SRS Labs SR560 User Manual
MODEL SR560
LOW-NOISE PREAMPLIFIER
1290-D Reamwood Avenue
Sunnyvale, CA 94089 U.S.A.
(408) 744-9040
Copyright © 1989, 1990, 1997, 1999
Stanford Research Systems
All Rights Reserved.
Revision 2.8
(12/2006)
TABLE OF CONTENTS
Introduction and Setup
Instrument Overview 1
Preparation for Use 2
Line Voltage 2
Line Fuse 2
Line Cord 2
Ventilation 2
Power-Up 2
Repackaging for Shipment 3
Use in Biomedical Applications 3
Warning Regarding Use with Photomultipliers 3
Accessories Furnished 3
Environmental Conditions 3
Symbols 4
Specifications
SR560 Low Noise Preamplifier Specifications Chart 5
Operation and Controls
Front Panel Operating Summary 7
Power 7
Source 7
Filters 8
Gain Mode 9
Gain 9
Output 9
Reset 9
Status 9
Rear Panel Operating Summary 10
AC Power Input 10
Amplifier Power Output 10
Battery Charger 10
Blanking Input 11
RS-232 Interface 11
Battery Care and Usage 11
Recharging 11
Battery Care 11
Circuit Description
Differential Low-Noise Front End 13
Configurable Filters and Gain 13
Output Stages 13
Overload Detection 14
Microprocessor 14
Battery Charger and Pre-regulators 14
Power Regulators 15
Rear Panel Interfaces 15
Batteries and P.E.M. 16
Front Panel 16
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TABLE OF CONTENTS
Calibration and Repair
Offset Adjustment 17
Calibration 17
Front End Replacement 17
SR560 Offset Adjustment Procedure 18
Battery Replacement 18
Fuse Replacement 18
Noise Contours 19
Input Voltage Noise 20
Dynamic Reserve 20
Appendix A
Remote Programming A-1
Introduction A-1
Commands A-1
Appendix B
Noise Sources and Cures B-1
Intrinsic Noise Sources B-1
Johnson Noise B-1
'1/f' Noise B-1
Others B-1
Non-Essential Noise Sources B-1
Capacitive Coupling B-2
Inductive Coupling B-2
Resistive Coupling ('Ground Loops') B-3
Microphonics B-3
Thermocouple Effect B-3
SR560 - Component List
Front Panel Board C-1
Main Board C-2
Miscellaneous Parts C-13
PCB - SR560 Revision F (9/89)
Schematics
E-1 to E-10
D-1
ii
INTRODUCTION AND SETUP
Figure 1: SR560 Block Diagram
INSTRUMENT OVERVIEW
The SR560 architecture is diagrammed
above. The instrument provides DCcoupled low-noise amplification of singleended and true differential input signals at
gains of 1 to 50,000. Two configurable R-C
filters are provided to selectively condition
signals in the frequency range from DC to 1
MHz. The user can choose high dynamic
reserve or low noise settings, and can invert
the output relative to the input. The SR560
normally operates with a fully floating
ground and can be viewed as an "in-line-
BNC amplifier" with the amplifier ground
isolated from the chassis and the AC power
supply. Opto-isolated input blanking control
and listen-only RS-232 interface lines are
provided for instrument control. Digital
noise is eliminated by shutting down the
microprocessor's oscillator except during
the short time required to alter the
instrument's configuration, either through a
front-panel pushbutton or through an RS232 command. Internal sealed lead-acid
batteries provide 15 hours of line-
1
INTRODUCTION AND SETUP
independent operation. Rear panel banana
jacks provide access to the internal
regulated power supplies (or batteries) for
use as a bias source.
PREPARATION FOR USE
**********CAUTION**********
This instrument may be damaged if
operated with the LINE VOLTAGE
SELECTOR card set for the wrong applied
AC input source voltage or if the wrong fuse
is installed.
