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THEORY
OF OPERATION
INSTRUCTION
MANUAL
Digital
Multimeter
Models
172.
173
2.
For
relatively
large
values
of
the
input,
the
time
required
for
the
integrating
capacitor
to be recharged
above
the
comparator
threshhold
is
relatively
small,
and
the
charging-discharging
cycle
described
will
occur
at
a high
frequency.
For
lower
values
of
input
current,
the
recharging
time
of
the
integrator
will
be correspond-
ingly
longer
and
the
events
described
will
occur
at
a lower
rate.
Thus
the
repeti-
tion
rate
of
reference
current
pulses
(and
digital
output
pulses)
is
a function
of
the
input
curre,,t.
3.
Notice
that
the
amount
of
charge
removed
from
the
integrating
capacitor
during.
each
discharge
cycle
is
equal
to
the
product
of
the
reference
current,
I , and one-
half
the
clock
period.
Through
appropriate
circuit
design
these
two
parLmeters
can
be made very
stable,
with
the
result
that,
whether
the
frequency
of
the
discharge
cycle
is
high
or
low,
each
increment
of
charqe
removed
(i.e.,
each
reference
current
pulse)
is
precisely
the
same size.
Since
the
total
charge
removed
from
the
capacitor
in
any
given
time
period
is
equal
to
the
total
charge
that
flowed
in
during
the
same time
period
(within
a resolution
of
one
discharge
increment)
the
uniformity
of
size
of
the
reference
current
pulse
guarantees
that
the
total
number
of
such
pulses
is
precisely
proportional
to
the
time-integral
of
the
input
current.
FIGURE 29.
Operating
Waveforms
of
A/D
Converter.
5-16
AB
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