ground loop problems2 - precision filters, inc. loop problems with measurement systems and how to...
TRANSCRIPT
Ground Loop Problems withMeasurement Systems and How
to Avoid Them
Alan R.SzaryPrecision Filters, Inc.Phone: 607-277-3550
E-mail: [email protected]: www.pfinc.com
Electromagnetic NoiseCoupling into Loops
• Two “Inconvenient Truths” of electromagneticnoise coupling.
• Inconvenient Truth #1: Any current carryingconductor will create a magnetic field.
B = μI / 2r
μ = permeability = 4 X 10 –7 H/m
I = current flowing in source conductor
r = distance to measurement point
Electromagnetic NoiseCoupling into Loops
• The magnitude of the magnetic field (B) at anypoint depends on the current flowing in the sourceconductor and the distance to the measurementpoint and is given by:
VN
Electromagnetic NoiseCoupling into Loops
• Inconvenient Truth #2: We know from Faraday’sLaw that any conductive loop being penetratedby a magnetic field will have a voltage induced“within” the loop.
VN= jw BA cos
Electromagnetic NoiseCoupling into Loops
• The magnitude of the induced voltage dependson the overall loop area and is given by:
VN
Area A
Electromagnetic NoiseCoupling into Loops
• Adhering strictly to Faradays Laws, the distributionof this voltage throughout the loop can not be exactlydetermined. For purposes of analysis we refer to thisas the “open circuit” or “Thevenin” voltage and placeit at any location where the loop is opened up.
VN Area A
Electromagnetic NoiseCoupling into Loops
VN
Area A
Electromagnetic NoiseCoupling into Loops
VNArea A
Electromagnetic NoiseCoupling into Segments
• In order to better analyze a cabling system we canexploit the rules of Partial Inductance which describesthat the Faraday loop can be decomposed intoindividual (partial) conductive segments...
Electromagnetic NoiseCoupling into Segments
• …and that voltage is induced in any conductivesegment exposed to a magnetic field.
VN
VN
Electromagnetic NoiseCoupling into Segments
• The induced voltage is dependent on the conductivemedium, and the distance and angular orientationbetween the two conductors. Collectively we will referto these as the conductor’s spatial geometry.
D
Electromagnetic NoiseCoupling into Segments
• For the frequencies of interest for modal analysis testing,it is a reasonable assumption that conductor segmentswithin a individual cable will have similar spatial geometryrelative to the wavelength of the magnetic field at everypoint throughout the cable run…
D1 D2
D1 D2
Electromagnetic NoiseCoupling into Segments
• …and will therefore have nearly equal induced voltages.
e1 e2
e2
e1
Electromagnetic NoiseCoupling into Segments
• Relative to the wavelength of the signals of interest,even a coaxial shield is spatially equivalent to it’scenter conductor.
e1
e2
e1 e2
Magnetic Coupling Errorsin Measurement Systems
• If this coaxial cable is connecting a floating IEPEaccelerometer to an amplifier...
...WHAT WILL HAPPEN?
–+
e1
e2Vsensor
e1 e2
Magnetic Coupling Errorsin Measurement Systems
• To answer this we develop a loop equationas seen by the input circuit of the signalconditioner.
–+
e1
e2Vsensor
Vout = e1 + Vsensor + e2
e1 e2
Magnetic Coupling Errorsin Measurement Systems
• It is also important to note that the spatial equivalenceof conductors within a cable will also cause the inducedvoltages to be of the same phase.
–+
e1
e2Vsensor
e1 e2
Magnetic Coupling Errorsin Measurement Systems
• We borrow from the notations in the field of magneticsand symbolize “in phase” signals with a solid DOT.
–+
e1
e2Vsensor
e1 e2
Magnetic Coupling Errorsin Measurement Systems
• Since the induced voltages are of equal amplitude andequal phase they will tend to cancel in a loop equationas seen from the input stage of a signal conditioner.
–+
e1
e2Vsensor
Vout = e1+Vsensor - e2 Vsensor
e1 e2
Magnetic Coupling Errorsin Measurement Systems
• It can be said that a single-ended connection to afloating sensor is relatively immune to magneticallyinduced signals if the high and low conductors arespatially equivalent.
–+
e1
e2Vsensor
Vout Vsensore1 e2
The Ground Loop Problem
• If however the cable shield is grounded at both ends,we have a new situation!
…The dreaded ”Ground Loop”
–+
Voute1
e2Vsensor
The Ground Loop Problem
• Whether it is a facility green wire ground, common earthground, or some other hard wired ground connection,there is now a new segment to be analyzed in ourcable system.
–+
Voute1
e2Vsensor
The Ground Loop Problem
• This segment will have it’s own unique induced voltageand most likely this voltage will be very different fromthat of our cable.
e3 e1 & e2
–+
Voute1
e2
e3Vsensor
Current Flow in Ground Loop
• We know that very high currents will flow when weconnect signal sources together if their voltagesare not equal.
