Stanford Research Systems SR445 User Manual

MODEL SR445

FAST PREAMPLIFIER

MODEL SR445

FAST PREAMPLIFIER

1290-D Reamwood Avenue

Sunnyvale, California 94089

Phone: [phone] • Fax: [phone]

email: [e-mail] • www.thinkSRS.com

Copyright © 1990 by SRS, Inc.

All Rights Reserved.

Revision 1.2 (10/2001)

INTRODUCTION

The Model SR445 Fast Preamp contains four
wide bandwidth, DC coupled amplifiers
designed to be used independently or cascaded
to provide gains of 5, 25, and 125.

STANFORD RESEARCH SYSTEMS DC-300 MHz AMPLIFIER

SRS

500

Ω

50

Ω

MODEL SR445

50 A = 5/CHANNEL

Ω

The fast rise time, low noise, and DC accuracy
of the SR445 make it the ideal instrument for
amplifying outputs of fast photomultiplier
tubes and photodiodes.

INININ OUT OUTOUT

v

Figure 1 - SR445 Front Panel

TYPICAL SPECIFICATIONS

INPUTS 50 Ω impedance, DC coupled, BNC connectors

(channel 1 selectable 50 or 500 Ω).

IN OUT

OUTPUTS DC coupled, BNC connectors (terminate into 50 Ω).
VOLTAGE GAIN 5.0 per channel (up to 3 channels can be cascaded).
BANDWIDTH DC to 300 MHz (-3 dB).
RISE/FALL TIME 1.2 ns.
PROPAGATION DELAY 2.2 ns.
RECOVERY TIME <4 ns for a times 20 overload.
OPERATING RANGE inputs ±200 mV, outputs ±1.0 V.
OVERSHOOT ±5%.
NOISE <50 µV rms referenced to the input, (2.2 nV/√Hz).
STABILITY 10 µV/°C referenced to the inputs (0 to 50°C).
OFFSET input ±50 µV (adjustable).
PROTECTION ±3.5 V DC, ±50 V transient.
MECHANICAL 7.7" x 6.7" x 2".
POWER 16W, 100/120/ 220/240 VAC, 50/60 Hz.
WARRANTY One year parts and labor on materials and workmanship.

OPERATION

The SR445 Fast Preamplifier is useful for
amplifying small signals to levels that allow
processing by other instruments such as the
SR430 Multichannel Scaler, or the SR400
Gated Photon Counter. The four channels may
be used independently or cascaded. The
following table shows the maximum input
voltage for linear operation as a function of
amplitude gain.

Number of
Channels Gain Maximum Input

1 5 200 mV
2 25 40 mV
3 125 8 mV

The input impedance of channel 1 can be

increased to approximately 500 Ω by using the

front panel switch. This can improve the
sensitivity to signals from current and charge
output devices such as photo-multiplier tubes.
However, under these conditions cable
capacitance must be kept as low as possible to
preserve the frequency response and rise time
of channel 1. The other three channels are
unaffected.

TROUBLESHOOTING
WARNING: Dangerous voltages are present

on the printed circuit board. Always turn the
power off and disconnect the line cord before
removing the cover or changing components.

There are two rows of 4 holes in the cover,
which access the offset calibration pots. The
first row is the input offset adjust, and the
second row the output offset adjust.

Allow at least 30 minutes for the unit to warm
up before calibrating. To calibrate the input
offset, connect a voltmeter to the input and
adjust the pot at the front hole in line with that
channel to 0.0 V ±10 µV. To calibrate the
output offset, connect a voltmeter to the
output and adjust the pot at the back hole in
line with that channel to 0.0 V ±50 µV.

CIRCUIT DESCRIPTION

Input signal coupling is provided by D1, D2,
D3, and D4 which are biased by R14, R8,
R9, and R27. Positive overload protection is
supplied by the action of Q6 and D10. When
the output reaches +1.25 Volts, Q6 begins to
conduct, stealing bias current from D1 and
D3. Thus, the input voltage is effectively
clamped at ±0.25 Volts. Similarly, Q5 and D8
provide negative overload protection. Input
offsets are nulled out with P1. SW1 allows the
channel 1 input impedance to be increased to

500 Ω by the use of bootstrapping. Q1 and Q2

comprise a fully symmetrical push-pull output
stage that is driven by the complimentary pair
of Q3 and Q4. U1 uses feedback to maintain
DC accuracy and P2 adjusts the output offset
voltage of the amplifier.

OVERLOAD BEHAVIOR

The diodes in the overload protection circuit
can be damaged by excessive voltages at the
input of any stage. To check for damage,
measure for 0.6 Volts across diodes D1
through D4. Recalibration is necessary if any
of the diodes are changed.

CALIBRATION

allowance of the application. A graph of
recovery time versus overload duration is

The SR445 accurately amplifies input signals
from -200 mV to +200 mV. Transient
overloads to ±50 Volts can be safely
accommodated, and DC inputs up to ±3.5
Volts that will not damage the unit. The time
required for the amplifier output to return
from an overload condition is a function of the
duration of the overload and the error

shown in Figure 2. The input referred error
bands are defined as the voltage difference