DaqScribe Solutions
P.O Box 958
Carson City NV. 89702
Author: Sam Herceg
sherceg@pyramid.net
A basic understanding of grounding and ground loops
In the world of data acquisition if there is one thing which causes more anguish that
anything else, it is grounding! Some sensor manufactures and most instrument providers
include a provision for a third wire either as part of the signal output or the signal input
path. The third wire has a variety of labels, shield, guard, common, ground, signal
ground etc. Often, the signal wires are shielded and contain a “drain wire” adding to the
connection complexity. This article attempts to provide a “simple” (if ground loops
could ever be called simple) explanation of this phenomenon.
Drain wire
Common Mode/Normal Mode
Any discussion of grounding and ground loops must contain at least a basic discussion
and some understaning of terms like single ended inputs, differential inputs, common
mode voltages and normal mode voltages. The following definitions are included to at a
minimun provide a very simple definition of these terms.
Single Ended Inputs. Single ended inputs are referenced to the module’s power supply
common. The singnel being measures is on the positive conductor and the negative
conductor is tied to the supply
common.
Single Ended input
+
Output
Differential inputs. Differential inpus are two single ended inputs referenced to a
common potential. The signal being measured is the difference between these two single
ended inputs.
Two Wire Differential input
+
_
Output
Typically these wires are twisted
Common Mode Voltage: When referenced to the local common or ground, a commonmode signal appears on both lines of a 2-wire cable, in-phase and with equal amplitudes.
Common mode voltages are induced by (A) Radiated signals coupled equally to both
wires. (B) An offset from the signal common created in the driver circuit. (C) A ground
differential between the transmitting and receiving locations.(D) Magnetic Influcenses,
things like large motors and transformers can radiate magnetic fields that can induce
signels into the wires.
Normal Mode Voltage: A normal mode voltage is any type of signal that appears
between a pair of wires, or on a single wire referenced to the earth/ground.
The Third wire and the Sensor
The third wire, generally the shield wire represents the common mode potential of the
signal. If not taken into account properly there will likely be an erroneous normal mode
signal generated. The “center or electrical balance point” of the signal probably will not
have identical impedance to each of the two signal wires. If a potential exist between the
shield and the signal, and unequal voltage will appear in series with each signal lead
caused by the current flowing from the shield through the unequal impedance of the
signal source. In this way, the voltage that exists between the sensor case and either of
the input leads will generated a false normal mode voltage. Balancing of the input leads is
crucial to the reduction of these common mode voltages. For this reason differential
input systems are generally preferred over single ended systems.
If the sensor is of a design that uses an internal high frequency voltage (Excitation) to
enhance the signal generating ability of the sensor, the unbalance effect will be greatly
exaggerated with the additional likelihood of cross modulation products of the shield
voltage frequency and sensor carried frequency appearing in the output signal.
In most applications, the sensor case mounting determines the common mode potential of
the sensor. Many transducers isolate the case from the signal path. Unfortunately at
higher frequencies insulating the case does little to assure signal noise integrity as the
capacitive reactance at higher frequencies significantly affects the case potential. There
are methods to address this problem but it is not the intent of this article to address high
frequency noise applications ( >100 Hz).
Signal Receiver
The third wire is used to present the common mode (CM) voltage existing at the sensor to
the receiving device. The difference between the average signal voltage and input circuit
reference voltage (common mode voltage) will generate an error in all receiving devices.
Receiving devices may be signal conditioning amplifiers, data acquisition digitizers,
recorders or data display devices (DMM, DVM, Scopes etc). Whether or not this error is
significant depends upon the amplitude and frequency of the common mode voltage and
the immunity of particular receiving devices. It is not unusual for two identical receiving
devices to exhibit entirely different apparent zero offset values as the signal wires are
moved from one device to the other because of differing and insufficient CM immunity.
In general, the very best devices will have common mode rejection ratios (CMRR) of
about 1,000,000/1 at low frequencies (<100 Hz). As the CM frequency increases, both
the CMRR and the maximum allowable CM voltage decrease at about 6dB/octave. At
higher CM frequencies (>100 kHz), more error is caused by slew limiting in the input
amplifier. The error appears as a DC offset. It is therefore not coherent with the CM
voltage and the term CMRR is meaningless. The onset of slew error is sudden and
serious and is easily recognizable.
Accurate receiving devices have input filters to help limit the CM voltage presented to
the input circuit and a method of driving the receiving device input circuit common. The
filter must also remove any signal components that might alias the sampling signal.