+V1 V2+
I
Current Flow in Ground Loop
• Similarly potentially high “loop current” Ig will flow whenthese differing induced voltages are connected togetherthrough the low impedance of the ground loop.
lg
–+
Voute1
e2
e3
Current Flow in Ground Loop
• This current will create a voltage eshld on the cableshield equal to Ig * Rshld, where Rshld is the end toend resistance of the cable shield.
–+
Voute1
e2
e3
eshld
The Ground Loop Problem
• We see in the new loop equation that eshld is seenat the channel input and will be amplified by anychannel gain.
–+
Voute1
e2
e3
Vout= e1+ Vsensor + eshld- e2 Vsensor + eshld
eshld
Cabling Difficulties withTriaxial Accelerometers
• Common “Gotcha #1”: Consider the seeminglysimple task of connecting a triaxial accelerometerto a distantly located signal conditioning system.
Cabling Difficulties withTriaxial Accelerometers
• The triaxial accelerometer employs three separate IEPE buffercircuits, each sharing a common return. In order to minimizesensor size and mass, a miniature 4-pin connector is oftenused. The connector contains the signal output of each ofthe X, Y and Z accelerometers and a common return thatis usually connected to the sensor case.
X
Z
ground
Y
TriaxialAccelerometer
TriaxialAccelerometer
–+
–+
–+
SignalConditioner
?
Cabling Difficulties withTriaxial Accelerometers
• How do we properly connect the three accelerometerchannels to three channels of DISTANT signalconditioning?
Cabling Difficulties withTriaxial Accelerometers
• One cabling option that performs well for very shortcable runs is to bundle all three signal carrying wiresin a single “shielded triplet” cable as shown below.
–+
–+
–+
e2
e1
e3e4
e1 e2 e3 e4
Cabling Difficulties withTriaxial Accelerometers
• This cabling scheme is only useful for very short cableruns where capacitive coupling between the signalconductors (crosstalk) is within acceptable levels.
–+
–+
–+
C 50 p F/ft
Rout = 50–200 ohms
eY
eX
eZ
0
–5
–10
–15
–20
–25
–30
–35
–40
–45
–5010 100 1000
Cable Length (Feet)
Cro
ssta
lk (
dB
)
10Khz Inter-Cable Crosstalk Vs Cable Length
Cabling Difficulties withTriaxial Accelerometers
• If we assume the IEPE accelerometer has 200 ohmsof output impedance, and the cable has 50pF/ft ofconductor to conductor capacitance, then we cancalculate the following crosstalk:
Cabling Difficulties withTriaxial Accelerometers
• We emphasize that this discussion considers the caseof a long interconnecting cable, where it is importantto provide shielding of the inner conductors.
TriaxialAccelerometer
–+
–+
–+
SignalConditioner
Distance >25 ft.
eY
eX
eZ
Cabling Difficulties withTriaxial Accelerometers
• We may start with a manufacturer supplied Breakoutcable which conveniently breaks out the four sensorpins to three coaxial cables with BNC connectors.
–+
–+
–+
GND
Note: The shields of each coaxial cable are tied together in the breakout cable
Cabling Difficulties withTriaxial Accelerometers
• We might then use BNC cables to connect tothe signal conditioner inputs.
–+
–+
–+
Cabling Difficulties withTriaxial Accelerometers
• This shows the resulting connections tothe IEPE circuits.
–+
–+
–+
eY
eX
eZ
Cabling Difficulties withTriaxial Accelerometers
• It is important to note that traditional IEPE signalconditioners utilize a single-ended input scheme,whereby input “low” is connected to ground at eachsignal conditioner input. This is usually an appropriateconnection since it provides a solid ground connectionto the shield of the coaxial cable.
–+
–+
–+
eY
eX
eZ
Cabling Difficulties withTriaxial Accelerometers
• We wish to determine how this connectionscheme would perform in the presence ofhigh magnetic fields?
–+
–+
–+
eY
eX
eZ
Cabling Difficulties withTriaxial Accelerometers
• Step 1: We begin by inserting induced voltagesources in each conductive segment.
–+
–+
–+
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshldeYcntr
eYshldeZcntr
eZshld
eY
eX
eZ
Cabling Difficulties withTriaxial Accelerometers
• Step 2: Determine whether there is current flowin the cable shield and an associated cableshield IR drop.
In this cable scheme…YES!
–+
–+
–+
eirX
eirY
eirZ
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshldeYcntr
eYshldeZcntr
eZshld
iX
iY
iZ
eY
eX
eZ
einZ = eZcntr + eZ + eirZ – eZshld = eZ + eirZ
–+
–+
–+
eirX
eirY
eirZ
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshldeYcntr
eYshldeZcntr
eZshld
eY
eX
eZ
Cabling Difficulties withTriaxial Accelerometers
• Step 3: We then draw the loop equation for eachchannel input.
