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The SR570 is a low-noise current preamplifier capable of current
gains as large as 1 pA/V. High gain and bandwidth, low noise, and
many convenient features make the SR570 ideal for a variety of photonic,
low temperature and other measurements.
Gain
The SR570 has sensitivity settings from 1 pA/V to 1 mA/V that can
be selected in a 1-2-5 sequence. A vernier gain adjustment is also
provided that lets you select any sensitivity in between.
Gain can be allocated to various stages of the amplifier to optimize
the instruments performance. The low noise mode places gain in the
front end of the amplifier for the best noise performance. The high
bandwidth mode allocates gain to the later stages of the amplifier
to improve the frequency response of the front end. In the low drift
mode, the input amplifier is replaced with a very low input-current
op amp, reducing the instrument’s DC drift by up to a factor
1000.
Filters
The SR570 contains two first-order RC filters whose cutoff frequency
and type can be configured from the front panel. Together, the filters
can be configured as a 6 or 12 dB/oct rolloff low-pass or high-pass
filter, or as a 6 dB/oct rolloff band-pass filter. Cutoff frequencies
are adjustable from 0.03 Hz to 1 MHz in a 1-3-10 sequence. A filter
reset button is included to shorten the overload recovery time of
the instrument when long filter time constants are used.
Input Offset and DC Bias
An input offset-current adjustment is provided to suppress any
undesired DC background currents. Offset currents can be specified
from ±1 pA to ±1 mA in roughly 0.1 % increments. The
SR570 also has an adjustable input DC bias voltage (±5 V)
that allows you to directly sink current into a virtual null (analog
ground) or a selected DC bias.
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SR570 Current Noise
vs. Frequency
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Toggle and Blanking
Two rear-panel, opto-isolated TTL inputs provide additional control
of the SR570. A blanking input lets you quickly turn off/on the
instrument's gain which is useful in preventing front-end overloading.
A toggle input inverts the sign of the gain in response to a TTL
signal allowing you to perform synchronous detection with a chopped
signal.
Battery Operation
Three rechargeable lead-acid batteries provide up to 15 hours of
battery-powered operation. An internal battery charger automatically
charges the batteries when the unit is connected to the line. The
charger senses the battery state and adjusts the charging rate accordingly.
Two rear-panel LEDs indicate the charge state of the batteries.
When the batteries become discharged they are automatically disconnected
from the amplifier circuit to avoid battery damage.
No Digital Noise
The microprocessor that runs the SR570 is "asleep" except
during the brief interval it takes to change the instrument's setup.
This ensures that no digital noise will contaminate low-level analog
signals.
RS-232 Interface
The RS-232 interface allows listen-only communication with the
SR570 at 9600 baud. All functions of the instrument (except power
on) can be set via the RS-232 interface. The RS-232 interface electronics
are opto-isolated from the amplifier circuitry to provide maximum
noise immunity.
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