Elenco 3 1/2 Digit Cap. / Trans. Kit - Manual (Page 3)

During autozero, a ground reference is applied as an
input to the A/D converter. Under ideal conditions the
output of the comparator would also go to zero.
However, input-offset-voltage errors accumulate in the
amplifier loop, and appear at the comparator output as
an error voltage. This error is impressed across the AZ
capacitor where it is stored for the remainder of the
measurement cycle. The stored level is used to provide
offset voltage correction during the integrate and read
periods.
The integrate period begins at the end of the autozero
period. As the period begins, the AZ switch opens and
the INTEG switch closes. This applies the unknown
input voltage to the input of the A/D converter. The
voltage is buffered and passed on to the input of the
A/D converter. The voltage is buffered and passed on
to the integrator to determine the charge rate (slope)
on the INTEG capacitor. At the end of the fixed
integrate period, the capacitor is charged to a level
proportional to the unknown input voltage. This voltage
is translated to a digital indication by discharging the
capacitor at a fixed rate during the read period, and
counting the number of clock pulses that occur before
it returns to the original autozero level.
As the read period begins, the INTEG switch opens
and the read switch closes. This applies a known
reference voltage to the input of the A/D converter. The
polarity of this voltage is automatically selected to be
opposite that of unknown input voltage, thus causing
the INTEG capacitor to discharge as fixed rate (slope).
When the charge is equal to the initial starting point
(autozero level), the read period is ended. Since the
discharge slope is fixed during the read period, the
time required is proportional to the unknown input
voltage.
The autozero period and thus a new measurement
cycle begins at the end of the read period. At the same
time, the counter is released for operation by
transferring its contents (previous measurement value)
to a series of latches. This stored stat is then decoded
and buffered before being used for driving the LCD
display.
-2-
AZ
READ
+REF
(FLYING
CAPACITOR)
UNKNOWN
INPUT
VOLTAGE+
INTEG
INTEG
AZ
AZ READ AZ
INTEG.
TO
DIGITAL
CONTROL
LOGIC
COUNTER OUTPUT
+.20
.15
.10
.05
0
0 10,000
1000 500
0
166.7mS
1500 2000
Figure 2 Dual Slope A/D Converter
AZ
COMPARATOR
INTEGRATOR
BUFFER
AMP EXTERNAL
INPUTS
A/D CONVERTER
A simplified circuit diagram of the analog portion of
the A/D converter is shown in Figure 2. Each of the
switches shown represent analog gates which are
operated by the digital section of the A/D converter.
Basic timing for switch operation is keyed by an
external oscillator. The conversion process is
continuously repeated. A complete cycle is shown in
Figure 2.
Any given measurement cycle performed by the A/D
converter can be divided into three consecutive time
periods: autozero (AZ), integrate (INTEG) and read.
Both autozero and integrate are fixed time periods.
A counter determines the length of both time
periods by providing an overflow at the end of every
1,000 clock pulses. The read period is a variable
time, which is proportional to the unknown input
voltage. The value of the voltage is determined by
counting the number of clock pulses that occur
during the read period.
1. The function of the A/D converter is to . . .
r A. convert digital to analog.
r B. divide analog signal by 2.
r C. convert analog to digital.
r D. convert AC to DC.
2. What type of divider network is used for voltage
measurements?
r A. Divide by 20
r B. Capacitance
r
C. Divide by 5
r D. Resistor
3. When the AC voltage is measured, it is first . . .
r A. divided down by 2.
r
B. converted to DC.
r C. coupled to a halfwave rectifier.
r D. low voltage.
4. When measuring current, the shunt resistors convert
the current to . . .
r A. +0.190 to –0.190.
r B. –1.199 to +1.199.
r C. –0.099 to +0.099.
r D. –199 to +0.199.
5. Which IC drives the LCD?
r A. 358
r B. LM324
r C. 7106
r D. 1N5232
6. Resistance measurements are made by . . .
r A. comparing voltage drops in the unknown
resistor and a reference resistor.
r B. measuring the current in the unknown resistor.
r C. measuring the current in the reference resistor.
r
D. equalizing the voltage drop in the unknown
and reference resistor.
7. Measurement cycles performed by the A/D converter
can be divided into what types of time periods?
r A. Long, short
r B. Auto zero, integrate, read
r C. Zero, read, interphase
r D. Autozero, read, cycle phase
8. A resistor with band colors green-black-green-brown-
green is what value?
r A. 50.5k
Ω
+ 5%
r B. 5.15k
Ω
+ 10%
r C. 5.05k
Ω
+ .5%
r D. 5.05k
Ω
+ 1%
9. When checking a transistor, the selector knob should
be in the . . .
r A. farad position.
r B. ohm position.
r C. diode position.
r D. hFE position.
10. Where do the leads need to be on the meter when
measuring 450mA?
r A. 10A, COM
r B. V
Ω
CAP, COM
r C.
μ
A/mA, 10A
r D.
μ
A/mA, COM
QUIZ
-33-
This marking adjacent to another marking or a terminal operating device indicates that the
operator must refer to an explanation in the operating instructions to avoid damage to the
equipment and/or to avoid personal injury.
This WARNING sign denotes a hazard. It calls attention to a procedure, practice or the like, which
if not correctly performed or adhered to, could result in personal injury.
This CAUTION sign denotes a hazard. It calls attention to a procedure, practice or the like, which
if not correctly adhered to, could result in damage to or destruction of part or all of the instrument.
This marking advises the user that the terminal(s) so marked must not be connected to a circuit
point at which the voltage, with respect to earth ground, exceeds (in this case) 500 volts.
This symbol adjacent to one or more terminals identifies them as being associated with ranges
that may in normal use be subjected to particularly hazardous voltages. For maximum safety, the
instrument and its test leads should not be handled when these terminals are energized.
5. SAFETY SYMBOLS
WARNING
CAUTION
500V max.
!
Answ ers: 1. C , 2. D, 3. B, 4. B, 5. C, 6. A, 7. B , 8. C, 9. D, 10. A