Line Voltage
When the AC power cord is connected to
the unit and plugged into an AC outlet, the
unit automatically switches the amplifier
power source from internal battery operation
to line operation. The internal batteries are
charged as long as AC power is connected.
The SR560 can operate from a 100 V, 120
V, 220 V or 240 V nominal AC power source
having a line frequency of 50 or 60 Hz.
Before connecting the power cord to a
power source, verify that the LINE
VOLTAGE SELECTOR card, located in the
rear-panel fuse holder of the unit, is set so
that the correct AC input voltage value is
visible.
Conversion from one AC input voltage to
another requires a change in the fuse
holder's LINE VOLTAGE SELECTOR card
position and a new fuse. Disconnect the
power cord, slide the fuse holder cover to
the left and rotate the fuse-pull lever to
remove the fuse. Remove the small printed
circuit board. Select the operating voltage
by orienting the printed circuit board. Press
the circuit board firmly into its slot, so the
desired voltage is visible. Rotate the fusepull lever back into its normal position and
insert the correct fuse into the fuse holder.
Line Fuse
Verify that the correct line fuse is installed
before connecting the line cord to the unit.
For 100 V and 120 V, use a 1 Amp fuse and
for 220 V and 240 V, use a 1/2 Amp fuse.
Line Cord
The SR560 has a detachable, three-wire
power cord with a three-contact plug for
connection to both the power source and
protective ground. The protective ground
connects to the accessible metal parts of
the instrument except for BNC shields.
To prevent electrical shock, always
use a power source outlet that has a
properly grounded protective-ground
contact.
Ventilation
Always ensure adequate ventilation when
operating the SR560. The unit will generate
heat while charging dead batteries.
Power-Up
All instrument settings are stored in
nonvolatile memory (RAM backed-up) and
are retained when the power is turned off.
They are not affected by the removal of the
line cord. If the power-on self test passes,
the unit will return to the settings in effect
when the power was last turned off. If an
error is detected or if the backup battery is
exhausted, the default settings will be used.
Additionally, if the RESET key is held down
when the power is turned on, the instrument
settings will be set to the defaults shown
below:
Parameter
SOURCE Channel A
COUPLING DC
INVERT OFF
Setting
2
INTRODUCTION AND SETUP
ROLLOFF bypassed
HIGH-PASS 0.03 Hz, +6 dB/oct
LOW-PASS 1 MHz, -6 dB/oct
GAIN MODE High Dynamic
Reserve
GAIN 20, calibrated
LISTEN ON
DEVICE ADDRESS As per SW601
Repackaging for Shipment
The original packing materials should be
saved for reshipment of the SR560. If the
original packing materials are not available,
wrap the instrument in polyethylene
sheeting or equivalent and place in a strong
box, cushioning it on all sides by at least
three inches of high-density foam or other
filler material.
Use in Biomedical Applications
Under certain conditions, the SR560 may
prove to be unsafe for applications involving
human subjects. Incorrect grounding,
component failure, and excessive commonmode input voltages are examples of
conditions in which the instrument may
expose the subject to large input currents.
Therefore, Stanford Research Systems
does not recommend the SR560 for such
applications.
Accessories Furnished
- Power cable
- Operating Manual
Environmental Conditions
OPERATING
Temperature: 10°C to 40°C
Relative Humidity: <90% Non-condensing
NON-OPERATING
Temperature: -25°C to +65°C
Relative Humidity: <95% Non-condensing
Warning regarding battery
maintenance
Batteries used in this instrument are seal
lead acid batteries. With usage and time
these batteries can leak. Always use and
store this instrument in the feet-down
position. To prevent possible damage to the
circuitboard, it is recommended that the
batteries be periodically inspected for any
signs of leakage.