We see that there is an induced noise signal eirZwhich will appear at the input and is amplifiedby any channel gain.
Cabling Difficulties withTriaxial Accelerometers
• Similarly each loop equation includes an errorcaused by IR drop in the cable shield.
–+
–+
–+
eirX
eirY
eirZ
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshldeYcntr
eYshldeZcntr
eZshld
eY
eX
eZ
Experimental Result Test #1
–+
–+
–+
D1,2,3 = 0 to 12 inches
B=50mGauss @1inch
I=20 APCB modelHTT356A02 Gain=100X
1V/G
D1
D2
D3
L=50 Feet
eY
eX
eZ
Cabling Difficulties withTriaxial Accelerometers
• One solution often employed to solve this problem is tosimply break the loops by disconnecting two of the threecable shields at the sensor side of the cable connection.
–+
–+
–+
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshldeYcntr
eYshldeZcntr
eZshld
eY
eX
eZ
Cabling Difficulties withTriaxial Accelerometers
• This does indeed prevent current flow in the cableshields, and the second induced cable shield errorvoltage, however a new problem is introduced.
–+
–+
–+
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshldeYcntr
eYshldeZcntr
eZshld
eY
eX
eZ
einX= eXcntr + eX - eYshld
Cabling Difficulties withTriaxial Accelerometers
• We can see below that the loop equation for einX now
involves shield noise of cable Y and the center conductor
noise of cable X. Since these are different cables, they
will likely have different induced signal amplitudes and
may NOT fully cancel.
–+
–+
–+
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshldeYcntr
eYshldeZcntr
eZshld
eY
eX
eZ
einZ = eZcntr + eZ - eYshld
Cabling Difficulties withTriaxial Accelerometers
• Similarly, a noise signal at einZ will containthe center conductor noise of cable Z and theshield noise of cable Y, and again these maynot fully cancel.
–+
–+
–+
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshldeYcntr
eYshldeZcntr
eZshld
eY
eX
eZ
einY = eYcntr + eY – eYshld eY
–+
–+
–+
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshldeYcntr
eYshldeZcntr
eZshld
eY
eX
eZ
Cabling Difficulties withTriaxial Accelerometers
• We notice that the loop equation for einY containscenter conductor and shield noise of cable Y and sincethese are of the same cable, we would expect theseto more fully cancel.
Experimental Result Test#2
Cabling Difficulties withTriaxial Accelerometers
• Breaking the ground loops at the signal conditionerside does not change matters, again we see withany ground return from a different cable the loopequations do not fully cancel and error signals arepresent in our data.
–+
–+
–+
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshldeYcntr
eYshldeZcntr
eZshld
eY
eX
eZ
Cabling Difficulties withTriaxial Accelerometers
• After analyzing all possible cabling schemes,it becomes apparent that there is NO cablingscheme to a 4-pin triaxial accelerometer whichoffers immunity from magnetic interference ifthe signal conditioner inputs have groundedlow inputs.
Then what do we do?
Optimal Signal Conditioner
• For a long distance, cabled connection to triaxialaccelerometers, the signal conditioner musthave isolated inputs...
Cable X
Cable Y
Cable Z
einY
einX
einZeY
eX
eZ
–+
–+
–+
Optimal Signal Conditioner
• ...and each channel must be individually programmableto “grounded” or “isolated” mode.
Cable X
Cable Y
Cable Z
einY
einX
einZ
–+
–+
–+
eY
eX
eZ
Optimal Signal Conditioner
• For triaxial accelerometers it is important to groundone channel and one channel only on each triaxialaccelerometer.
Cable X
Cable Y
Cable Z
einY
einX
einZ
–+
–+
–+
eY
eX
eZ
Optimal Signal Conditioner
• This single ground connection provides theelectrostatic shield ground for all cablesand the accelerometer case.
Cable X
Cable Y
Cable Z
einY
einX
einZ
–+
–+
–+
eY
eX
eZ
Optimal Signal Conditioner
• We wish to determine how this cabling schemewould perform in a high magnetic fieldenvironment.
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshld
eYcntr
eYshld
eZcntr
eZshld
–+
–+
–+
eY
eX
eZ
Optimal Signal Conditioner
• Step 1: We start by drawing in the inducedsegment voltages.
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshld
eYcntr
eYshld
eZcntr
eZshld
–+
–+
–+
eY
eX
eZ
Optimal Signal Conditioner
• Step 2: Since only one of the shields is connected toground at the signal conditioner inputs, we do not haveloops in the cable shield connections and therefore noI*R induced signals in the cable shield.
