t.
System Cabling Errors and DC Voltage Measurement Errors in Digital Multimeters
Digital Multimeter Measurement Errors Series
System Cabling Errors and DC
Voltage Measurement Errors
in Digital Multimeters
Application Note AN 1389-1
When making measurements with a digital multimeter
(DMM), common errors will crop up. The following
discussion will help you eliminate potential measure-
ment errors and achieve the greatest accuracy with
a DMM. This paper covers system cabling errors and
dc voltage measurement errors. For an overview of
ac voltage measurement errors, see Application Note
AN 1389-3. For a discussion of resistance; dc current;
ac current; and frequency and period measurement
errors, see Application Note AN 1389-2. (NOTE: The
Agilent 34401A, a 6-1/2-digit, high-performance DMM
with both benchtop and system features, will be used
as an example throughout this article).
Introduction Radio Frequency Interference
Most voltage-measuring
instruments can generate false
readings in the presence of large,
high-frequency signal sources
such as nearby radio and television
transmitters, computer monitors,
and cellular telephones. High-
frequency energy can also be
coupled to the multimeter on
the system cabling. To reduce
interference, try to minimize the
exposure of the system cabling to
high-frequency RF sources. If the
application is extremely sensitive
to RFI radiating from the multi-
meter, use a common mode choke
in the system cabling, as shown in
Figure 1, to attenuate multimeter
emissions.
Thermal EMF Errors
Thermoelectric voltages, the most
common source of error in low-
level voltage measurements, are
generated when circuit connections
are made with dissimilar metals
at different temperatures. Each
metal-to-metal junction forms a
thermocouple, which generates
a voltage proportional to the
junction temperature. It is a
good idea to take the necessary
precautions to minimize
thermocouple voltages and
temperature variations in low-
level voltage measurements.
The best connections are formed
using copper-to-copper crimped
connections. Figure 2 shows
common thermoelectric voltages
for connections between dissimilar
metals.
Noise Caused by Magnetic Fields
When you make measurements
near magnetic fields, take pre-
cautionary steps to avoid inducing
voltages in the measurement
connections. Voltage can be
induced by either movement of
the input connection wiring in
a fixed magnetic field, or by
a varying magnetic field. An
unshielded, poorly dressed
input wire moving in the earths
magnetic field can generate
several millivolts. The varying
magnetic field around the ac
power line can also induce
voltages up to several hundred
millivolts. Be especially careful
when working near conductors
carrying large currents.
Where possible, route cabling
away from magnetic fields, which
are commonly present around
electric motors, generators,
televisions and computer monitors.
In addition, when you are
operating near magnetic fields,
be certain that the input wiring
has proper strain relief and is tied
down securely. Use twisted-pair
connections to the multimeter to
reduce the noise pickup loop area,
or dress the wires as closely
together as possible.
2
System Cabling Errors
Figure 1.
Figure 2.
Copper-to-
Approx. µV/ °C
Copper
< 0.3
Gold
0.5
Silver
0.5
Brass
3
Beryllium Copper
5
Aluminum
5
Kovar or Alloy 42
40
Silicon
500
Copper-Oxide
1000
Cadmium-Tin Solder
0.2
Tin-Lead Solder
5 Noise Caused by Ground Loops
When you measure voltages in
circuits where the multimeter and
the device-under-test (DUT) are
both referenced to a common
earth ground, a ground loop is
formed. As shown in Figure 3,
any voltage difference between
the two ground reference points
(V
ground
) causes a current to flow
through the measurement leads.
This causes errors, such as noise
and offset voltage (usually power-
line related), which are added to
the measured voltage.
The best way to eliminate ground
loops is to maintain the multimeters
isolation from earth; do not
connect the input terminals to
ground. If the multimeter must
be earth-referenced, be sure to
connect it and the DUT to the
same common ground point.
This will reduce or eliminate any
voltage difference between the
devices. Also, whenever possible,
make sure the multimeter and
DUT are connected to the same
electrical outlet.
