Adc; Ma Low Range; Detector Interface - GE Medical Systems PRODIGY Service Manual

2.5.28 ADC

2.5.29 mA Low Range

2.5.30 Detector Interface

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PRODIGY Service Manual (Rev C - 2000)
A jumper and/or DNP'd resistor pad is provide to drive the IQ_HVPS line,
which the firmware reads to determine which HVPS it is intended to operate.
The PWB provides lemo style connector pads for use with the traditional 0311/
0312 HVPS and MAX CCA. For the 7681 HVPS the lemos are DNP'd and a
single DB-25 connector is used to control the HVPS. Tranzorb pads are
provide on the PWB and are expected to be populated only if deemed
necessary by EMC testing. An opto bank is also provided to support the digital
interface to the 7681. See Lunar dwg 7681-SPC for details.
The Burr Brown ADS8320 16 bit, serial, single channel ADC was chosen for
the cSBC for its high resolution, and excellent accuracy.
An Analog devices AD586 +5V voltage reference is used. The part was
selected for it low noise and high accuracy
Overall the ADC and reference give the cSBC +/-5mV analog accuracy 11.2
DAC's
A single LTC1454 12 bit, serial, dual channel, DAC is used to provide the kV
and mA program voltages. The part was chosen for it low cost, ready
availability at national distributors, and excellent DNL specifications. The DAC
is used in the x2 configuration such that the full scale output is twice the
reference voltage. Voltage outputs feed back to the ADC MUX such that
firmware can calibrate out DAC INL errors.
A single LTC1661 10 bit, serial, dual channel, DAC is used to provide the arc
threshold and filament current limit input voltages to the new 7681 HVPS. The
part was chosen for it low cost, ready availability at national distributors, and
reasonable DNL specifications. Voltage outputs are not fed back to the ADC
MUX as high accuracy is not required on these threshold inputs.
To support future scan modes, an mA low range circuit is provided. The FLEX
PLD provides a control bit by which the firmware can switch the mA DAC
reference voltage from 2.048V to 0.5V, hence decreasing the LSB size, hence
allowing the firmware to take smaller voltage steps when ramping to low uA
settings. A second control bit is provided to switch the ADC from 5.0 to 0.5V
reference.
The cSBC provides a RS-422 communications post to the Detector Mother
Board. The CSBC does not participate in any Detector data manipulation, all
analysis and Detector control is performed at the DMB.
System Overview 2-61