Warning Regarding Use with
Photomultipliers
The front-end amplifier of this instrument is
easily damaged if a photomultiplier is used
improperly with the amplifier. When left
completely unterminated, a cable connected
to a PMT can charge to several hundred
volts in a relatively short time. If this cable
is connected to the inputs of the SR560, the
stored charge may damage the front-end
FETs. To avoid this problem, provide a
leakage path of about 100 kΩ to ground
inside the base of the PMT to prevent
charge accumulation.
3
4
SPECIFICATIONS
SR560 LOW-NOISE PREAMPLIFIER SPECIFICATIONS CHART
Inputs
Impedance
Maximum Inputs
Maximum Output
Noise
CMRR
Gain
Frequency Response
Gain Stability
DC Drift
Filters
Distortion
Power
Battery Life
Charge Time
Mechanical
Dimensions
Warranty
Single-ended or true differential
100 MΩ + 25 pF, DC-coupled
1 VDC before overload; 3 V peak to peak max AC coupled;
protected to 100 VDC
10 Vpp
<4 nV/√Hz at 1 kHz
>90 dB to 1 kHz, decreasing by 6 dB / octave (20 dB / decade)
above 1 kHz
1 to 50,000 in 1-2-5 sequence
vernier gain in 0.5% steps
Gains up to 1000, small signal
±0.5 dB to 1 MHz
±0.3 dB to 300 kHz
-3 dB at 1 MHz, 1 Vpp output
200 ppm /°C
5 µV/°C referred to input (DC coupled)
0.03 Hz to 1 MHz, 10% typical accuracy
0.01% typical
100, 120, 220, 240 VAC (50/60 Hz), 60 Watts Max
Internal Batteries: 3 x 12 V, 1.9 Ah sealed
lead-acid (rechargeable)
±12 VDC in / out through rear panel banana jacks.
15 hours nominal
250-1000 charge / discharge cycles
4 hours to 80% of capacity
1/2 Rack-Mount width, 3 1/2" height, weight 15 lbs.
14-7/8" x 8-1/8" x 3-1/2"
1 year parts and labor on materials and workmanship
5
SPECIFICATIONS
6
OPERATION AND CONTROLS
Figure 2: SR560 Front Panel
FRONT PANEL OPERATING SUMMARY
The operation of the SR560 Low-Noise
Preamplifier has been designed to be as
simple as possible. The effect of each
keypress on the front panel is reflected in
the change of a nearby LED. The front
panel LED’s will remain lighted at all times
unless dip switch SW601 (accessible
through the bottom cover of the unit)
positions 3 and 4 are placed in the "off"
position. All front panel functions can be
controlled through the rear-panel RS-232
interface.
Power
The SR560 is turned on by depressing the
POWER switch. When disconnected from
AC power, the unit will operate for
approximately 15 hours on internal sealed
lead-acid batteries. Up to 200 mA of
unregulated battery power is available at the
rear panel banana jacks as long as the
power switch is in the ON position. Battery
life will be reduced when the unit is
providing external power through the rear
panel jacks. When operating on batteries,
the front panel "BATT" indicator will be
lighted. As the batteries near exhaustion,
this indicator will change from green to red,
indicating that the unit should be connected
to AC power to charge the batteries.
When connected to an AC power source,
amplifier power is derived from regulated
line power, and the internal batteries are
automatically charged. When operating on
AC power, the front panel "LINE" indicator is
on to indicate the source of amplifier power.
Charging status is indicated on the rear
panel by the "CHARGE" and "MAINTAIN"
LED indicators.
Source
There are two input connectors located in
the SOURCE section of the front panel.
The pushbutton located between them
selects either single-ended (A or B) or
differential (A-B) inputs.
The A and B inputs are voltage inputs with
100 MΩ, 25 pF input impedance. Their
connector shields are completely isolated
from chassis ground, but can be made
7
OPERATION AND CONTROLS
common with chassis ground by connecting
the "AMP GROUND" and "CHASSIS
GROUND" banana jacks on the rear panel
of the SR560. When connected to AC
power, the chassis of the unit is always
connected to the grounding conductor of the
AC power cord. The inputs are protected to
100 VDC but the DC input should never
exceed 10 Vp. The maximum DC input
before overload is 1 V peak.
