Ground Loop Problems2 - Precision Filters, Inc. Loop Problems with Measurement Systems and How to...

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


• 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


• 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


• The magnitude of the induced voltage dependson the overall loop area and is given by:

VN

Area A


• 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


VN

Area A


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...


• …and that voltage is induced in any conductivesegment exposed to a magnetic field.

VN

VN


• 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


• 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


• …and will therefore have nearly equal induced voltages.

e1 e2

e2

e1


• 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


• To answer this we develop a loop equationas seen by the input circuit of the signalconditioner.

–+

e1

e2Vsensor

Vout = e1 + Vsensor + e2

e1 e2


• 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


• We borrow from the notations in the field of magneticsand symbolize “in phase” signals with a solid DOT.

–+

e1

e2Vsensor

e1 e2


• 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


• 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


• 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


• 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


• 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


• 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


• 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.


• 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


–+

–+

–+

SignalConditioner

?


• How do we properly connect the three accelerometerchannels to three channels of DISTANT signalconditioning?


• 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


• 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


• 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:


• We emphasize that this discussion considers the caseof a long interconnecting cable, where it is importantto provide shielding of the inner conductors.


–+

–+

–+

SignalConditioner

Distance >25 ft.

eY

eX

eZ


• 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


• We might then use BNC cables to connect tothe signal conditioner inputs.

–+

–+

–+


• This shows the resulting connections tothe IEPE circuits.

–+

–+

–+

eY

eX

eZ


• 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


• We wish to determine how this connectionscheme would perform in the presence ofhigh magnetic fields?

–+

–+

–+

eY

eX

eZ


• 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


• 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


• 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.


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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

–+

–+

–+


• ...and each channel must be individually programmableto “grounded” or “isolated” mode.

Cable X

Cable Y

Cable Z

einY

einX

einZ

–+

–+

–+

eY

eX

eZ


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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

–+

–+


• 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

–+

–+


• 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

–+

–+

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

Ground Loop Problems2 - Precision Filters, Inc. Loop Problems with Measurement Systems and How to...

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