Dc Outputs And Scaling - Stanford Research Systems SR810 Manual

SR810 Basics

DC OUTPUTS and SCALING

The SR810 has X and Y outputs on the rear panel
and a Channel 1 output on the front panel.
X and Y Rear Panel Outputs
The X and Y rear panel outputs are the outputs
from the two phase sensitive detectors with low
pass filtering, offset and expand. These outputs
are the traditional outputs of an analog lock-in.
The X and Y outputs have an output bandwidth of
100 kHz.
CH1 Front Panel Output
The front panel output can be configured to output
voltages proportional to the CH1 display or X.
If the output is set to X, the output duplicates the
rear panel X output.
If the output is set to Display, the output is updated
at 512 Hz. The CH1 display can be defined as X,
R, X Noise, Aux Input 1 or 2, or any of these
quantities divided by Aux Input 1 or 2. If the
display is defined as simply X, this display, when
output through the CH1 output BNC, will only
update at 512 Hz. It is better in this case to set
output to X directly, rather than the display.
X, Y and R Output scales
The sensitivity of the lock-in is the rms amplitude
of an input sine (at the reference frequency) which
results in a full scale DC output. Traditionally, full
scale means 10 VDC at the X, Y or R BNC output.
The overall gain (input to output) of the amplifier is
then 10 V/sensitivity. This gain is distributed
between AC gain before the PSD and DC gain
following the PSD. Changing the dynamic reserve
at a given sensitivity changes the gain distribution
while keeping the overall gain constant.
The SR810 considers 10 V to be full scale for any
output proportional to simply X, Y or R. This is the
output scale for the X and Y rear panel outputs as
well as the CH1 output when configured to output
X. When the CH1 output is proportional to a
display which is simply defined as X or R, the
output scale is also 10 V full scale.
Lock-in amplifiers are designed to measure the
RMS value of the AC input signal. All sensitivities
and X, Y and R outputs and displays are RMS
values.
Phase is a quantity which ranges from -180° to
+180° regardless of the sensitivity. The measured
phase is only available from the interface.
X, Y and R Output Offset and Expand
The SR810 has the ability to offset the X, Y and R
outputs. This is useful when measuring deviations
in the signal around some nominal value. The
offset can be set so that the output is offset to
zero. Changes in the output can then be read
directly from the display or output voltages. The
offset is specified as a percentage of full scale and
the percentage does not change when the
sensitivity is changed. Offsets up to ±105 % can
be programmed.
The X, Y and R outputs may also be expanded.
This simply takes the output (minus its offset) and
multiplies by an expansion factor. Thus, a signal
which is only 10 % of full scale can be expanded
to provide 10 V of output rather than only 1 V. The
normal use for expand is to expand the
measurement resolution around some value which
is not zero. For example, suppose a signal has a
nominal value of 0.9 mV and we want to measure
small deviations, say 10 µV or so, in the signal.
The sensitivity of the lock-in needs to be 1 mV to
accommodate the nominal signal. If the offset is
set so to 90% of full scale, then the nominal 0.9
mV signal will result in a zero output. The 10 µV
deviations in the signal only provide 100 mV of DC
output. If the output is expanded by 10, these
small deviations are magnified by 10 and provide
outputs of 1 VDC.
The SR810 can expand the output by 10 or 100
provided the expanded output does not exceed full
scale. In the above example, the 10 µV deviations
can be expanded by 100 times before they exceed
full scale (at 1 mV sensitivity).
The analog output with offset and expand is
Output = (signal/sensitivity - offset) x Expand x10V
where offset is a fraction of 1 (50 %=0.5), expand
is 1, 10 or 100, and the output can not exceed 10
V.
In the above example,
Output = (0.91mV/1mV - 0.9) x 10 x 10V = 1V
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