SRS Labs SR554 User Manual

Model SR554

Transformer Preamplifier

Model SR554

Transformer Preamplifier

1290-D Reamwood Avenue

Sunnyvale, California 94089

Phone: [phone] • Fax: [phone]

email: [e-mail] • www.thinkSRS.com

Copyright © 1999 by SRS, Inc.

All Rights Reserved.

Revision 1.2 (07/2004)

SR554 SPECIFICATIONS

Inputs Single ended or differential

Input Impedance 0.5 Ω

Maximum Inputs Transformer and Buffer: 14.0 mV RMS (±20 mV peak)
Transformer only: 350 mV RMS (±0.5 V peak)

Common Mode Range: ± 100 VDC
Rejection: 140 dB @ 100 Hz

Isolation > 40dB DC to 500 MHz

Input Noise 120 pV/√Hz @ 10 Hz (typical)
(Transformer and Buffer) 100 pV/√Hz @ 100 Hz
100 pV/√Hz @ 1000 Hz
(see noise contours, pg. 5)

Gain Transformer and Buffer: 500 (nominal)
Transformer only: 100 (nominal)
See Amplitude-Frequency Response Curve (pg 6).

Gain Accuracy 5% (with fixed source resistance)

Gain Stability 100 ppm/ °C

Outputs Single ended or differential

Output Impedance Transformer and Buffer: < 1.0 Ω
Transformer only: > 5000 Ω

Maximum Output Transformer and Buffer: 7.1 V RMS (10 Volts peak)
Transformer only: 35.3 V RMS (50 Volts peak)

Power Provided by any SRS lock-in amplifier via the supplied

connector cable or from a ± 20 volts DC @ 100 mA source.

Mechanical 2.9" X 3.8" X 7.5" (HxWxL), 4 lb.

Warranty One year parts and labor on materials and workmanship.

1

2

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 lock­in 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
kΩ) 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 kΩ 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

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1.6 µF. The real portion of the input
impedance (0.5 Ω) determines the noise
performance. See page 7 for detailed
information on input impedance.

The input can be floated up to ±100 VDC
with respect to the chassis. The
maximum AC input before overload is 14
mV RMS (±20 mV peak) when the unit is
in the buffered mode. In either mode, the
input is clamped at about 350 mV RMS
(±0.5 V peak). Care should be taken when
the unit is used in the bypassed mode,
since a ±0.5 V peak input becomes ±50 V
at the output.

In the buffered mode, the OVERLOAD
indicator will light when the preamplifier
overloads. An overload that occur after
the preamplifier will be indicated by the
lock-in amplifier’s overload indicator. The
SR554’s overload indicator is only

functional when the unit is in the buffered
mode. If the unit is operated in the
bypassed mode and power is applied, the
overload LED may light even when the
unit is not overloaded. This does not
indicate an overload, but is due to leakage
current of the buffer amplifier and its
protection circuitry. To avoid this, power
should not be connected when the unit is
used in the bypassed mode.

OUTPUT CONNECTION

For single ended operation the (A) Output
of the SR554 should be connected to the
(A) Input of the lock-in amplifier. The
center conductor carries the signal and
the shield is ground. For most
applications, this single connection will be
adequate.

For situations with potential noise pick-up
on the cable, it may be better to operate in

4

the differential mode. In differential
operation the (A) and (B) center
conductors carry the signal and shielded
preamp ground, and the shields are tied to
the SR554 chassis. The (A) and (B)
cables should be twisted together to
prevent inductive pick-up.

For most experiments it is preferable to
use the SR554 in the buffered mode. If
the preamplifier is used in the bypass
mode, care must be taken to not load the
output. The output resistance of the
transformer is at least 5 kΩ (for a 0 Ω
source) and is typically 10,000 times the
input resistance. Therefore, a 50 Ω
source impedance will become 500 kΩ. If
the instrument that the SR554 is
connected to has an input impedance of 1
MΩ, 1/3 of the signal is lost. Any

significant cable capacitance will create a
low-pass filter with this output resistance
as well, so short cables should always be
used.

