System Dacs - Agilent Technologies 33120A Service Manual

Chapter 5 Theory of Operation

System DACs

System DACs
Block 3 on block diagram page 129; Schematic on page 133.
All output amplitudes are derived from the internal voltage reference of
System DAC U303. The system dac track/hold amplifier outputs are used
to provide controllable bias voltages to various analog circuits including
AM modulation depth, square wave amplitude, square wave duty cycle,
output dc offset, and output amplitude level. The System DAC is
programmed and responds to the main controller via the internal 3-wire
serial data bus SERCLK, SERRBK, and SERSTB. The System DAC is
multiplexed to 7 track/hold amplifiers through U304. Each track/ hold
amplifier is refreshed approximately every 3 ms to maintain its output
setting. Changes to track/hold amplifier outputs are accomplished by
dwelling on that position for an extended period.
96