Operation And Controls; Introduction; Overview; Quick Start Instructions - Stanford Research Systems SR570 User Manual

INTRODUCTION

Why use a Current Amplifier?
Many people wonder why current amplifiers are
necessary. Why not simply terminate a current
source with a resistor and amplify the resulting
voltage with a voltage preamplifier? The answer
is twofold. First, to get a large voltage from a
small current, large resistors are necessary. In
combination with cable capacitance and other
stray capacitance, this can lead to unacceptable
penalties in frequency response and phase
accuracy. Current amplifiers have much better
amplitude and phase accuracy in the presence of
stray capacitance. Secondly, using resistive
terminations forces the current source to operate
into possibly large bias voltages–a situation that
is unacceptable for many sources and detectors.
Current amplifiers can sink current directly into
a virtual null, or to a selected DC bias voltage.

Overview

The SR570 is a low-noise current preamplifier,
providing a voltage output proportional to the
input current. Sensitivities range from 1 mA/V
down to 1 pA/V. The general architecture is
diagrammed in figure 1 on the following page.
The DC voltage at the input can be set as a
virtual null or biased from -5V to +5V. An input
offset current from 1pA to 1 mA may also be
introduced. The user can choose between low
noise, high bandwidth, and low drift settings,
and can invert the output relative to the input.
Two configurable R-C filters are provided to
selectively condition signals in the frequency
range from DC to 1 MHz.
The SR570 normally operates with a fully
floating ground with the amplifier ground
isolated from the chassis and the AC power
supply. Input blanking, output toggling and
listen-only RS-232 interface lines are provided
for remote instrument control. These lines are
optically isolated to reduce signal interference.
Digital noise is eliminated by shutting down the
processor clock when not executing a front-
panel button press or an RS-232 command.
Internal sealed lead-acid batteries provide up to
15 hours of line-independent operation. Rear
panel banana jacks provide access to the internal
regulated power supplies (or batteries) for use as a
voltage source.
Use this procedure as a quick orientation to the
instrument's features and capabilities. If you
encounter problems, read the detailed discussions
on operation.
1) Make sure that the correct line voltage has been
selected on the rear panel power entry module.
2) With the unit's power switch "OFF", hold the
"FILTER RESET" key down and turn the unit
"ON". This will return all instrument settings to their
default state.
3) Select a filter from the "FILTER TYPE" menu.
Then use the up/down arrows of the "FILTER
FREQ" menu to choose the filter 3 dB points.
4) If an input offset current is desired, choose a
current level from the "INPUT OFFSET" menu with
the up/down arrow keys. The current will be applied
when the "ON" led is lit.
5) When the bias voltage is off, the amplifier input
is a virtual null. To set a bias voltage, use the
up/down arrow keys of the "BIAS VOLTAGE"
menu. The test point will always reflect the selected
bias voltage, but the bias will only be applied when
the "ON" led is lit.
6) Set the sensitivity and gain mode to the desired
settings for the the amplitude of the signal to be
measured.
7) Adjust the "FREQ COMP" pot near the input
BNC to compensate the amplifier's frequency
response for any input capacitance. An external
square wave signal from the source under test can be
used for precise calibration.
8) Connect the signal to be measured to the
"INPUT" BNC. The signal will be converted to a
voltage, filtered and amplified. The amplifier output
voltage can be accessed from the "OUTPUT" BNC
connector.
1

Operation and Controls