SRS Labs SR445A, Manual

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SR445A Quad Fast Amplifier
Description
The SR445A is a four-channel, 350MHz bandwidth, dc coupled, 50
Ω amplifier with a
gain of 5× (or +14dB) in a half-width 1U chassis. Two, three or four channels may be
cascaded for gains of 25×, 125×, or 625×, (+28dB, +42dB, or +56dB.) The unit uses
BNC connectors for inputs and outputs. The unit is powered from the ac mains by a
universal input power supply.
The full scale input is ±200mV. The input noise is typically 5.2nV/
√ Hz. The output is
linear over ±1V and should be terminated into a 50 Ω load. Output rise and fall times are
1.3ns. The output will recover from a 10× full scale overload in 3ns. The unit is protected
from ±50V, 1µs input overloads.
There are five LEDs on the front panel; one indicates that the unit is powered and “on”
and four indicate when the output signal for the corresponding channel is outside its
linear range, typically ±1.3Vdc. Brief overloads (<5ns) trigger a 10ms flash.
Operation
The input impedance for each channel is 50 Ω . (There is a front panel switch that allows
the input resistance of Channel 1 to be set to either 500 Ω or 50 Ω . The 500 Ω setting
provides high transimpedance gain for current source—such as photomultiplier tubes—
that may be useful when the amplifier is located close to the signal source.) The dc input
voltage must be limited to ±4V to avoid damaging the amplifier front-end. The amplifier
is internally protected from 50V transients of 1µS duration. The 50
Ω input impedance is
intended to terminate 50 Ω coaxial cable such as RG-58.
The amplifiers perform well when cascaded due to their high input return loss and flat
frequency response characteristics. Referenced to the input, the broadband noise (1Hz to
300MHz) is 80µV
rms
. Peak-to-peak noise is typically 5× the RMS value and so one would
expect to see about 10mV
pp
at the output of two cascaded amplifiers, 50mV
pp
at the
output of three cascaded amplifiers, and 250mV
pp
at the output of four cascaded
amplifiers.