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Operation and Controls
the batteries from an SR570 with no AC power
connected will reset the unit to the default state.
The default settings are:
Sensitivity = 1 µA/V, calibrated
Invert = off
Input Offset = +1 pA, calibrated, off
Bias = 0 V, off
Filters = none
Hi Freq = 0.03 Hz
Lo Freq = 1 MHz
Gain Mode = Low Noise
Bias Voltage
In the default configuration, the SR570 is a
virtual null at the input BNC. The bias voltage
provides a variable -5V to +5V voltage (12 bit,
1.22 mV resolution) at the input. This voltage
can be used to bias a photodiode or similar
device. The voltage level is set by the up/down
arrows in the bias voltage section of the front
panel. The up arrow increases the voltage
towards +5V, and the down arrow decreases the
voltage towards -5V. To enable the bias voltage,
simply push the button directly below the bias
ON LED. The selected voltage can be monitored
at the TEST point with a DC voltmeter whether
the bias voltage is turned on or not.
Input Offset Current
The SR570 can provide a DC current offset to
suppress any background currents at the input.
The offset range can be changed from 1 pA to 5
mA (both positive and negative) in discrete
increments. Use the up/down arrow keys in the
Input Offset section to change the current level.
In addition to these fixed settings, the user may
specify arbitrary currents through the UNCAL
feature. To set an uncalibrated offset current, the
user must press both up and down buttons
simultaneously, lighting the UNCAL LED. In
this mode, by pressing the up or down
pushbuttons, the user may reduce the calibrated
current in roughly 0.1% increments from 100%
down to 0% of the selected offset value. 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 button is depressed. Simultaneously
pressing both Offset buttons once again will
restore the unit to the previously calibrated current
setting, and turn off the UNCAL LED. The sign of
the current is set with the button directly below the
POS and NEG LEDs. A positive offset current is
defined to be a current that will produce a positive
output voltage with no signal connected to the input
BNC and INVERT not selected. The button below
the input offset ON LED turns the offset on and off.
The current level can be adjusted whether the offset
current is turned on or not.
Invert
The INVERT pushbutton allows the user to invert
the output of the instrument with respect to the
input. A positive current will give a negative voltage
and visa versa. The INVERT LED displays the
output sense relative to the input unless the
TOGGLE feature is being used.
Filters
The SR570 contains two identical 1st-order R-C
filters whose cutoff frequencies and configuration
(high-pass or low-pass) are controlled from the front
panel. The maximum bandwidth of the instrument is
1 MHz.
The FILTER CUTOFFS can be configured in the
following six ways:
i. high-pass filter at +6 dB / octave
ii. high-pass filter at +12 dB / octave
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
Filter settings are chosen by the FILTER TYPE
pushbutton. Each time the FILTER TYPE
pushbutton is pressed, the instrument configures the
two R-C filters in the progression shown above.
LEDs give a visual indication of the filter
configuration.
The filter cutoff frequencies are controlled by the
up/down arrows in the FILTER FREQ section.
When the FILTER TYPE section is configured
solely as high-pass or low-pass (i, ii, iv and v ), the
cutoff frequency is illuminated by one of sixteen
4
iii.
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