3
OPERATION
The SR554 Transformer Preamplifier is
designed to be used with all SRS Lock-in
amplifiers. It can reduce input noise of a
lock-in amplifier dramatically (as low as
100 pV/
√
Hz) and extends the lock-in’s full
scale sensitivity (without expand). It also
nearly eliminates noise radiated back from
the lock-in amplifier to the users
experiment. When used as a remote
preamplifier, the SR554 can eliminate the
effects of noise pickup on long signal
cables. The SR554 transformer is
designed to be used with its internal
buffer, but the buffer may be bypassed for
transformer only operation. When used
as a simple transformer, no power
connection is required.
CONNECTING THE SR554
When the transformer and buffer are used
together, power is supplied to the SR554
via the 9 pin connector and cable. This
cable mates with all SRS lock-in amplifiers
through the rear panel connector. To use
the SR554 without an SRS lock-in, the
user must provide their own ±20 VDC
(100 mA) source. Always connect the
power cable to the SR554 while the lock-
in power is off. Attach one end of the
cable to the connector on the rear of the
SR554, and connect the other end to the
PRE-AMP connector on the rear of the
lock-in. If a longer cable is required, any
standard 9 pin cable will suffice since all
connections are straight through. When
the lock-in power is on, the POWER
indicator on the SR554 will light.
The SR554 output switch selects between
buffered mode with a gain of 500,
(transformer and buffer) or bypassed
mode with a gain of 100 (transformer
alone). In the buffered mode, the
transformer secondary goes into a low
noise buffer which drives the output cable.
This is to reduce loading due to the
transformer output impedance (about 5
k
Ω
) and cable capacitance (10's of pf). If
the user prefers not to use the buffer or if
power isn’t available for the preamp, then
the SR554 can be used in the bypassed
mode. In the bypassed mode, the output
impedance of the pre-amp is 5 k
Ω
and
care must be taken to avoid loading the
output with too much cable capacitance.
Power should not be connected to the
SR554 when used in the bypass mode.
INPUT CONNECTION
Signals into the SR554 can be connected
either single ended through the (A) input,
or differentially through the (A-B) inputs.
In the single ended configuration the
shield and center conductor of the (A)
input are connected to the transformer.
For differential connection, the shields of
the input BNC’s are connected to the
SR554 case and the (A) and (B) center
conductors connect to the transformer.
The input impedance is a combination of
0.5
Ω
and 0.5 H (in series) in parallel with
