HP 8112A, Operating, Programming And Servicing Manual

Page: 248 / 406

Parameter Control
The time difference between LRC, enabling U23, and LRC, enabling
new data for U20, is the propagation delay through U19.
Offset DAC
The offset level output from U21 can be attenuated using R42.
The offset DAC U20 is a 1 2-bit multiplying device which provides
two output currents.
loutl
is the summed current derived from the
reference voltage via the selected networks.
(Ix
in "Digital to
Analog Converters" ) .
Iout2
is the sum of the unselected currents and
hence:
Ioutl + f out2
=
fin
The remaining DACs, U4-U6, Ul2, Ul3 and U23, convert the digital
vernier values input to the microprocessor from the instrument front
panel or system controller, into analogue signals for use by:
•
Timing ICs on Al Main Board which generate the required period,
delay and width delay signals.
•
Slope IC on Al Main Board which generates the leading and
trailing edges of the output pulse.
•
Shaper IC on the Al Main Board which governs the final
amplitude of the output pulse.
Range switching circuits are included in the outputs of these DACs,
that are selected automatically by the microprocessor in conjunction
with the Timing range decoder.
10.5-4 Servicing the Control Board