Measurement
Tutorial
DC
VOLTAGE
MEASUREMENT
.
lWx
Rs
CIRCUIT LOADING ERROR
IN
Oh
=
Rs
+Ri
where Rs
=
Source impedance
Ri
=
8840A input impedance
(Ri
=>10,000 Mi2 in 200 mV, 2V, and 20V ranges)
(Ri
=
10 MR in 200V and 1OOOVranges)
EXAMPLE
When measuring the voltage across the 10 kR leg of a90 mover 10 knvoltagedivider, thecircuit loading error
is
I
than 0.1% in the upper ranges, and less than 0.0001% in the lower ranges:
SOURCE
,
8840A
VOLTAGE
-
R
i
Error in the 200V and
lOOOV
ranges
=
100 x
9kQ
9kn+10Mn
=
0.09%
Error
in
the 200 mV. 2V
and
20V ranges
=
100 x
9Kn
9
=O-MQ=m
Figure
4-1.
Circuit
Loading
Error Calculation
5.
Remove the
1
MQ
resistor.
6.
Measure the voltage of the circuit under test.
Note that this procedure
does
not correct for circuit load-
ing error.
Also
note that if input bias current error is not
corrected for, it may be added to the circuit ioading error.
4-5.
RESISTANCE MEASUREMENT
The
8&40A
allows you to measure resistance in both
2-wire and &wire configurations. Each has its benefits.
The
test cullrent and full-scale voltage for each resia
range are shown in Table
4-1.
Since the
HI
INPU'
lead is psieve with respect to the
LO
INPUT
lead,
test leads aie not interchangeable when a semiconc
device is being measured.
4-7.
Cowcting
for
Test
Lead
Resistance in
2-Wire
Ohb
In 2-wire ohms, the resistance of the test lead!
introduce efror when measuring low resis$ances.
TI
4-6.
2-Wire
Ohms
test
leads
rbay add
as
much as 0.58 to 2-wire
readings.
Two-Wire ohms measurements are simple
tc
set up and
yield good results for most measurement conditions. Mea-
with
~WA,
it is
easy
to
corren
for
this enor
surements are made
as
shown in Figure
4-3.
An
internal
the
button:
current source (the "ohms current source") passes a known
test current (Itest) through the resistance being tested
1,
Select
the
2-wire ohms function.
(Runknown). The 8840A measures the voltage drop across
Runknown, calculates Runknown using Ohm's law
2. Touch the
test
ltd~
together.
The
t#4oA
s
(Runknown
=
Vtafitest), and displays
the
result.
indicate the resistance of the test leads.
4-2
--
-
-
--
---
--
--
---
---
--
lnce
test
lese
ctor
can
lical
Ims
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
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