User
Guide
for
Axoclamp
900A
instantaneous
current
switches
from
zero
to
some
finite
value
at
the
sample
rate.
This
is
because
for
30%
of
each
period
CHANNEL
1
is
used
for
passing
current
while
for
the
remaining
70%
of
each
period
no
current
is
passed
and
the
IR
voltage
drop
due
to
the
previous
current
is
allowed
to
passively
decay.
On
the
other
hand,
the
sampled
current
is
the
true
membrane
current.
It
is
recovered
from
the
instantaneous
current
by
a
circuit
that
samples
the
current
pulses,
retains
the
samples
during
the
passive
‐
decay
period,
then
scales
the
samples
to
yield
the
average
current
for
the
whole
period.
The
sampled
current
output,
selected
from
either
the
Scaled
Output
(Membrane
Current
signal)
or
from
the
Current
Output,
is
smoothed
by
the
output
filter.
The
whole
current
into
the
bath
can
be
separately
measured
using
a
virtual
‐
ground
headstage
(see
below).
Virtual
‐
Ground
Current
Measurement
A
Virtual
‐
Ground
headstage
can
be
used
to
ground
the
preparation
bath.
Connect
the
headstage
to
the
AUXILIARY
1
or
2
HEADSTAGE
input.
All
of
the
current
flowing
into
the
Virtual
‐
Ground
input
is
measured
and
a
voltage
proportional
to
the
current
is
provided
at
the
10
AUX1
or
10
AUX2
OUTPUT.
The
output
gain
is
1
mV/nA
when
VG
is
×10,
and
0.1
mV/nA
when
VG
is
×100.
R
0
(M
Ω
)
Full scale range
Conversion
VG-9A ×10
1
±10 µA
1 mV/nA
VG-9A ×100
0.1
±100 µA
0.1 mV/nA
Table
5.
Current
recording
ranges
for
virtual
ground
headstages.
A
Virtual
‐
Ground
headstage
is
not
required
for
normal
use
of
the
Axoclamp
900A
because
built
‐
in
current
‐
measurement
circuits
are
provided
for
each
microelectrode.
However,
in
TEVC
mode
the
current
output
of
the
Virtual
‐
Ground
headstage
has
slightly
less
high
‐
frequency
noise
than
the
output
of
the
built
‐
in
current
‐
measurement
circuit.
112
