Datron 4708 User Handbook Manual page 31

Some
typical sources
are:
-
Proximity of
large
electric fields
-
Fluorescent
lighting
-
Inadequate
screening,
filtering or
grounding of
power
lines
- Transients
from
local switching
-
Induction and
radiation
fields
of
local
E-M
transmit-
ters
-
Excessive
common mode
voltages
between
source
and
load
The
disturbances
may
be magnified by
the user's
hand
capaci-
tance. Electrical
interference
has
greatest effect in
high imped-
ance
circuits.
Separation of
leads
and
creation
of
loops in the
circuit
can
intensify the disturbances.
Lead Impedance
The impedance
of
the connecting
leads
can drop
significant
voltages
between
the source
and
load,
and
generate adverse
phasing
effects particularly
if
the leads are
long
or the current
in
them
is
high.
Lead
Insulation
Leakage
This can cause
significant errors in
measurement
circuits at
high
voltages.
Some
insulating materials suffer greater losses than
others
e.g.
PVC
has
more
leakage than
PTFE.
AVOIDANCE
TACTICS
Thermal
EMFs
Screen thermal
juntions
from
draughts.
Allow
time
for thermal equilibrium
to
be reached before
taking
readings.
Use
conductors,
joints
and
terminals with a good margin
of
current-carrying capacity.
Avoid
thermo-electric junctions where
possible.
e.g.
Use
untinned
single-strand
copper wire of high
purity.
Avoid making
connections through Nickel, Tin, Brass and
Aluminium.
If
oxidation
is
a problem use
gold-plated
copper
terminals,
and
replace the terminals before the
plating
wears
off. If joints
must be
soldered,
low-thermal
solders are available, but
crimped
joints are preferred.
Use
low-thermal switches and
relays
where
they
form
part
of
the
measuring
circuits.
Balance one thermal
EMF
against
another
in
opposition,
where
possible (switch
and
relay contacts, terminals,
etc.).
E-M
Interference
Choose
as 'quiet'
a
site
as possible (a screened cage
may
be
necessary
if interference
is
heavy).
Suppress
as
many
sources as possible.
Always
keep
interconnecting leads as short as
possible, espe-
cially
unscreened
lengths.
Run
leads together as twisted pairs
in a
common
screen
to
reduce
loop pick-up
area,
but beware
of leakage
problems and
exces-
sive capacitance.
Where
both source and load
are
floating,
connect
I-
to
ground
at
the source
to
reduce
common mode
voltages.
Lead Impedance
Keep
all
leads as short as possible.
Use
conductors with a good
margin
of
current-carrying capacity.
Use Remote
Sense and
4-
wire connections
where
necessary
to establish
the
4708
output
specification
at
the load.
Use
4-wire connections
for
values of
resistance
below IkQ.
Lead
Insulation
Leakage
Choose
low-loss insulated leads
-
PTFE
is
preferred
to
PVC.
When
running
leads together
in
screened
pairs,
avoid
large
voltages between
leads
in
the
same
screen, especially
if
using
PVC
insulation.
Remote/Local Sense
Configurations
The 4708
terminals are configured
as follows:
Voltage ranges
lOOpV, ImV, lOmV,
lOOmV
-
Local
sense only.
IV, lOV, lOOV,
lOOOV
-
user
selects
Local
or
Remote
sense
All Current ranges
-
Local
sense only.
All Resistance ranges
-
Remote
Sense
gives 4-wire connection
-
Local Sense
provides 2-wire connection
capability
The key
LED
indicates the true connection:
Lit
=
Remote,
Unlit
=
Local
N,B,
When
changing
to
Q
function, the
4708
is
automatically
forced into
Remote
Sense
for 4-wire
operation.
4708
-
CONNECTIONS TO THE LOAD
General Considerations
The
choice of connection
method
is
influenced
by
several
factors:
a.
Loading
Effects
4-wire connections should be used
for
low
load imped-
ances.
For
high
impedance
loads,
2-wire connections
can be employed.
The
ratio: Total
Lead
Resistance
Load
Resistance
gives
the
approximate
error for 2- wire connection
at
low
frequencies.
e.g.
Two
1/2
Ohm
leads
with a load of
lOOkO
to
produce
an
error
of approx. lOppm.
At
frequencies higher than about lOOkHz,
the error
is
also modified
by
reactive effects.
b.
Noise and
Output Level
Providing
the
E-M
environment
is
reasonably
quiet,
interference
due
to
noise
pickup
in
the load connection
is
4-2