HP 3438A Operating And Service Manual page 58

Section Vll l
-
FEEDBACK
-7"
Figure 8-2. -7 V Regulator.
8-21. +6.5 V (V DISP) Power Supply. 1-he V
D I S P
Supply is fullwave rectified by C R 4 0 2 and CK401 and
regulated by series regulator Q402. 4 4 0 2 gets its
reference from the
+
7 V Supply.
8-22. + 9 V (V D) Power Supply. The V
11
Supply is
fullwave rectified by CK60I a n d CK602and regulated by
U60I. U601 is a three pin 5 V regulator. CK607 references
U601 at 3.92 V instead of ground. I'herefore, the output
voltage is a non-adjustable 8.55 t o 9.45 volts.
8-23.
+
5 V (Vc) Power Supply. The VC Supply is
fullwave rectifiied by CK605 and CK606 and regulated
by U602. U602 is again a three pin 5 V regulator,
however. in this a p p l ~ c a t i o n it is referenced to ground.
8-24. V
BG
Power Supply. 1-he V
MI
Supply is a Lener
regulated (CK608 - 5.62 V) supply that is fullwave
rcctil.icd by CK603 and CK604. 1-he output is adjustable
lrom -2 V to -5 V by K603. 1-he V
H ( i
supply should be set
to thc voltage stamped o n 11725.
8-25. Analog Theory.
8-26. lnput Switching. The
input switches are
separated into two groups Function (S2 thru S6) a n d
Range ( S 7 thru S14). The function switches provide
correct paths for the input signals to the analog circuitry
and at the same time output a three line function code
which programs the Digital Control IC (U 13), the lnput
Hybrid (U I I), and the Integrator Hybrid (U 12). The
simplified analog schematic (Figure 8-12) shows the
input switching configuration for each function. Table 8-
I shows the input switching configuration for each
function. I-able 8-1 shows the three line function codes
for each of the five Multimeter functions.
Model 3438A
Table 8-1. Function Code.
F u n c t ~ o n
D C V (S2)
A C V (S3)
DCI (S41
ACI (S5)
i l (S6) 1 1 1
8-27. I'he range switches ( S 7 thru S13) output a three
line range code to U 13, U 11, and U12 when the A U T O
(S14) switch is not depressed. If (S14) is depressed, S 7
thru S 13 are open a n d the range code information then
comes from the Control IC (U 13). Table 8-2 shows the
range codes and Figure 8-3 shows a block diagram of the
logic interface during A u t o and Manual ranging.
Table 8-2. Range Codes.
8-28. Voltage a n d ohms functions can be A u t o o r
Manually ranged. The current function (dcl and a c l ) are
manually range only. S 8 thru S 12 are used to select the
correct current shunt for the five current ranges.
Range
20 m V (S7)
2 0 0 r n V ( S 8 )
2
v
(S9)
20
v
(S10)
200
v
(S11)
1200
v
(S12)
2 0 Mi2 (S131
A u t o (S14)
8-29. D C Voltmeter. I'he S i m p l i f i e d A n a l o g
Schematic (Figure 8-12) shows the D C Voltmeter circuit
configuration. f h e function of the analog portion of the
Multimeter is to convert voltage, current, o r resistance
information at the input terminals to a d c voltage a t the
input to the Analog to Digital Converter ( A to D
Converter). In the d c voltmeter configuration, the
voltage a t reference point
@
can vary from 0 Vdc to k
1200 Vdc.
8-30. The voltage a t the input to the A to D Converter
@
needs to stay within the limits of -1 Vdc to + I Vdc t o
avoid setting the Multimeter display to a n overload ( O L )
Code
MANUAL
(S14
OFF) AUTO
(S14
ON)
CONTROL CONTROL
FROM 5 7 - S l 3 {
i!E:-r
u402
,
i
L'
RGD RGE RGF
INPUT INTEGRATOR INPUT INTEGRATOR
34661
R -
L6
R G D
0
0
1
1
1
0
0
Open
Figure 8-3. Range Code Logic Interface.
8-2
A u t o
1
1
1
1
1
1
1
0
R G E
1
0
0
1
1
1
0
Open
R G F
0
0
0
0
1
1
1
Open