Signal Conditioning - Fluke 8050A Instruction Manual

THEORY OF OPERATION
SIGNAL CONDITIONING
AND
FUNCTION SWITCHES
3-20. Signal
Conditioning
3-21.
Some 8050A
inputs
must
be
scaled
and/or
conditioned before being presented
to the
a/d
converter.
For
example,
high voltage
levels
must
be attenuated,
and
ac inputs
must
be attenuated and converted
into the
equivalent
dc
voltage
levels.
The
a/d
converter has
two
ranges:
±200
mV
full-scale
and
±2V
full-scale.
The
following
paragraphs
describe the signal conditioning
circuits.
3-22,
VOLTAGE SIGNAL CONDITIONING
3-23.
As
Part
A
of Figure 3-5 shows, the voltage signal
conditioning
is
accomplished with
an
input voltage
divider
network.
The
division factor of the
network
is
determined by
the
range
selected:
I/lOO
for the
20
and
200V
ranges,
1
/
1000
for the
lOOOV dc (750V
ac)
range.
If
the
AC/
DC
switch
is
in
the
AC
position, the
output
of the
divider
network
will
be routed through
the
true-rms
converter
to
the a/d converter.
If the
AC/
DC
switch
is
in
the
DC
position, the output of
the divider
network
is
routed
directly to the
a/d
converter.
If
the kCl
switch
is
selected
(ohms
or conductance), the input divider
resistors
are
used
as the reference
resistors.
3-24,
CURRENT SIGNAL CONDITIONING
3-25.
As
Part
B
of Figure 3-5 shows,
current
measurements
are
made
using a
selected
value current
shunt
to
perform
the
current-to-voltage conversion
required
by
the
a/d
converter.
The
range
switches
determine
the
value of the current shunt, thus
determining
the scale of the voltage
level
developed
across
the shunt.
If
the
AC/
DC
switch
is
in the
DC
position, the
output of the current shunt
is
applied
to
the input of the
a/d converter.
If
the
AC/
DC
switch
is
in the
AC
position,
the voltage
level
developed across the shunt
is
applied to
the input of
the
rms
converter.
3-26,
RESISTANCE/
CONDUCTANCE
SIGNAL
CONDITIONING
3-27, Resistance
and conductance measurements made
on
the 2
mS,
2000, and
2
kO
ranges use a
direct ratio
technique.
Other
ranges use a subtraction and
ratio
technique
to indirectly derive
a
ratio.
3-6