal. This means that in practice the estimation of the series resistance
