Axon Axopatch 200B, Теория работы

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94 • PRINCIPLES OF OPERATION
Axopatch 200B, Copyright 1997-1999, Axon Instruments, Inc.
When the Axopatch 200B probe is in the PATCH configuration, it has a 1 pF
capacitor as its feedback element (Figure 21).
INTEGRATOR
1 pF
10 k Ω
S
1
+
-
C
m
V
STEP
V
o
R
s
Figure 21. Integrator driving bilayer model.
When a voltage step, V
step
, is applied to the command input, the output, V
o
, of the
integrator, will attempt to become:
C
m
V
o
= -V
step
___
C
f
where C
m
is the membrane capacitance in pF and C
f
is the feedback capacitor, the
value of which is 1pF.
If V
o
≥
10 V ( e.g. , 100 mV on 100 pF), a reset of the integrator is initiated
internally; an external reset pulse is not needed. In the reset state, switch S
1
is
closed and the 1 pF capacitor is shunted by the 10 k
Ω
reset resistor. This all
happens within 2 µs from the time the integrator r 10 V or - 10 V.
During the time the integrator is in reset (10 µs), it can pass up to 1 mA (10
6
nA) of
current to charge the membrane. For example, if C
m
= 100 pF and R
s
= 10 k
Ω
, the
charging time constant (
τ
) is 1 µs. Therefore, the integrator will be in its reset
mode for 10
τ
. This is long enough for the membrane to essentially reach its final
value.
If the voltage step is small enough so that it does not cause a reset (V
o
<10 V), it is
recommend that a reset signal be applied externally. This is not required in order