The COUPLING pushbutton selects the
method of connecting the A and B inputs to
the amplifier. The inputs can be AC (0.03
Hz - 3 dB) or DC-coupled, or the inputs to
the amplifier can be internally grounded with
the A and B input BNC’s left floating. This
feature makes for simple offset nulling,
particularly useful when operating the
amplifier DC-coupled at high gains. Please
refer to CALIBRATION AND REPAIR -OFFSET ADJUSTMENT for information on
the offset nulling procedure.
NOTE
: When the coupling is set to AC, a
0.03 Hz cutoff high-pass filter is always
engaged. All high-pass filter modes can still
be selected while AC-coupled, but the 0.03
Hz filter will always be in, even if the filters
are set to DC. Because one of the two filter
sections is always used as a high pass
when AC coupling is selected, low-pass
filters are only available with a 6 dB / octave
rolloff.
The INVERT pushbutton allows the user to
invert the output of the instrument with
respect to the input when operating with
single-ended or differential inputs. The
INVERT LED displays the output sense
relative to the input for all SOURCE
settings.
Filters
The SR560 contains two identical 1st-order
R-C filters whose cutoff frequencies and
topology (high-pass or low-pass) are
controlled from the front panel. The
maximum bandwidth of the instrument is 1
MHz. The filters in the FILTER CUTOFFS
section can be configured in the following
six ways:
i. high-pass filter at +12 dB / octave
ii. high-pass filter at +6 dB / octave
iii. high-pass filter at +6 dB / octave,
and low-pass filter at -6 dB /
octave (band-pass)
iv. low-pass filter at -6 dB / octave
v. low-pass filter at -12 dB / octave
vi. no filters in the signal path
The filter settings are controlled by the
ROLLOFF, HIGH-PASS and LOW-PASS
pushbuttons. Each time the ROLLOFF
pushbutton is pressed the instrument
configures the two R-C filters to conform to
the progression shown above. The four
ROLLOFF LED’s give a visual indication of
the current filter configuration. For the
HIGH-PASS filter the left pushbutton serves
to decrease its cutoff frequency. The two
pushbuttons for the LOW-PASS filter
function in an analogous manner.
When the FILTER CUTOFFS section is
configured solely as high-pass or low-pass
(i, ii, iv and v ), the cutoff frequency is
illuminated by one of sixteen LED’s in the
range from 0.03 Hz to 1 MHz, and the slope
of the rolloff is shown by one of the four
ROLLOFF LED’s. When the filter section is
configured as band-pass (iii), the cutoff
frequencies are illuminated by two LED’s.
The frequency setting on the left marks the
cutoff for the high-pass filter, and the setting
on the right is the cutoff for the low-pass
filter. The two 6 dB / oct ROLLOFF LED’s
are also illuminated. In this case the two
cutoffs can be set to the same frequency to
provide a narrow bandpass. When both
filters are removed from the signal path (vi)
all rolloff and cutoff frequency LED’s are
extinguished from the FILTER CUTOFFS
section and the DC LED is on.
NOTE
for the four highest frequency settings. See
the note under Source: Coupling for
: High pass filters are not available
8
OPERATION AND CONTROLS
information on using filters with the amplifier
in AC coupled mode.
Gain Mode
The allocation of gain throughout the
instrument is set using the GAIN MODE
pushbutton The Gain Mode is displayed by
two indicator LED’s: HIGH DYNAMIC
RESERVE and LOW NOISE. For a given
gain setting, a HIGH DYNAMIC RESERVE
allocates the SR560's gain toward the
output stages after the filters. This prevents
signals, which are attenuated by the filters
from overloading the amplifier. The LOW
NOISE setting allocates gain toward the
front-end in order to quickly "lift" low-level
(nV range) signals above the instrument's
noise floor.
