Hologic DISCOVERY A, Technical Manual

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Discovery QDR Series Technical Manual
2-20
2.10 Data Acquisition System (C, W, and SL)
The Discovery Data Acquisition System (DAS) consists of a single board that has 128
solid state (diode) detectors, the integration circuitry along with the analog-to-digital
conversion circuitry, and multiplexing circuitry to select the outputs in a sequential
fashion. The solid-state detectors are photodiodes with a cadmium tungstate (CdWO
4
)
crystal attached. X-rays striking the cadmium tungstate crystals are converted into photons
of visible light. The diodes sense this light and convert the light into a current, which is
amplified in a current to voltage converter. The converter output is applied to an integrator
through analog switches. The analog switches all operate in parallel and are turned on
during the X-ray pulse. These switches are turned off during the integrator hold time to
prevent the introduction of integration noise into the data. The turning on and off of these
switches is controlled by the INTEGRATE signal under control of the software. The
signal from the detectors is then integrated with the final voltage obtained held in a
sample/hold circuit. This voltage is sampled, in photodiode order, by the multiplexor
circuitry under control of the software. The output voltage is then sent to the analog-to-
digital conversion circuitry where it is changed to a digital value usable by the software.
The Detector Assembly is located in the upper end of the C-arm directly above the X-ray
source. There is a lead radiation shield between the detector board and the cover to stop
any X-rays that may get through the detectors.
2.10.1 Solid State Detector
The Solid State Detector converts X-rays into signals that are applied to the Integrator/
Multiplexor section of the Detector Assembly board. Each Detector Assembly contains
128 high-resolution detectors. Each detector is 2mm wide at the detector. This equates to
slightly less than 1mm resolution in an AP spine.
2.10.2 Integrator/Multiplexor Subsection
The Integrator/Multiplexor subsection of the Detector Assembly receives up 128 signals
in parallel from the Solid State Detector Boards. This subsection integrates and stores
those signals and then applies the stored signals to the Analog to Digital subsection of the
Detector Assembly.
Switched signals charge integrating capacitors in this subsection during a given charging
time. After the charging time, the switched signals are turned off and the charges are held
on the capacitors. Each integrator is sampled, in photodiode order, by a multiplexor and
the output sent to the Analog to Digital subsection. At the end of sampling, all the
integrating capacitors are discharged in parallel by shorting them out with analog
switches.
Each integrator has an additional input into which a test signal (TESTLVL) can be
applied when there are no X-rays present. This test signal is used to verify the operation of
the integrators and multiplexors by the SQVERIFY diagnostic program.