Theory
of
Operation
This
section
describes
the
overall
operation
of
the
4339B
and
the
operation
of
each
assembly
.
Overall
Measurement
Theory
The
4339B
measures
the
resistance
of
the
Device
Under
T
est
(DUT)
in
the
following
manner
.
Figure
4-1.
Ohm's
Law
In
Figure
4-1,
a
DUT
is
connected
in
series
with
test
voltage
source
V
,
and
ammeter
A.
If
the
ammeter
measures
I
amperes
when
the
source
voltage
is
V
,
the
DUT's
resistance
R
is
expressed
by
(when
ignoring
r):
R
=
V
I
This
equation
shows
that
if
the
DUT
resistance
R
is
extremely
high,
the
current
owing
the
circuit
is
extremely
low
.
F
or
example
,
if
R
=
1
G,
and
V
=
1
V
,
the
current
owing
is
1
nA
(10
09
A).
The
4339B
High
Resistance
Meter
is
essentially
a
set
of
a
voltage
source
and
a
very
sensitive
current
meter
.
Note
In
practice
,
ammeter
input
resistance
r
,
which
is
approximately
1
k,
decreases
the
current
owing
through
the
DUT
.
The
actual
current,
I,
owing
through
the
DUT
,
R,
is
expressed
by:
I
=
V
R
+
r
In
the
current
measurement
mode
,
the
4339B
displays
the
actual
current
I
for
the
measurement
result.
F
or
example
,
if
R
=
100
k
,
and
V
=
1
V
,
the
measurement
result
will
be
9.9
A
(
1
100 k
+1k
)
rather
than
10
A
(
1
100 k
).
In
the
resistance
measurement
mode
,
on
the
other
hand,
the
4339B
displays
the
\derived"
DUT
resistance
which
coincides
with
the
DUT
resistance
.
In
other
words
,
the
4339B
shows
the
resistance
calculated
by:
R
=
V
I
0
r
F
or
example
,
if
R
=
100
k,
and
V
=
1
V,
the
measurement
result
will
be
100
k,
even
the
actual
current
owing
through
the
DUT
,
thus
through
the
ammeter
with
r
is
9.9
A.
T
roubleshooting
4-3
