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3Ć28
The Acquisition system consists of two boards: the A27 Acquisition Analog
board and the A28 Acquisition MPU board.
The A27 Acquisition Analog board consists of the following major blocks:
Programmable gain amplifiers
A/Dconverters
Measurementhardware
Timingand sampling head control
The A28 Acquisition Analog board consists of the following major blocks:
Shared RAM
Timebase/controller interface
Microprocessor
Measurementhardware
Timingand sampling head control
See Figures 9Ć14 and 9Ć15 for the block diagrams of these boards.
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The four sampling head outputs are multiplexed to two amplifier channels
through the inputmultiplexer. The microprocessor selects a sampling head
channel to be acquired and connects it to one of the amplifier channels. The
programmable gain amplifier then applies offset (VertPos) and amplifies or
attenuates the signal (VertSize). The output of the amplifier is applied to the
analog to digital (A/D) converter for conversion to digitized data.
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The output of each programmable gain amplifier is applied to an 8Ćbit flash
A/Dconverter. The converter compares its analog input to two reference
voltages (+1 V REF and -1 V REF) and outputs an 8Ćbit binary number that
indicates the value of the analog input signal in relation to the reference
voltages. The digital output from the A/D converters is stored in the shared
RAM and made available for transfer to the WaveformMemory through the
TimeBase/Controller.
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This circuitry has the selected sampling head output from the input multiĆ
plexer applied to a precisionĆstrobed voltage comparator.This voltage
comparator compares the sampling head output voltage to a reference
voltage generated by a 14Ćbit digitalĆtoĆanalog converter (DAC).The
comparator output is then filtered by a filter and transition counter circuit on
the A28 Acquisition MPU board to remove false transitions generated by
noise and to count the transitions.
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