System Considerations
The overall data acquisition system design must not allow the receiving device to be
presented with the normal or CM signals that have voltage levels or slewing rates that are
incompatible with the device’s input circuit limitations. The system design must also
provide a method to connect floating signals ohmically to the receiving device circuit
common. This is necessary to provide a pump-out current source and to limit the CM
signal voltage at the device input. Many systems, utilizing devices that exhibit good test
bench accuracy fail when long signal leads, (typical of many test environments), are used.
Before an installed system is used with certainty, checks must be made to assure that the
total system is not being exposed to and responding to unknown and undesired signal
sources.
Grounding (earth)
This term is purposely avoided in the above discussion as almost never is an instrument
or sensor ever tied directly to ground. In most cases when the term ground or grounding
is used it refers to a method of connecting to a location which at some point is connected
to a piece of metal which is actually tied to an earth connection somewhere down the line.
It may be a facility grounding bus tied to a copper stake driven 6’ into the earth out side
the building, or tied to a U’fer ground connection in the building foundation, or it may be
tied to the facility AC ground connection in the buildings electrical panel. The electrical
panels ground is tied to the power company’s earth connection (and who knows where
that is). The earth connection may be at the power pole located where the power line
enters the facility or may be located some distance away. This in itself produces a
potential difference in the ground reference.
All instruments containing “electronics” are sensitive to higher frequencies. Usually the
higher the frequency, the more effect. The signal presented to an instrument is
recognized by the instrument relative to its reference (ground), not necessarily the system
designer’s designated ground. It is therefore incumbent upon the system designer to
consider all signals, both wanted and unwanted, and their effect upon the individual
instrument as well as the total system accuracy.
A good wiring practice for a voltage-input channel is shown in figure 1
Isolated Sensor Ground Connections
Output
+
_
Sensor
Figure 1
In this case, the sensor is not grounded and the cable shield is connected to the midpoint
of the sensor as well as to the “ground” connection at the data acquisition system. This
shield connection also meets the requirements of most instrumentation front-ends.
(THERE MUST BE A RETURN PATH TO THE INSTRUMENTATION COMMON SO
THAT THE INPUT CURRENT (as low as it might be)WILL NOT CAUSE THE INPUT
PAIR TO “FLOAT” OUTSIDE THE COMMON-MODE RANGE) Failure to have this
return path is a common cause of common mode problems.
The situation is more complex if the sensor is grounded. The customary recommendation
in this case is to connect the shield at the sensor and not at the DAQ system input to
avoid a ground loop. Refer to Figure 2.
Sensor-end shield grounded configuration
+
_
Sensor
Output
Figure 2
If adequate CMMR is not achieved with this method another method to be tried is to
leave the shield unconnected at the sensor end and connect it at the receiving end, as is
shown in the following diagram. Refer to Figure 3
Amplifier/DAQ end shield connection configuration
+
_
Sensor
Figure 3
Output
For many applications, the circuit shown in the following diagram may produce the best
results, Note the shields are connected at both ends, even though this creates a ground
loop. This is an example of a good ground loop.
Sensor and Signal Conditioning shield connection configuration
+
_
Sensor
Output
Figure 4
In many cases, this double grounding has been shown to give superior performance over
configurations with the shield open at either end.
How can it be that a ground loop substantially improves performance? A ground loop is
generally bad if it involves a signal carrying conductor. An example of this was shown in
Figure 2, where the voltage produced by the ground current translated one to one into a
normal mode voltage because the shield is a “return” for the signal. A ground loop is
often good if it does not involve a signal carrying conductor. High frequency “hash” that
enters the system through reduced common mode rejection and nonlinearities in the
instrumentation amplifier at high frequencies may or may not be of concern. Often
filtering is provided in the amplifier to eliminate this wide band noise.
The cable capacitance and other factors reduce the transmission of high frequency noise
when the shield is connected to the circuit elements at both ends. Indeed, with a
connection to ground at both ends, current flows through the shield, particularly at power
line frequencies, and the signal effect of this transformer action is greatly reduced
because these voltages cancel out in the differential input instrumentation amplifier.
Note: The double grounding cannot be applied if there is a substantial potential difference
between the two circuit commons. For this case, an isolated input channel, as shown in
Figure 5 should be used.
Power
Source
Isolated data acquisition input circuit
Isolation Amplifier
Shunt
No Connection
Figure 5
If the data system uses more than one equipment rack, the racks should be bonded to each
other. The rack sub-systems should be connected to a good ground. Often the electrical
conduct is used as the tie point. Care must be taken to assure the conduit and all its
connections are sufficient to achieve good continuity to the facility power ground. Often
loose connections, or poor connections of the screws at the conduit coupling produce
high impedance junctions, resulting in higher CM potentials. Ideally the impedance
should be “0” Ohms. A better ground connection would be attaching directly to the
ground wire (green wire) inside the conduit. In some facilities the electrician may have
used the conduit as the ground instead of pulling a third wire (green wire). If the
impedance of the ground connection is too high, a better direct connection must be found.
