Stanford Research Systems SR844, User Manual

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Outputs, Offsets and Expands 1-15
SR844 RF Lock-In Amplifier
7 Press CH1
Offset Modify.
The offset of the CH1 display quantity is shown on
the Reference display. The reading is in percent of
full scale. In this case, the Xoffset should be about -
54% (of 1 Vrms).
Note!
The entered offset percentage does not change when
the sensitivity is changed. However, it does change
if the reference phase is changed (see above).
8 Use the knob to adjust the offset until the X
display is 0.1 V.
The displayed value of X should be close to
0.1 Vrms. The offset should be about 44% and the
CH1 output voltage (see the formula in step 4
above) should be
(0.54 V/1.0 V - 0.44)
×
1
×
10V = 1.0 V
or nearly so.
9 Press CH1
Expand
.
With an expand of
××××
10
, the display has one more
digit of resolution (100 mV full scale). The
Expand indicator turns on at the bottom of the
Channel 1 display to indicate that the displayed
quantity has been expanded. The output voltage
should now be
(0.54 V/1.0 V - 0.44)
×
10
×
10V = 10 V
or nearly so.
The expand allows the output gain to be increased
by 10 or 100. To use output Expand, it is necessary
to have a display reading that is less than 10% or
1% of the full scale sensitivity. This can be
achieved using offsets if necessary.
The maximum output is limited to 110% of the
display full scale (Sensitivity
÷
Expand). Any
greater output will turn on the OVLD indicator
above the CH1 OUTPUT connector. The
OVLD
indicator within the CH1 display will also turn on.
With offset and expand, the output voltage gain and
offset can be programmed to provide control of
feedback signals with proper bias and gain for a
variety of situations.
Offsets add and subtract from the display. Expand
increases the resolution of the display and the gain
of the analog output.