GAIN OF SR554

The actual gain of the SR554 is a function
of the source impedance, frequency and
the set gain. In the bypass mode (x100),
the gain will be affected by loading on the
output. The gain is fairly flat over a range
of input impedances (<10 Ω) and
frequencies (5 Hz-10 kHz). The actual
gain can be determined from the
amplitude-frequency response curves on
page 4. The plot assumes operating the
SR554 in the buffered mode or with no
loading on the output in the bypassed
mode.

5

EXTRA LOW NOISE MEASUREMENTS

When making extremely low noise
measurements, it is a good practice to
connect the grounding plug of the SR554
to a ground point near the experiment. If a
good ground is not available near the
experiment, connect a wire from the lock­in chassis (using a lug under one of the
chassis screws) to the grounding lug of
the SR554.

NOISE FIGURE

The noise figure describes the noise
contribution of an amplifier in a
measurement when compared to an ideal
amplifier.

The expression:

where N is the measured noise, A is the
pre-amp gain and e
noise of the source impedance, describes
the noise figure contours shown below.
The optimum operating frequency can be
determined from this graph.

nrs is the Johnson

USING THE SR554 WITH SRS LOCK­INS

The SR554 is not sensed through the 9
pin cable by SRS lock-in amplifiers.
Therefore the lock-in does NOT
compensate for the gain of the preamp.
Measurements made using the preamp
must be divided by the gain of the SR554.
The actual gain can be obtained from the
amplitude response curves on page 4.

USING THE SR554 WITH THE
SR810/830/850

For typical measurements the lock-in input
should be set to AC coupled, with the
shield grounded. For low frequency
measurements (<1Hz), set the lock-in to
DC coupled, with the shield grounded,
since the SR554 can sense signals below
the lock-in’s AC coupling frequency
(0.16 Hz).

USING THE SR554 WITH THE
SR510/530

The SR510/530 is AC coupled from 0.5
Hz to 100 kHz. Measurements below 0.5
Hz are not recommended with the
SR510/530.

6

USING THE SR554 WITHOUT AN SRS
LOCK-IN

The SR554 can be powered with an
external power supply. Power is applied
through the 9 pin connector as described
below.

PIN

VOLTAGE CURRENT
1 +20 V 100 mA
6 -20 V 100 mA
7,8 Ground

Both voltages are required. Pins 7 and 8
should be tied together and grounded. All
other pins should be left open.

COMMON MODE REJECTION RATIO

The SR554 has an extremely high CMRR
at low frequencies (up to 160 dB below 10
Hz). It drops off at higher frequencies due
to capacitive coupling between the
primary and secondary windings and
reduced signal gain. See the graph below
for the relationship between CMRR and
frequency.

RADIATED NOISE

The SR554 reduces radiated noise from
the lock- in amplifier’s input by 40 dB
(100x) over most frequencies (DC to 500
MHz). To minimize radiated noise, a thick
(low impedance) wire should be
connected from the ground plug to a quiet

ground point. If a ground is not available
near the experiment, connect a wire to the
lock-in using a lug under one of the
chassis screws.

INPUT IMPEDANCE

The input impedance of the SR554
appears as a combination of 0.5 Ω and

0.5 H (in series) in parallel with 1.6 µF and
several parasitic impedances. The
transformer primary has a DC resistance
of 0.2 Ω and a primary inductance of 0.5
H. The secondary has a DC resistance of
3 kΩ and a capacitance of about 160 pF.
When the secondary impedance is
converted over to the primary side of the
transformer by the turns ratio (1:100), the

0.5 Ω, 0.5 H and 1.6 µF values are
obtained. The actual values of the
magnitude and phase of the input
impedance is shown in the graph below.

7

8

PERFORMANCE TESTS

Performance tests are designed to verify
that the unit is performing within the
specifications.

Necessary Equipment:

1. Lock-In Amplifier
Freq Range 0.1 Hz - 100 kHz
Output Ampl 4 mV rms - 1 Vrms
Output Z 50 Ω
Recommended SRS

2. 50 ΩTerminator

3. 50 Ω, 20 dB Attenuator

The instructions here apply to SRS DSP
lock-ins (SR850/830/810). Other lock-in
amplifiers may be used. However they will
require substantially longer time constants
to arrive at accurate measurements.

