Digital Controller; The Display; Input Signal Conditioners - Fluke 8010A Instruction Manual

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