Gain
The instrument's gain is increased or
decreased using the GAIN pushbuttons.
Gain settings from 1 to 50,000 are available
and are displayed as the product of a factor
1, 2 or 5 and a multiplier (none (i.e. 1), 10,
100, 1,000 or 10,000). In addition to these
fifteen fixed gain settings, the user may
specify arbitrary gains through the UNCAL
feature. To set an uncalibrated or arbitrary
gain the user must press both Gain buttons
simultaneously, lighting the UNCAL LED. In
this mode by pressing the Gain Up or Gain
Down pushbuttons, the user may reduce the
calibrated gain in roughly 1% increments
from 100% down to 0% of the selected gain.
In contrast to other front-panel functions,
when in UNCAL the instrument's key-repeat
rate will start slowly and increase to a limit
as long as either Gain button is depressed.
Simultaneously pressing both Gain buttons
once again will restore the unit to the
previously calibrated gain setting, and turn
off the UNCAL LED.
Output
The outputs of the instrument are located
within the OUTPUT section of the front
panel. Two insulated BNCs are provided: a
600Ω output and a 50Ω output. The
amplifier normally drives high impedance
loads and the instrument's gain is calibrated
for high impedance loads. When driving a
600Ω load via the 600Ω output (or a
50Ω load via the 50Ω output) the gain of the
amplifier is reduced by two. The shields of
all the front-panel BNC’s are connected
together and form the amplifier's floating
ground.
Reset
The OVLD LED indicates a signal overload.
This condition can occur when a signal is
too large or the dynamic reserve is too low.
Reducing the gain, reducing the input signal
and/or switching to the HIGH DYNAMIC
RESERVE setting should remedy this
condition. If an overload occurs with filter
settings of long time constants, the RESET
pushbutton will speed the SR560's recovery
from overload.
Status
The ACT LED indicates communications
activity over the SR560's optoisolated RS232 port. Please refer to Appendix A:
Remote Programming for further details on
programming the instrument via RS-232.
The BLANK LED indicates the optoisolated
BLANKING input (on the rear panel of the
SR560) is active. The SR560 responds to a
blanking input by internally grounding the
amplifier signal path after the front end and
before the first filter stage.
9
OPERATION AND CONTROLS
Figure 3: SR560 Rear Panel
REAR PANEL OPERATING SUMMARY
The SR560 rear panel is pictured in Figure
3. Various interface and power connectors
are provided, along with fuses and charger
status LEDs.
AC Power Input
The power entry module contains the
receptacle for the AC line cord and fuse.
The line fuse should be a 1 A slow-blow for
100/120 VAC operation, or a 1/2 A slowblow for 220/240 VAC operation.
Amplifier Power Output
The -12 V, +12 V, and AMP GROUND
banana jacks provide external DC power up
to 200 mA for use as a bias source
referenced to the amplifier's floating power
supplies.
The CHASSIS GROUND banana jack is
provided to allow the amplifier's ground to
be referenced to the chassis. If the unit is
connected to an AC power source via a
three prong grounding plug, the chassis
ground is connected to the AC line ground
conductor.
Battery Charger
The two 3 A slow-blow fuses protect the
battery supply and charging circuitry. If
these fuses are blown, battery power will be
unavailable, and charging of the batteries
will not be possible.
When both the positive and negative supply
batteries are dead, the red "CHARGE" LED
will be on brightly, and the batteries will be
charging at a fast rate. When the batteries
approach a fully charged condition, the
charging current will be reduced to complete
the charge and maintain the batteries.
Because the batteries charge at different
rates, the indicators on the rear panel can
reflect the charge status of the positive and
negative batteries independently. When
one set of batteries switches to the
"MAINTAIN" mode, the red "CHARGE" LED
will be reduced to half brightness, and the
yellow "MAINTAIN" LED will turn on at half
brightness. When both batteries switch to
"MAINTAIN", the red "CHARGE" LED will
10
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