HP 8112A, Operating, Programming And Servicing Manual

Page: 246 / 406

Timing Range Decoder
DAC Reference Circuit
Digital to Analog
Converters
The timing range decoder U2, controlled by RCT from U l , latches
data from the data bus which is then used to select the timing ranges
required by the period, delay, width and transition time control
sources.
Refer to Figure 10.5-4. The references are used by DACs. Op amps
U lOA and UlOB with associated components, provide constant
current sources of
-9 V and - 1 V, derived from the -15 V and
+5
V
supplies.
Normal operation
0V
0V
0V
Figure 1 0.5-1 . Principle of DAC Operation
The amplitude control DAC
(
U23
)
operates on the principle
summarised in Figure 10.5-1. The current Iin comes from a
reference voltage and is repeatedly divided by two at each branch
of the resistance network. This process provides a series of binary
current-fractions which are switched to ground or the summing
point
X. Each switch is operated by the data bit which has the same
significance as the current it controls
(
A=Most Significant Bit
)
. The
total current summed at X therefore represents the data value as a
fraction of lin· For example, in Figure 10.5-1 with all three switches
on
(
all three data bits on
)
:
I
fin fin fin
7 [ x = 2 + 4 + 8 = 8 in
10.5-2 Servicing the Control Board