If there are wire ducts or conduit carrying the signal cables, the recommendation is that
these be bonded to the ground reference for the sensors and the racks that contain the data
front-end. Note that this creates another ground loop, usually the good kind. This
approach is controversial when the sensors are in a rather hostile electrical environment.
The concern often expressed is that ground bonding at both ends will cause the electrical
noise at the sensor to be transmitted to the data system ground and create more
interference. Generally, the noise reduction resulting from the sensors and front end
being at nearly the same potential will far outweigh the introduction of noise by the
ground loop. An additional benefit of ground bonding is that it will reduce the chance of
damage to the electronics in the presence of lighting or other voltage transients. The
lower the electrical impedance between the various parts of circuit, the lower the
potential difference in the event of large voltage transients induced into the
instrumentation elements.
If the sensors are not connected to any part of an electrical circuit or ground, as shown in
Figure 6 the preferred technique is to bond the grounds at the sensors and the DAQ front
end using the input cable conduit, This will minimize the effect of any electrostatic
coupling of noise to the sensors. Again, this is to keep the entire data acquisition front
end, including the sensors, in as uni-potential an environment as possible. A similar
approach involves the use of double shielded cables, where the inner shield is connected
as in figure 6 and the outer shield is connected to ground as the sensors and to the front
end equipment chassis ground. The guiding principle is to keep all parts of the analog
subsystem at the same potential.
Shielded Twisted Pair
Output
+
_
Sensor
Figure 6
Other approaches can be used if the primary common mode (signal to ground)
interference is primarily high frequency in nature. One configuration uses a trifilar
transformer, which is a tightly coupled three-winding transformer, with one winding in
series with each of the two signal conductors and the third winding in series with the
guard connection. This is quite effective in enhancing common mode rejection at high
frequencies.
Another approach is to use a capacitor to connect the circuit ground to the shield at the
instrumentation front end. This provides a high frequency ground while reducing the
current caused by power line frequencies. The effectiveness of the capacitor depends
upon the particular situation. Also, some front ends provide a guard signal for connection
to the shield. The guard voltage is derived from a special instrumentation amplifier
output that monitors the common mode voltage and produces a signal to cancel it.
Often asked is what are the correct cabling and ground connections when the signal
source is an amplifier output instead of a passive sensor. Properly wired, this output
circuit can be single-ended and still produce a good signal to noise ratio. If the amplifier
output circuit drives a two contact and a shield type connector. The connections are
shown in figure 7. This configuration provide a good balance because the output
impedance of an instrumentation or operational amplifier is generally less than 1 ohm at
low frequencies
Driving a differential input from an unbalanced source
+
_
Output
Figure 7
Many signal conditioning chassis contain BNC single contact connectors at their output
and are intended to be used with coaxial cable. This can present problems in correctly
wiring a coaxial cable to a differential input front end. If the output shield connection on
the signal conditioning unit is isolated from ground or has a resistance path to ground of
1000 ohms or greater, then the connections shown in Figure 8 should be used. If the
shield conductor is grounded at the source, then the diagram shown in Figure 9 can be
employed to prevent a ground loop in a signal carrying conductor. This approach may be
satisfactory unless the data acquisition system contains a high frequency filter. Also it is
desirable the ground frames of the chassis associated with the signal transmitter and
receiver are mounted in the same rack or nearby racks and are electrically bonded
together so that the ground noise between them is minimized. A good wiring alternative,
particularly if the two units are not in the same rack, is to convert the cable to a shielded
twisted pair type as close to the source as possible as shown in Figure 10
Output
+
_
R
GND
Floating source Connection BNC Input Connection
Figure 8
Output
+
_
Grounded source Connection BNC Input Connection
This bond is very important
Figure 9
This connection may or may
not be required depending
upon the difference in the
ground potentials
+
_
Twin-Ax Cable configuration
Figure 10
Conclusion
Output
As we have discussed there are many considerations when it comes to connecting a
sensor to a DAQ, but the main objective should always be getting the ground reference at
the sensor to be at the same level as the ground reference at the receiving end. The use of
the third wire become somewhat of an art, but certainly an art that can be if not mastered
at least controllable by most instrumentation engineers. The afore mentioned and
describe connection diagrams will generally provide an acceptable grounding solution for
most input configurations.