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Theory of Operation
1780R-Series Service Manual
3–103
Analog Multiplexers. U346 and U348 are Analog Multiplexers. Each has one
input and eight outputs; the input is the analog signal level from the DAC. The
outputs are normally high impedance disconnects, except when one is selected
by the digital word at inputs A, B, and C.
U248 is a latch that takes a “snapshot in time” of the information on the data
bus, and uses it as steering information to the Analog Multiplexers. By properly
timing the steering information, the voltage from the DAC is routed to the
desired output. Only one Analog Multiplexer is enabled at one time by bringing
the INH pin low.
The Analog Multiplexers time share the DAC signal. In addition, the DAC input
from the Microprocessor Data Bus is time shared with the rest of the instrument.
Because of the input and output time sharing, the outputs rarely are connected to
the DAC. To compensate for this, each output of the Analog Multiplexers drives
a sample-and-hold capacitor. The capacitors hold the DAC value between
samples; their output drives a Buffer Output Amplifier.