Theory of Operation 4
Keysight E3633A and E3634A Service Guide 105
D-to-A Converter
All reference voltages of power circuits are derived from the internal voltage
reference of system DAC U22. The system DAC track/hold amplifier outputs are
used to provide controllable reference voltages to power circuits. The system DAC
is programmed and responds to the main controller via internal 3-wire serial data
bus SERCLK, SERRBK, and SERSTB. The system DAC is multiplexed to 6 track/
hold amplifiers through U26. Each track/hold amplifier is refreshed approximately
every 1 msec to maintain its output setting. Changes to track/hold amplifier
outputs are accomplished by dwelling on that position for an extended period.
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