Design of the Optical Module
10-4 DSX™ System Service Manual
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The following components are included in the control of the system:
MICROPROCESSOR – Overall operation of the module is based on the 84-pin
PLCC version of the Hitachi H8/3337 single-chip microcomputer, which provides
60Kbytes of flash memory (program/data space) and 2Kbytes of RAM. The H8/3337
has the following internal peripheral capabilities. The I/O ports are configured to allow
maximum software control of all AM functions within the limitations of timing and
functionality of the H8/3337 and to minimize external hardware.
ANALOG/DIGITAL CONVERTER – The Analog to Digital Converter is a single
chip, 20-bit, charge digitizing IC manufactured by Burr-Brown which is configured in
the bipolar mode to accept both positive and negative inputs to allow for internal
circuit offsets. The converter obtains its sensitivity and low noise by performing
multiple integrations of the current produced by the photodiode light sensors in each
optical channel. Each photodiode output is individually switched to the A/D input by
the use of an analog multiplexer.
MULTIPLEXER – The 1 reference and 12 channel photodiodes are selected for A/D
conversion using a DG406 (1 of 16) Analog multiplexer. The multiplexer allows the
use of a single A/D converter to read multiple analog inputs.
WELL PHOTODIODES – A single photodiode is used for each of the 12 optical
channels to read the 12 wells in the test plate row. The photodiode currents are
measured in sequence from channel 1 to channel 12.
REFERENCE PHOTODIODE – The Reference Diode is an additional photodiode
which is used to regulate the lamp intensity during plate reads. The diode is exposed to
a portion of the filtered lamp illumination at all times and is part of the closed loop
lamp intensity regulation system composed of the diode, an op-amp gain stage and a
Pulse Width Modulation driver IC.
LAMP PULSE WIDTH (PWM) MODULATION – The lamp PWM IC provides
control of the lamp intensity by rapidly switching the voltage to the lamp ON and
OFF. The ratio of the ON to OFF time determines the lamp's intensity. An external
power transistor is used to perform the actual voltage switching. The control input to
the PWM IC is a variable analog voltage supplied by the D/A output of the
microcomputer.
LAMP REGULATION – The intensity of the lamp is maintained by the PWM IC,
which compares the input control voltage from the microcomputer to the amplified
signal from the Reference Diode. As the lamp intensity drifts lower or higher, the
signal from the Reference Diode follows. The difference between the amplified diode
signal and the applied control voltage is sensed by the PWM IC which then increases
or decreases the ON time to the lamp to bring the resultant diode signal back to the
required matching value. This action occurs very rapidly so that the lamp intensity
deviations are minimized and are not reflected in the plate reading results.
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