8
of
10
4.
Using
the
SR555/SR556
with
a
Lock
In
Power
is
supplied
to
the
SR555/SR556
via
the
9
pin
cable
which
is
supplied
with
the
unit.
If
it
is
used
as
a
pre
‐
amplifier
for
a
SRS’
lock
‐
in
amplifier,
attach
one
end
of
the
cable
to
the
connector
on
the
rear
of
the
SR555/SR556.
With
the
lock
‐
in
power
off,
connect
the
other
end
of
the
cable
to
the
“PREAMP”
connector
on
the
rear
of
the
lock
‐
in.
Both
ends
of
the
cable
are
equivalent.
If
a
longer
cable
is
required,
any
standard
cable
with
male
9
‐
pin
D
‐
sub
connectors
on
both
sides,
wired
pin
‐
to
‐
pin,
will
suffice.
The
Output
A
of
the
SR555/SR556
should
be
connected
to
the
(A)
Input
of
the
lock
‐
in.
The
Output
B
should
be
connected
to
the
(B)
Input
of
the
lock
‐
in.
Equal
‐
length,
BNC
‐
terminated
coax
cables
should
be
used
for
both
connections,
and
for
best
results
the
two
cables
may
be
twisted
together
to
minimize
inductive
pickup.
The
lock
‐
in
Input
Selector
should
be
set
to
(A–B).
Note
that
the
output
of
the
SR555/SR556
is
a
balanced,
differential
signal
presented
across
the
A
and
B
outputs,
making
it
important
to
receive
the
output
signal
differentially.
For
applications
in
which
the
input
current
node
should
be
left
unbiased
(0
V
potential),
the
“Bias
Enable”
switch
should
be
left
in
the
down
position
on
the
SR555/SR556.
For
applications
in
which
a
non
‐
zero
potential
is
to
be
established
at
the
input
current
node,
the
“BIAS
IN”
BNC
connector
is
used.
This
is
a
DC
input
that
programs
the
non
‐
inverting
node
of
the
input
opamp,
and
sets
the
voltage
of
input
current
signal.
The
DC
bias
voltage
is
subtracted
by
the
differential
amplifier
and
will
not
be
sensed
by
the
lock
‐
in
amplifier.
The
overall
sensitivity
of
the
lock
‐
in
plus
the
preamplifier
is
not
displayed
on
the
lock
‐
in
front
panel.
It
is
necessary
to
divide
the
displayed
sensitivity
by
the
preamplifier
gain:
10
V/A
for
the
SR555,
or
10
V/A
for
the
SR556.
5.
Using
the
SR555/SR556
without
a
Lock
In
The
SR555/SR556
can
be
powered
with
an
external
power
supply.
Power
is
applied
through
the
9
pin
connector
as
described
in
the
table
below:
Pin
Voltage
Current
1
+20
VDC
±10%
80
mA
6
–20
VDC
±10%
50
mA
7,8
Ground
Both
DC
voltages
are
required.
Either
Pin
7
or
pin
8,
or
both
may
be
used
as
ground.
All
other
pins
are
not
used.
There
are
two
protection
diodes
connected
between
the
internal
regulators
and
the
+20V
and
‐
20V
input
pins.