3
Figure 3.
R
L
= Lead Resistance
R
i
= Multimeter Isolation Resistance
V
ground
= Voltage Drop on Ground Bus Common Mode Rejection
Ideally, a multimeter is completely
isolated from earth-referenced
circuits. However, there is finite
resistance between the multimeters
input LO terminal and earth
ground, as shown in Figure 4.
This can cause errors when you
measure low voltages that are
floating relative to earth ground.
Noise Caused by Injected Current.
Residual capacitances in the
multimeters power transformer
cause small currents to flow from
the LO terminal to earth ground.
The frequency of the injected
current is the power line frequency
or possibly harmonics of the
power line frequency. The injected
current is dependent upon the
power line configuration and
frequency. A simplified circuit
is shown in Figure 5.
4
DC Voltage Measurement Errors
Figure 4.
V
f
= Float Voltage
R
s
= DUT Source Resistance Imbalance
R
i
= Multimeter Isolation Resistance (LO-Earth)
C
i
= Multimeter Input Capacitance ( 200 pF LO-Earth)
Error (v) = V
f
x R
s
R
s
+ R
i
Figure 5.
Injected Current
(50/60 Hz ac line leakage current) With Connection A (see Figure 6),
the injected current flows from
the earth connection provided by
the circuit to the LO terminal of
the DMM, adding no noise to the
measurement. However, with
Connection B, the injected current
flows through the resistor R,
thereby adding noise to the
measurement. With Connection B,
larger values of R will worsen
the problem.
Loading Errors Due to Input
Resistance
Measurement
loading errors occur when
the resistance of the DUT is an
appreciable percentage of the
multimeters own input resistance.
Figure 7 shows this error source.
To reduce the effects of loading
errors, and to minimize noise
pickup, set the Agilent 34401As
input resistance to greater than
10 G for the 100 mVdc, 1 Vdc,
and 10 Vdc ranges. The input
resistance is maintained at 10 M for the 100 Vdc and 1000 Vdc
ranges.
5
Figure 7.
V
s
= Ideal DUT Voltage
R
s
= DUT Source Resistance
R
i
= Multimeter Input Resistance (10 M or >10 G )
Error (%) = 100 x R
s
R
s
+ R
i
Figure 6.
Note: The
measurement
noise caused by
injected current
can be significantly
reduced by setting
the integration
time of the DMM
to 1 power line
cycle (PLC) or
greater. Loading Errors Due to Input Bias
Current The multimeters input
capacitance will "charge up" due
to input bias currents when the
terminals are open-circuited (if
the input resistance is 10 G ).
The multimeters measuring
circuitry exhibits approximately
30 pA of input bias current for
ambient temperatures from 0 °C
to 30 °C. Bias current will double
(x2) for every 8 °C change in
ambient temperature above 30 °C.
This current generates small
voltage offsets dependent upon
the source resistance of the DUT.
This effect becomes evident for a
source resistance of greater than
100 k , or when the multimeters
operating temperature is signifi-
cantly greater than 30 °C.
6
Figure 8.
i
b
= Multimeter Bias Current
R
s
= DUT Source Resistance
C
i
= Multimeter Input Capacitance
Error (v) i
b
x R
s
= A modern DMM such as the
34401A has many features to help
protect you from many common
sources of error. However, when
making low-level or precise
measurements, you must be
more aware of sources that
contribution to measurement
errors. Environmental conditions
such as changes in temperature,
RF signals and electromagnetic
fields will have a significant
impact on the quality of DMM
measurements. Once the
measurement environment
has been optimized, proper
cabling can significantly reduce
measurement errors. The remaining
sources of error can be calculated
and added to the measurement
uncertainty.
For more information about
the Agilent 34401A, go to
www.agilent.com/find/34401a
7
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Product specifications and descriptions in this document subject to change without notice.
© Agilent Technologies, Inc. 2002
Printed in the USA January 31, 2002