HP 3438A Operating And Service Manual page 61

Model 343812 Section V l l l
Range
2 0 M 5 V ' 1 0 M 1 0 M 00005 - 5 V 500 K
2 M 5 V 1 M 1 M 0005 5 V 500 K
2 0 0 K 5 V 1 0 0 K 1 0 0 K 005 5
V
500 K R a m p
(+)
2 0 K 5 V I O K 1 0 K 05 5 V
500 K
2 K 5 V 1 K I K 5 V 500K O V
200 5 V 100 100 5 V 500 K
2 0
5 V 100 10
05
V 50
K
'10 M i 1 Ellect~ve Res~stance
Figure 8-7.
8-37. Analog t o Digital Converter ( A t o
D
Converter).
Refer to Figures 8-8, 8-12, and 8-14. The A t o 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)
f o r a
complete measurement cycle a s shown in Figure 8-8.
These conditions exist f o r each of the five Multimeter
functions.
8-39, D u r i n g A u t o Z e r o
a
the exact potential a t the
lntegrator s u m m i n g junction is s t o r e d on C205. This
potential hhould be nearly zero volts. However. any offset
voltages a t t h e input t o the lntegrator will be s t o r e d
d u r i n g
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 t o 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 1 0 0 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, Q H l 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 m a y vary f r o m z e r o t o 200
milliseconds d e p e n d i n g o n t h e charge built u p o n C202
Ohm Gain.
d u r i n g run-up. D u r i n g run-down the discharge rate of
C202 is fixed (fixed slope). Therefore. the greater the
charge o n C 2 0 2 (positive o r negative), the longer the
discharge time. This conversion method f r o m voltage t o
time is called D u a l S l o p e Integration. A counter is started
a t the beginning of run-down-and runs until the o u t p u t of
the lntegrator crosses zero. Phe accumulated time is
directly proportional t o the d c voltage a t the i n p u t t o the
A t o L) Converter. Phis time is processed by the
Controller a l o n g with the range a n d function
information that is already established t o become t h e
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 t o 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 a n Algorithmic
S t a t e Machine ( A S M ) controller. It controls the MOS
FE I s w i t c h i n g o n the Input a n d Integrator Hybrids.
1 1 13
o u t p u t s drive signals for the display digits.
8-45. At the end of run-down, the o u t p u t of the A to D
Converter ( C o n i p a r a t o r ) is a
.stale
c./~ar,,yo HI t o LO o r
1-0
t o H I , d e p e n d i n g o n the polarity of the lntegrator
o u t p u t . As previously discussed, the display counter has
been counting since thc beginning of run-down. Now, the
c o u n t e r must be stopped exactly a s the C o m p a r a t o r state
changes t o ensure accurate A t o D Conversion. 'The
c o m p a r a t o r o u t p u t s t o p s the controller counter. The
information now stored in the counter is a true
representation of the Multimeter Input.