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Operation
Voltage
divider
checks
For
thick film
resistor
networks
that are to
be used
as
voltage
dividers,
it
may
be
desirable to
test
them
using
voltages
that
simulate
actual
operating
conditions.
This
is
a
particularly useful
test
for resistor
networks
that
have
a voltage
coefficient specification-
The
test
system
in Figure 4-4
uses a Keithley
Model
230
to
source voltage
and
the
Model
196
to
measure
voltage.
A
consideration
in
these checks
is
the
effect
of
the
Mod-
el
196 input
impedance on
voltage
measurements.
The
input
impedance
is
shunted
across the
resistor
being
measured.
The
resultant
divider resistance
is
the paral-
lel
combination
of
the
resistor
under
test
and
the
input
impedance.
As
long
as the
input
impedance
is
much
larger
than
the
resistor
being
tested,
the error
intro-
duced
into the
measurement
will
be
iniriimal.
Mini-
mum
input
impedance
requirements
are,
of course,
determined
by
the accuracy
needed
in the measiure-
ment.
The
input
impedances
of the
Model
196
are as
follows:
300mV
and 3V
ranges,
IGQ;
30
V
range,
IIMO;
300V
range,
lO.lMO. For
better
input
impedance
re-
quirements,
the
Keithley
Model
617
Electrometer
can
be
incorporated
into the
test
system
to
measure
volt-
age.
Another
factor to
be
considered
when
checking
low
voltage
dividers
is
thermal
EMFs
generated
by
the
ma-
trix
card,
(The
Model
7012 has
been
designed
to
keep
relay
EMF
at
a
minimal
level.)
A
matrix card
crosspoint
can
generate
up
to
5pV
of
thermal
EMF.
Thus,
when
making low
voltage
measurements be
sure
to
accormt
for
this
additional
error.
Even
though
four-terminal
connections are
made
at
the
Model
196
and
the
resistor
networks, the sense
leads
are
internally
disconnected
from
the
input
of
the
DMM
when
the
volts
function
is
selected.
The
simplified
test
system
is
shown
in
Figure
4-6.
The
thick film
is
tested
by
applying a
voltage across
the
resistor
network and measuring
the voltage across
each
resistor
element
and/or
across
combined
ele-
ments. In Figure
4-6,
crosspoints
l!3!l
and
1!4!4
are
closed
to
apply
voltage across the
network,
and
cros-
spoints
1!1!3
and
1!2!4
are
closed
to
measure
the
volt-
age drop
across R3.
4-8
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