2222
Technical Description
This monitors the incoming signal and is used to inhibit the clock when the signal is within the
ranges preset.
Window Comparator
The Counter provides the DAC with a six bit binary output which increments until inhibited by
the Window Comparator.
Counter
The Digital to Analogue Converter, provides the emitter control circuitry with the necessary drive
voltage when the received signal falls within the ranges set by the Window Comparator.
Digital to Analogue Convertor (DAC)
This section regulates the drive current to the transmitter LED's and ensures that the drive
current is sufficient to set the receive signal within the limits set by the Window Comparator.
Emitter Control
The Amplifier allows the multiplexer to operate at maximum resolution, this signal amplified
from the DAC allows the RD2 main unit to obtain the LED transmitter intensity.
Amplifier
This circuitry stores the signal from the receive Photodiode when selected by the TARE line. The
TARE is selected periodically until the received signal stabilises and the test can begin.
Sample and Hold
The second Differential Amplifier compares the signal from the Sample and Hold section with
the receiver Photodiode signal and amplifies the difference.
This provides the RD2 via the multiplexer with the change in dc level since the last TARE signal.
Differential Amplifier -2
The Multiplexer alternates the output between the LED drive current and signal output.
Multiplexer
The Differential Amplifier is used to remove the majority of the dc offset from the input amplifier.
This is necessary as the dc signal from the Photodiode is much larger than the change that is
to be studied.
Differential Amplifier-1
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