1) Connect the 9-pin power cable from

the rear of the lock-in amplifier to the
SR554. Connect the 20 dB attenuator
to the lock-in reference output and a
BNC cable from that to the SR554
(A) input. Connect another BNC cable
from the SR554's (A) output to the
lock-in’s (A) input. Set SR554 to
buffered mode (X500), (A) input.

2) Power on the lock-in in the default

condition. (SR810/830 Power on
Setup, SR850 Power on Bksp) This
places the lock-in in a known state.
The power LED of the SR554 should
light.

3) Set the input to DC coupled, grounded

shield. Set the time constant to 300
ms (1 s for 1 Hz measurement), 12
dB/oct filter and turn on synchronous
filtering (if available). Set the display
type to magnitude (R) and the
reference sinewave amplitude to 10
mV rms.

SR850/830/810

4) For each frequency, the following
amplitude should be observed, ± 5%.

Frequency

1.0 Hz 31 mV

10 Hz 252 mV
100 Hz 438 mV
200 Hz 442 mV
500 Hz 439 mV
1 kHz 414 mV
10 kHz 96 mV
100 kHz 11 mV

5) Disconnect the lock-in reference from
the SR554 input. Connect the 50 Ω
terminator to the SR554 (A) input.

6) Set the lock-in to measure noise.

7) For each frequency and time constant,
the following rms noise voltage should
be observed (±10%). Set the lock-in
sensitivity to about 5 times the
expected reading and allow it to settle
(10-20 time constants) before making
readings.

Frequency

1.0 Hz 3.0 s 480 nV

10 Hz 1.0 s 260 nV
100 Hz 0.1 s 460 nV
200 Hz 0.1 s 460 nV
500 Hz 0.1 s 460 nV
1 kHz 0.03 s 420 nV
10 kHz 0.03 s 120 nV
100 kHz 0.03 s 60 nV