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshld
eYcntr
eYshld
eZcntr
eZshld
–+
–+
–+
eY
eX
eZ
Optimal Signal Conditioner
• Step 3: The isolated input structure causes the loopequations for each channel to include the shield noiseand center conductor noise of the SAME cable.
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshld
eYcntr
eYshld
eZcntr
eZshld
–+
–+
–+
eY
eX
eZ
Optimal Signal Conditioner
• We note that since channel Y’s cable shield isconnected to ground at the signal conditioner, thiswill force cable noise eYshld to exist at the commonconnection at the accelerometer case.
eY
eX
eZ
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshld
eYcntr
eYshld
eZcntr
eZshld
–+
–+
–+
eYshid
Optimal Signal Conditioner
• This signal, however, will appear as a common modesignal to channel X and Z inputs and will be eliminatedby the CMRR of the differential amplifiers.
eY
eX
eZ
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshld
eYcntr
eYshld
eZcntr
eZshld
–+
–+
–+
eYshid
Optimal Signal Conditioner
• For this scheme to function properly the conditionermust have independently isolated input stagesand must have high CMRR.
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshld
eYcntr
eYshld
eZcntr
eZshld
–+
–+
–+
eY
eX
eZ
Optimal Signal Conditioner
• The isolation provides additional protection fromground loops in the event that the accelerometercase is grounded.
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshld
eYcntr
eYshld
eZcntr
eZshld
–+
–+
–+
eY
eX
eZ
Optimal Signal Conditioner
• To eliminate the ground loop, all three channels wouldbe isolated from ground and all three channels wouldbe referenced to the distant sensor ground.
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshld
eYcntr
eYshld
eZcntr
eZshld
–+
–+
–+
eY
eX
eZ
egnd
Optimal Signal Conditioner
• Any differential ground signal (egnd) will beeliminated by the CMRR of the differentialinput stages.
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshld
eYcntr
eYshld
eZcntr
eZshld
–+
–+
–+
eY
eX
eZ
egnd
Optimal Signal Conditioner
• The isolated current sources guarantee that all IEPEcurrent returns through it’s own low connection and notthrough other return paths.
Cable X
Cable Y
Cable Z
einY
einX
einZ
eXcntr
eXshld
eYcntr
eYshld
eZcntr
eZshld
–+
–+
–+
eY
eX
eZ
egnd
Experimental Result Test #3
Optimal Signal Conditioner
• A final improvement that is useful for high magneticfield environments results from using shieldedtwisted pair cable in place of the coaxial cables.
Cable X
Cable Y
Cable Z
eXcntr h&leXshld
eYcntr h&leYshld
eZcntr h&leZshld
–+
eY
eX
eZ
–+
–+
Optimal Signal Conditioner
• Due to the tight twisting of the high and lowsignal lines, these pairs will have almostidentical induced signals.
Cable X
Cable Y
Cable Z
eXcntr h&leXshld
eYcntr h&leYshld
eZcntr h&leZshld
–+
eY
eX
eZ
–+
–+
Optimal Signal Conditioner
• Due to the improved match of the high and lowsignals, their error terms will more completelycancel in the loop equations written for eachdifferential input amplifier.
Cable X
Cable Y
Cable Z
eXcntr h&leXshld
eYcntr h&leYshld
eZcntr h&leZshld
–+
eY
eX
eZ
–+
–+
Experimental Result Test #4
Conclusions
• Segment analysis can be a useful analysis tool forunderstanding magnetically induced noise signalsin cabled measurement systems.
• A voltage is induced in each wire in our cablingsystem exposed to a magnetic field. This isunavoidable! Our job is to assure the sum ofinduced voltages sums to zero in the amplifiersloop equation.
Conclusions
• The triaxial accelerometer presents a particularlydifficult cabling problem. Even if the case isfloating, we can still have induced shield currentsdue to the shared ground at the low connection.
• The shielded triplet cable is a good solution forshort cable runs, but excessive X,Y,Z cross-talkmay disallow this for longer cable runs or higherfrequencies.
Conclusions
• For longer cable runs it was shown that there areNO suitable cabling schemes for connecting triaxialaccelerometers to grounded single ended IEPEconditioner systems.
• A signal conditioner topology was proposed whichsolves these problems. This requires independently(per channel) isolated input stages with balanced(high CMRR) differential amplifiers.
Conclusions
• The signal conditioning technique described isemployed in the Precision Filters Model 28316C.The 28316C is a 16-channel IEPE conditioner plugin card for the system 28000.
• 16 cards can be installed in the 16-slot chassis toprovide 256 channels of independently isolatedIEPE conditioning in 7U of rack space.
Precision Filters, Inc.
240 Cheery Street
Ithaca, New York 14850
Phone: 607-277-3550
E-mail: [email protected]
Web: www.pfinc.com