Varian ProStar 340, Operation Manual

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True double beam operation is provided by a fiberoptic beam
splitter. A reference photodiode continuously monitors the light
from one leg of the beam splitter. The other leg is imaged by a
lens through a sample cell onto the sample photodiode.
The photodiodes are connected to individual preamplifiers. The
preamp output voltages are directly proportional to the light
intensity at the photodiode active surfaces (see Figure 2).
The outputs of the preamplifiers are sent to an analog
ratiometer. The output of this circuit is a voltage that is directly
proportional to the negative logarithm of the ratio of the sample
signal to the reference signal. This voltage is supplied to the rear
panel as the 1V/AU integrator output and as the adjusted
absorbance full scale recorder output.
SAMPLE
AMPLIFIER
AUTO
ZERO
REFERENCE
AMPLIFIER
AUTO
ZERO
PHOTODIODES
LOG
S/R
RANGE
EVENT
OUTPUT
FILTER
INTEGRATOR
OUTPUT
1AU/V
RECORDER
OUTPUT
Figure 2 Electronics Block Diagram