THEORY
OF
OPERATION
CIRCUIT
DESCRIPTIONS
3-15.
Assume that the
A/D
Converter
is
just
before
its
integrate
(INT) period, and
an unknown
voltage
equal
to
1
/
2
range
is
applied to the A/
D
Converter.
The auto-zero
(AZ) period
is just
ending
so
the A/D Converter
should
be ready to
receive a new input. The controller
opens
the
.
AZ FETs and
closes
the INT FET applying the
unknown
voltage to
the integrator.
The integrator output
changes,
changing the comparator
output
(CM) to
switch
to
a
supply voltage. The
integrate
period lasts for
a
count
of
1000.
At the end of
this time,
the controller will
open
the
INT FET and
close the READ FET.
At
this time.
Cl has
charged to a
level
proportional to the level
of the
unknown
input voltage.
3-16. Closing the
READ
FET applies the
known
reference
voltage to the
input of the integrator.
The
polarity of
this reference voltage is selected by the digital
controller to be
opposite
the polarity of the unknown
input
voltage. Capacitor
Cl
starts to discharge
at a
known
rate.
The digital
controller starts to
count clock
pulses
at the
beginning
of the read period. When
Cl
discharges
to the level it was
when the integrate
period
started,
the comparator
swithces CM to the other
polarity
supply.
This
change
in the
level
of CM causes the
controller to
stop
counting
clock pulses, open
the READ
FET, and
close the AZ FET.
The controller now
contains
a
digital count that
is
proportional to the level of
the
unknown
input voltage.
During the read period,
the
controller can
count
from
0
to 1999. Since we used
an
unknown input
voltage equal to
1/2
range,
the digital
count will
be
1000.
The
controller
drives the
display logic
so
that 1000
is
displayed on
the LCD. The
location
of the
decimal
point is determined by
the position
of
the
range
switches. The LCD
displays
the unknown signal with
the
proper
units.
3-17.
When
the unknown
input
voltage
equals
full
range,
the read period is
extended. When
CM changes
polarity,
the digital
controller
has counted
1999 clock
pulses. This
is
the maximum
reading for
any range.
3-18.
If
the unknown
input
voltage is greater
than
full
range
(overrange),
everything
iS’ normal
until the
digital
controller counts 1999
clock
pulses. At this time.
Cl is still
not
discharged
(i.e., the
output
of the comparator
has
not
changed
polarity). The
digital
controller then
counts one
more
clock
pulse creating a
2000
count.
A
count
of
2000
generates an
overrange
pulse and the overrange
indicator
appears on
the LCD.
The
overrange pulse
will also
cause
the digital
controller
to
restart
the
auto-zero
period.
3-19.
During
the auto-zero
period,
a
ground
reference
is
applied to
the input of
the A/D Converter. Under
ideal
conditions,
the
output
of the A/D
Converter
would
be
zero.
However,
input-offset-voltage
errors accumulate
in
the
amplifier
loop
and appear at
the
comparator
output
as
an error
voltage.
This error
voltage
is charged on C2
where it is
stored
for
the rest
of the conversion cycle. The
stored
level is
used
to
provide offset
voltage
correction
during the
integrate and
read
periods. The auto-zero
period
starts
at the end of READ and ends at the
beginning
of INT.
The length of
AZ can
vary from a
minimum
of
17,000
counts
(overrange)
to a maximum of
about
19,000
counts
(zero input).
3-20. The INT
period is
1000 counts. With
a
clock
frequency
of
60
kHz, the INT period
is
exactly
1 cycle of a
60 Hz
period. All inputs to the A/D
Converter
are
integrated. So,
the positive half cycle of the
60
Hz
noise
cancels
out the
negative half cycle
of the
60
Hz noise.
As
stated
before,
this method is highly
accurate,
fast, and
noise-free.
3-21.
If
your
instrument is a
60 Hz
version
and you
select one of
the ranges that enables the
±200
mV
measurement
function of the internal voltmeter, the
A/ D
Converter
functions the same with two exceptions
to
the
timing
of
the conversion
cycle.
With the ±200 mV
function,
the INT
period
is
10,000
counts long.
Auto-zero
is
correspondingly
shorter. At a clock frequency
of 60
kHz,
the
10,000
count
length
of
the
INT period means
that it is 10
cycles of a 60 Hz period. Noise is canceled.
3-22. Instruments
configured
for 50 Hz operation
function
the
same has the 60 Hz versions with the
exception
of a
different frequency quartz
crystal being
used as
the clock
reference. The resulting
basic
clock
frequency
is 50
kHz. This
means that
50
Hz
noise
is
canceled by
the dual slope integration of the A/D
Converter.
3-23.
DIGITAL
CONTROLLER
3-24.
The
Digital Controller is an integral part
of
the
custom IC.
It uses
the reference frequency from the
crystal
as
the basic
clock. It
controls the
A/D Converter
and LCD
Driver.
It also monitors
the
input
for an
overrange
condition, and
turns
on the overrange
indication
when
necessary.
3-25,
THE
DISPLAY
(LCD)
3-26.
The
LCD is
located in
a mounting bracket
fastened to the
Main PCB Assembly. Its
drivers are
contained
inside the custom IC. Overrange indications
and
numerical
values originate from the custom
IC.
The
decimal
point is
controlled
separately
by the range
switches
and
a
CMOS IC,
3-27.
Input
Signal Conditioners
3-28. The
A/D Converter in your instrument
has
an
input
voltage
limit from -2V dc
to
2V
dc. Any other input
will result in
an
overrange condition.
If you are measuring
a dc
voltage
that falls within
this range, the
signal
conditioners
are not used
and
the voltage
is
applied
directly to the A/D
Converter,
The input signal
conditioners are used
when measuring:
higher dc voltages
(outside of
±2V),
ac voltages,
current
(ac/dc), resistance,
and conductance.
3-4
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