Model 343812 Section Vlll
Range
20M
5V
'10M
10M
00005
-
5
V
500
K
2M
5V
1M
1M
0005
5
V
500
K
200K
5V
100K
100K
005
5
V
500
K
Ramp
(+)
20K
5V
IOK
10K
05
5
V
500
K
2K
5V
1K IK
5V
500K
OV
200
5V
100
100
5
V
500
K
2
0
5V
100
10
05
V
50
K
'10
Mi1
Ellect~ve
Res~stance
Figure
8-7.
8-37.
Analog to Digital Converter (A to
D
Converter).
Refer to Figures 8-8, 8-12, and 8-14. The
A
to
D
Converter
converts dc voltage into a proportional timer control signal.
This circuit consists of an Integrator
(U2), a Slope Arnpli-
fier (U3). a Comparator (U4), and an Auto Zero Loop.
8-38. There are four basic conditions
(m)
for a
complete measurement cycle as shown in Figure
8-8.
These conditions exist for each of the five Multimeter
functions.
8-39, During Auto Zero
a
the exact potential at the
lntegrator summing junction is stored
on
C205. This
potential
hhould be nearly zero volts. However. any offset
voltages at the input to the lntegrator
will be stored
during
conditions.
8-40. At the beginning of run-up
@)
a dc voltage propor-
tional to the Multimeter input is applied across one of
the run-up resistors (depending on the Multirneter function
selected). This run-up voltage is integrated across
C202. The
polarity of the Integrator output is opposite to the run-up
voltage polarity. The run-up voltage polarity is dependent
upon the Multimeter function, range selected, arid the input
polarity. Figure 8-8 shows the Integrator output for three
different input levels and the polarity for different func-
tions and
input polarities.
8-41. Run-up is a fixed time of 100 milliseconds. At the
end of run-up the run-up resistor is disconnected from the
integrator summing junction. There is now a 1.6 milli-
second hold or settling time
@
before run-down is ini-
tiated. During this time the Controller senses the polarity
of the Integrator output and selects the proper
rindown
current. If the integrator output is positive at the end of
run-up,
QHl will be closed and QH2 open during run-down.
If the integrator output is negative
QH1 and QH2 will be
open.
8-42. Run-down
@)
time may vary from zero to 200
milliseconds depending on the charge built up
on C202
Ohm
Gain.
during run-up. During run-down the discharge rate of
C202 is fixed (fixed slope). Therefore. the greater the
charge on C202 (positive or negative), the longer the
discharge time. This conversion method from voltage to
time is called Dual Slope Integration. A counter is started
at
the beginning of run-down-and runs until the output of
the lntegrator crosses zero. Phe accumulated time is
directly
proportional to the dc voltage at the input to the
A to
L)
Converter. Phis time is processed by the
Controller along with the range and function
information that is already established to become the
Multimeter display readout.
8-43. The Slope Amplifier and Comparator amplify the
output of the Integrator by a factor of
X80000. This pro-
vides a very accurate zero crossing detector. If the output
of the Integrator is positive during run-up, the Comparator
output will be positive. This voltage is sensed and processed
by the controller to provide correct run-down and display
information. The comparator output will remain positive
until the output of the lntegrator runs down and crosses
zero volts. The comparator then changes to zero volts
output.
8-44.
Controller.
ti
13 functions
as
an Algorithmic
State Machine (ASM) controller. It controls the
MOS
FE
Iswitching on the Input and Integrator Hybrids.
11
13
outputs drive signals for the display digits.
8-45. At the end of run-down, the output of the A to D
Converter (Coniparator) is a
.stale
c./~ar,,yo
HI to
LO
or
1-0
to HI, depending on the polarity of the lntegrator
output. As previously discussed, the display counter has
been counting since thc beginning of run-down. Now, the
counter must be stopped exactly as the Comparator state
changes to ensure accurate A to
D
Conversion. 'The
comparator output stops the controller counter. The
information now stored in the counter is a
true
representation
of the Multimeter Input.
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