Section Vll
l
-
FEEDBACK
-7"
Figure 8-2.
-7
V Regulator.
8-21.
+6.5 V (V DISP) Power Supply.
1-he V
DISP
Supply is fullwave rectified by CR402 and CK401 and
regulated by series regulator
Q402. 4402 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 and 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 to 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 appl~cation 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 on 11725.
8-25.
Analog
Theory.
8-26. lnput Switching.
The input switches are
separated into two groups Function (S2 thru S6) and
Range (S7 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.
Funct~on
DCV (S2)
ACV (S3)
DCI (S41
ACI (S5)
il
(S6) 1 1 1
8-27. I'he range switches (S7 thru S13) output a three
line range code to U 13, U
11, and U12 when the AUTO
(S14) switch is not depressed. If (S14) is depressed, S7
thru
S 13 are open and 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 Auto and Manual ranging.
Table 8-2. Range Codes.
8-28. Voltage and ohms functions can be Auto or
Manually ranged. The current function (dcl and acl) are
manually range only. S8 thru S 12 are used to select the
correct current shunt for the five current ranges.
Range
20
mV (S7)
200rnV(S8)
2
v
(S9)
20
v
(S10)
200
v
(S11)
1200
v
(S12)
20
Mi2
(S131
Auto (S14)
8-29. DC Voltmeter.
I'he Simplified Analog
Schematic (Figure 8-12) shows the DC Voltmeter circuit
configuration.
fhe function of the analog portion of the
Multimeter is to convert voltage, current, or resistance
information at the input terminals to a dc voltage at the
input to the Analog to Digital Converter (A to
D
Converter). In the dc voltmeter configuration, the
voltage at reference point
@
can vary from 0 Vdc to
k
1200 Vdc.
8-30.
The voltage at the input to the A to D Converter
@
needs to stay within the limits of
-1
Vdc to +I Vdc to
avoid setting the Multimeter display to an overload (OL)
Code
MANUAL
(S14
OFF) AUTO
(S14
ON)
CONTROL CONTROL
FROM
57-Sl3{
i!E:-r
u402
,
i
L'
RGD RGE RGF
INPUT INTEGRATOR INPUT INTEGRATOR
34661
R-
L6
RGD
0
0
1
1
1
0
0
Open
Figure 8-3. Range Code Logic Interface.
8-2
Auto
1
1
1
1
1
1
1
0
RGE
1
0
0
1
1
1
0
Open
RGF
0
0
0
0
1
1
1
Open
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