Gain Mode; Sensitivity; Output; Filter Reset - Stanford Research Systems SR570 User Manual

LEDs in the range from 0.03 Hz to 1 MHz. High
pass filters are not available for the four highest
frequency settings. When the filter section is
configured as band-pass (iii), the cutoff
frequencies are illuminated by two LEDs. The
lower frequency setting marks the cutoff for the
high-pass filter, and the higher setting is the
cutoff for the low-pass filter.
To change the values of the bandpass cutoff
frequencies, use the up arrow button to change
the lowpass cutoff and the down arrow to
change the highpass cutoff. If the displayed
frequency is already at the highest or lowest
possible choice, then pushing the button again
will cause the frequency to "wrap around" to the
opposite extreme frequency. In this case the two
cutoffs can be set to the same frequency to
provide a narrow bandpass. The highpass
frequency can never exceed the lowpass
frequency. When both filters are removed from
the signal path (vi) all FREQ LEDs are
extinguished and the NONE LED is lit.

Gain Mode

The allocation of gain throughout the instrument
is set using the GAIN MODE pushbutton. The
gain mode feature controls the tradeoffs between
dynamic reserve, bandwidth, and noise in the
amplifier circuits. The Gain Mode is displayed
by three indicator LEDs: LOW NOISE, HIGH
BW, and LOW DRIFT. For a given gain setting,
the LOW NOISE mode allocates gain toward
the front-end in order to quickly "lift" low-level
signals above the instrument's noise floor. The
LOW DRIFT mode allocates the gain just as the
LOW NOISE mode, except the front-end op
amp is switched to one with a very low input
bias current for high sensitivity settings. The
HIGH BW setting allocates more gain toward
the output stages after the filters. Since smaller
values of feedback resistance are needed for the
front-end gain, the bandwidth of the amplifier is
increased over that of the other two settings.
This also prevents signals which are attenuated
by the filters from overloading the amplifier. See
Appendix B for further details of op amp
selection for the different gain modes.

Sensitivity

The instrument's sensitivity is increased or decreased
using the SENSITIVITY section pushbuttons.
Sensitivity settings from 1 pA/V to 1 mA/V are
available and are displayed as the product of a factor
1, 2 or 5 and a multiplier (x1, x10, x100) with the
appropriate units. In addition to these fixed settings,
the user may specify arbitrary sensitivities through
the UNCAL feature.
To set an uncalibrated or arbitrary sensitivity, 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 sensitivity in roughly
1% increments from 100% down to 0% of the
selected sensitivity. 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 sensitivity button is depressed.
Simultaneously pressing both sensitivity buttons
again will restore the unit to the previously
calibrated sensitivity setting, and turn off the
UNCAL LED.

Output

The output of the instrument is an insulated BNC
with a 50 ohm output impedance. In most
applications, the instrument will be used to drive
high impedance loads (e.g. voltmeters or
oscilloscopes). Therefore, the instrument's gain is
calibrated for high impedance loads. When driving
a 50 ohm load, the gain of the amplifier is reduced
by a factor of two. The shields of the two front-panel
BNCs are connected together and form the
amplifier's floating ground. In addition, a balun is
used at the output to reduce common mode noise.
See the end of Appendix A for more details about
the output balun.

Filter Reset

If an overload occurs with filter settings of long time
constants, the FILTER RESET pushbutton will
speed the SR570's recovery from overload. The
filters will be discharged by momentarily grounding
the filter capacitors.
The FILTER RESET button is also used to return
the unit to its default settings. Simply hold down the
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Operation and Controls