Amplitude

Time Constant Noise Voltage

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Part List

Ref. SRS part Value Description

C 101 5-00254-501 130P Capacitor, Ceramic Disc, 50V, 10%, SL
C 102 5-00023-529 .1U Cap, Monolythic Ceramic, 50V, 20%, Z5U
C 103 5-00023-529 .1U Cap, Monolythic Ceramic, 50V, 20%, Z5U
C 104 5-00005-501 150P Capacitor, Ceramic Disc, 50V, 10%, SL
C 105 5-00023-529 .1U Cap, Monolythic Ceramic, 50V, 20%, Z5U
C 106 5-00312-503 .005U Capacitor, Ceramic Disc, 50V, 20%, Z5U
C 107 5-00016-501 470P Capacitor, Ceramic Disc, 50V, 10%, SL
C 108 5-00044-509 47U Capacitor, Electrolytic, 50V, 20%, Rad
C 109 5-00100-517 2.2U Capacitor, Tantalum, 35V, 20%, Rad
C 110 5-00100-517 2.2U Capacitor, Tantalum, 35V, 20%, Rad
C 111 5-00023-529 .1U Cap, Monolythic Ceramic, 50V, 20%, Z5U
C 112 5-00312-503 .005U Capacitor, Ceramic Disc, 50V, 20%, Z5U
C 113 5-00016-501 470P Capacitor, Ceramic Disc, 50V, 10%, SL
C 114 5-00044-509 47U Capacitor, Electrolytic, 50V, 20%, Rad
C 115 5-00100-517 2.2U Capacitor, Tantalum, 35V, 20%, Rad
C 116 5-00281-521 220U Capacitor, Electrolytic, 25V, 20%, Rad
C 117 5-00281-521 220U Capacitor, Electrolytic, 25V, 20%, Rad
D 101 3-00226-301 1N5822 Diode
D 102 3-00226-301 1N5822 Diode
D 103 3-00403-301 1N459A Diode
D 104 3-00403-301 1N459A Diode
D 105 3-00011-303 RED LED, T1 Package
D 106 3-00010-303 GREEN LED, T1 Package
J 101 1-00003-120 BNC Connector, BNC
J 102 1-00003-120 BNC Connector, BNC
J 103 1-00003-120 BNC Connector, BNC
J 104 1-00003-120 BNC Connector, BNC
J 104 1-00014-160 9 PIN D Connector, D-Sub, Right Angle PC, Female
J 109 1-00229-102 BINDING Binding Post
L 101 6-00174-630 6611 TYPE 43 Ferrite Beads
L 102 6-00174-630 6611 TYPE 43 Ferrite Beads
PC1 7-00613-701 SR554 PREAMP Printed Circuit Board
R 101 4-00141-407 100 Resistor, Metal Film, 1/8W, 1%, 50PPM
R 102 4-00188-407 4.99K Resistor, Metal Film, 1/8W, 1%, 50PPM
R 103 4-00134-407 1.24K Resistor, Metal Film, 1/8W, 1%, 50PPM
R 104 4-00138-407 10.0K Resistor, Metal Film, 1/8W, 1%, 50PPM
R 105 4-00142-407 100K Resistor, Metal Film, 1/8W, 1%, 50PPM
R 106 4-00166-407 200K Resistor, Metal Film, 1/8W, 1%, 50PPM
R 107 4-00166-407 200K Resistor, Metal Film, 1/8W, 1%, 50PPM
R 108 4-00142-407 100K Resistor, Metal Film, 1/8W, 1%, 50PPM
R 110 4-00063-401 3.0K Resistor, Carbon Film, 1/4W, 5%
R 111 4-00045-401 2.0K Resistor, Carbon Film, 1/4W, 5%
R 112 4-00056-401 22 Resistor, Carbon Film, 1/4W, 5%
R 113 4-00056-401 22 Resistor, Carbon Film, 1/4W, 5%
R 114 4-00056-401 22 Resistor, Carbon Film, 1/4W, 5%
SW101 2-00022-217 DPDT Switch, On-None-On, Toggle, Right Angle
SW102 2-00027-214 DPDT Switch, Miniature Bat Toggle
T 101 6-00169-610 SR554 Transformer
T 102 6-00173-614 T68-17 Iron Powder Core
T 103 6-00173-614 T68-17 Iron Powder Core
U 101 3-00535-340 AD743 Integrated Circuit (Thru-hole Pkg)
U 102 3-00193-340 LM339 Integrated Circuit (Thru-hole Pkg)
U 103 3-00118-325 78L15 Transistor, TO-92 Package
U 104 3-00124-325 79L15 Transistor, TO-92 Package
Z 0 0-00025-005 3/8" Lugs

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Part List

Ref. SRS part Value Description

Z 0 0-00043-011 4-40 KEP Nut, Kep
Z 0 0-00079-031 4-40X3/16 M/F Standoff
Z 0 0-00089-033 4" Tie
Z 0 0-00128-053 4" #24 Wire #24 UL1007 Strip 1/4x1/4 Tin
Z 0 0-00150-026 4-40X1/4PF Screw, Black, All Types
Z 0 0-00187-021 4-40X1/4PP Screw, Panhead Phillips
Z 0 0-00208-020 4-40X3/8PF Screw, Flathead Phillips
Z 0 0-00209-021 4-40X3/8PP Screw, Panhead Phillips
Z 0 0-00221-000 SR440FOOT Hardware, Misc.
Z 0 0-00263-052 3" #22 Wire #22 UL1007
Z 0 0-00266-052 8-1/2" #22 BLK Wire #22 UL1007
Z 0 0-00304-043 7/8X3/8X1/16 Washer, nylon
Z 0 0-00386-003 SOLDR SLV RG174 Termination
Z 0 0-00440-052 2-1/2" #22 RED Wire #22 UL1007
Z 0 7-00604-720 SR554-1 Fabricated Part
Z 0 7-00605-720 SR554-2 Fabricated Part
Z 0 7-00606-720 SR554-3 Fabricated Part
Z 0 7-00607-709 SR554 Lexan Overlay
Z 0 9-00267-917 GENERIC Product Labels

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