Ohms Function - Fluke II Series Service Manual

Theory
of
Operation
DETAILED DESCRIPTION
Figure
2-2.
Ohms
Function
ROTARY
SWITCH
2-1
0.
Rotary switch SI
FRONT
selects
and
routes the input
signals.
Function codes
for
switch
SI
REAR
are
shown
in
Table 2-1.
Range
switch
S2
signals the
microcomputer
U2
for
the
manual
ranging and automatic
Touch
Hold®
function.
CR1
acts as protection for
U1
if
the battery
is installed
backwards.
C2
is
part
of
the
power-on
reset for
microcomputer U2.
A/D
CONVERSION
2-1
1
.
Analog-to-digital
(a/d)
conversion
is
accomplished
within
U1
using a modified
dual-
slope a/d converter circuit
(See Figure 2-3,
AC
and
A/D
Converter.)
Since the a/d conversion process
is
essentially
a dual slope method, two
voltages are
required
to
complete
a
measurement
cycle.
One
is
the
unknown
input and
the other
is
the reference voltage.
Conditioned
input signals are routed
to
the
a/d
converter
in
Ul, where
they are
integrated.
The
reference voltage developed
by
reference supply
VR1,
R15, R16, and
R8
is
routed
to the a/d
converter
in
Ul, where
it is
used
for
the
integrate reference
(de-integrate) portions
of
the
measurement
cycle.
C7
stores offsets
of
the buffer, integrator,
and comparator
amplifiers of
the a/d
converter.
The
gain of the buffer
is
determined by
the
resistors
of Z1 between
pins
8, 9,
and
10.
C8
is
the integrator capacitor.
®
Touch
Hold
is
a
registered
trademark of the
John
Fluke Mfg. Co.,
Inc.
2-5