HEKA Elektronik EPC 10 USB, Руководство по оборудованию, Страница: 50 / 81

44 Compensation Procedures
Together, the two parts of the
EPC 10 USB
Rs-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
τ
c
= (1
−
α )
∗
τ
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
EPC 10 USB
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
EPC 10 USB
, the estimation of
series resistance has been combined with the transient cancellation, in that the Rs 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 consists of adjusting
C-slow and R-series to cancel
the transient currents due to the cell membrane capacitance. Once this has been done, the relative amount of
Rs-compensation can then be selected with the %-comp control.
Theoretically, it is desirable to compensate as much of the series resistance as possible. In practice, however, a
degree of compensation above 90% can involve considerable technical problems, and in some recording situations
a value below 90% is preferable. To illustrate one technical problem, consider the case when a 100 mV potential
change is commanded and 90% compensation is in use. This degree of compensation means that the cell membrane
capacitance will be charged 10 times faster than normally. The rapid charging is accomplished in the compensation
circuitry by forcing the pipette potential to (very transiently) reach a potential of 1 V. The resulting large current
causes the membrane capacitance to charge quickly to its final value of 100 mV. In general, when a voltage step
of size ∆
V is commanded, the pipette potential actually receives an initial transient of size ∆ V / (1
−
α ) due to the
compensation effect. The technical problem comes from the fact that the maximum pipette potential excursion in
the
EPC 10 USB
is about ± 1.2 V, implying that 90% compensation can be used for steps only up to about 120 mV
in amplitude. Overload of amplifiers (obvious in practical use due to the loss of proper transient cancellation) will
occur if larger pulses are applied, unless the %-comp setting is reduced.
The degree of Rs-compensation is also limited by stability considerations. Stable Rs-compensation requires that
the
C-fast control is properly set to cancel the fast capacitance transients; when the series resistance is high, say
above 10 M Ω, maladjustment of C-fast can easily cause oscillation. In cases where Rs is this size or larger, it is
often advisable to use the slower settings of the Rs switch which, in slowing down the speed of the compensation
feedback, makes it less susceptible to high-frequency oscillations. In cases where Rs is relatively small, on the other
hand, it is sometimes not possible to use full 90% compensation because of the limited speed of the compensation
feedback, even in the fastest, 2
µs setting of the switch. This problem arises when the time constant τ is smaller
than about 100 µs , and comes from the fact that compensated membrane time constant τ
c
cannot be made smaller
than a value that depends on the speed of the Rs-compensation feedback. If you turn up the %-comp control to
try to obtain a smaller
τ
c
, you will observe overshoot or ringing in the current monitor signal, due to an overshoot
in the membrane potential. The minimum value for τ
c
is given approximately by
τ
c ( min )
=
√
τ
u
τ
f
where τ
f
is the effective time constant of the feedback loop. The corresponding maximum α values are given by
α
max
= 1
−
q
τ
f
/τ
u
The table gives maximum α values (i.e., %-comp settings) and the resulting τ
c
values in the 2 µs setting for some
values of the uncompensated time constant τ
u
. At the 10 µs setting, full 90% compensation may be used without
http://www.heka.com