7
of
10
3.
Description
The
block
diagram
below
shows
the
SR555/SR556
current
amplifier
which
converts
input
current
into
a
differential
output
voltage.
The
total
differential
output
voltage
is
given
by
.
A
current
amplifier
is
also
called
a
Transimpedance
Amplifier,
or
a
TIA.
Figure
7
A
block
diagram
of
SR555/SR556
current
amplifiers
Unlike
a
simple
resistor,
the
transimpedance
amplifier
presents
a
low
‐
impedance
node
to
the
source.
This
node
is
generally
a
virtual
ground.
In
the
SR555/SR556,
this
node
voltage
can
be
set
at
the
user
provided
voltage
by
switching
to
“Bias
Enable”.
This
bias
voltage
will
not
be
present
in
the
output
signal
since
it
is
subtracted
out
by
the
post
differential
amplifier.
The
SR555/SR556
can
be
located
close
to
the
source
to
eliminate
long
cables,
capacitive
loading
and
noise
pickup.
The
two
output
signals
should
be
used
differentially
for
best
results.
The
capacitance
from
long
input
cables
or
intrinsic
capacitance
from
the
input
source
will
add
up
to
the
total
source
capacitance
.
This
will
react
with
the
opamp
voltage
noise
e
n
(shown
in
Figure
5)
to
add
an
effective
current
noise
of
2
to
the
equivalent
input
noise,
as
shown
in
Figures
3
and
4
(so
‐
called
“noise
gain
peaking”).
It
should
be
noted
that
has
virtually
no
effect
on
the
signal
gain,
as
shown
in
Figure
1.
The
SR555
and
SR556
are
ideal
low
‐
noise
current
preamplifiers
for
SRS’
lock
‐
in
amplifiers,
and
can
also
be
used
for
any
other
small
current
detection
purposes.
