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  • TECHTIP 60201Grounding and Shielding Considerations for Thermocouples,Strain Gages, and Low-Level Circuits

    www.iotech.com IOtech 25971 Cannon Rd Cleveland, OH 44146 (440) 439-4091 Fax (440) 439-4093 sales@iotech.com

    www.iotech.com

    INTRODUCTION

    Signal to Noise RatioElectrical noise is present in all measurement systems to somedegree. Instrument engineers typically design systems thatmake the measured signal much larger than the noise thataccompanies it, whatever that source of noise may be. Theyspecify this parameter as signal-to-noise ratio SNR and typi-cally measure it in dB. In terms of voltage, dB = 20 log (V2/V1),where V2 is greater than V1, and V1 is the noise level. Forexample, 120 dB is a ratio of 1,000,000:1 and 160 dB is100,000,000:1, very respectable numbers for SNRs.

    Design rules that instrument manufacturers follow to makenoise-free and quality equipment include proper circuit designand layout on a printed circuit board as well as grounding,shielding, and guarding. One common method for measuringlow-level signals where noise can be a problem is to use differ-ential input amplifiers. The topology of such circuits tends tocancel certain types of noise. Both terminals of a two-terminalsensor connect to the differential amplifier input, and shieldingand guarding circuits typically connect to signal ground andspecial guard-shield terminals. This eliminates current flowbetween the grounds, common, or returns of the sensor and theinstrument input amplifier, which can generate noise signals.

    One drawback when using differential inputs is that only halfthe number of sensor inputs is available compared to a single-ended configuration. That is, one differential amplifier consumestwo single-ended input connections.

    DISCUSSION

    High SNR is particularly critical for strain gages and thermo-couples, which inherently have relatively low signal levelscompared to most sensors. These sensors deal with millivoltsand microvolts, signals already close to the values of typicalnoise levels that can be measured in test setups. By comparison,

    many transducers have 1V, 5V, and 10V standard output levels.Thus, 10V compared to 1 microvolt is 10/0.000001, that is,10 million to one or 20 log (10/0.000001) = 20 log (10,000000)= 20(7) = 140 dB. But a 1.0 mV signal compared to a 1 microvoltlevel of noise is much more serious. Here, 0.001V is comparedto 0.000001V, or 20 log (0.001/0.000001) = 20 log (1,000)= 20(3) = 60 dB. In other words, the SNR for the first example is10 million to one, and the second is only 1 thousand to one. So,maintaining a high signal-to-noise ratio is paramount in low-level signal circuits to prevent the measured variable from beingcontaminated or yielding inaccurate results.

    PROCEDURE

    Data acquisition system designers try to obtain the highest SNRas possible within their equipment with the resources availableto them, but users also have an equal responsibility for reducingnoisy signals and maximizing instrument measurement accu-racy. Several such factors under user control that play a signifi-cant role in reducing noise levels, increasing SNR, and improv-ing accuracy include transducer lead length, application ofgrounding points and shields, and the temperature coefficientof copper wire.

    General PrecautionsShort leads between sensors and amplifier inputs are the firstconsideration in minimizing noise. Long leads act as antennasand can pick up a variety of electric and magnetic interference.Although strain gages and thermocouples have characteristi-cally low impedances, it is still good practice to use short leadsand twist them together. Figure 1 illustrates a condition whereunshielded, parallel wires connect the signal source to theamplifier input. The parallel wires pick up noise through induc-tion from the radiating wire principally because the mutualinduction M1 and M2 are unequal as are the distances, d1 and d2.Figure 2 shows how to cancel or minimize the interfering noiseby twisting the wires. Moreover, thermocouples should usethermocouple lead wire rather than ordinary copper wire since

  • www.iotech.com IOtech, Inc. 25971 Cannon Rd Cleveland, OH 44146 (440) 439-4091 Fax (440) 439-4093 sales@iotech.com

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    TC wire is designed just for this applica-tion. In most cases, the special lead wiremay be obtained from the thermocouplemanufacturer or supplier.

    The second consideration is the groundpoint. Usually, a ground connection is madeat either the amplifier input or preferablythe sensor, but definitely not at both. Whenboth devices are grounded, a small butmeasurable ground-loop current can flowbetween them and manifest in an unwantednoise signal at the amplifier input termi-nals. The best policy is to follow the manu-facturers grounding recommendations forits particular instrument and sensor.

    The circuit in Figure 3 shows a potentialdifference between earth ground #1 andearth ground #2. As a result, two groundloop paths are generated when the shield,sensor return, and sensor shield are allgrounded to earth ground #1, and the otherend of the shield, the sensor return, thecommon amplifier terminal, and amplifierchassis are connected to earth ground #2.One circulating current runs through theshield conductor, and the other runsthrough the signal return (or common)wire. Figure 4 shows that the ground loopcurrent through the shield is eliminatedwhen its ground connection is removedfrom earth ground #2. Likewise, the loopcurrent in the signal return path is eliminatedwhen the ground connection is removedfrom terminal 2 of the amplifier.

    In addition, wires shielded with copper,aluminum, and tin can minimize mostelectrical interference, but ferrous materi-als may be needed to shield wires wherehigh magnetic fields may be present. Cop-per or aluminum shields alone may notbe effective enough. Moreover, ground-ing these shields require as much care asother shields to minimize ground loopcurrents that can appear as signals to theamplifier input terminals.

    Copper wire has a relatively high tempera-ture coefficient of resistance. However, ittends to be neglected when the sensor sig-nals are so large that any changes in imped-ance due to the wires temperature coeffi-cient have negligible effect. But in highimpedance, low-level circuits, or circuitswith long lead wires, the wire resistance byitself or with a change temperature can besignificant and affect the measurements byas much as 10% or more. In many cases,

    Amplifier

    Distributing wire

    IX

    Signal source

    EinRL

    Iin

    Ground plane

    M1 M2 d1 d2

    Amplifier

    Distributing wire

    IX

    Signal source

    EinRL

    Iin

    Ground plane

    M1 M2 d1 d2

    Figure 1: Induced Noise: Parallel Wires

    Parallel signal wires can pick up noise by induction when the mutual inductances M1 and M2are unequal.

    Figure 2: Induced Noise: Twisted Wires

    Twisted input signal wires tend to bring the values of the mutual inductances M1 and M2closer together and cancel the inductive noise.

    Desktop ornotebook computer

    SignalprocessingAmp

    Earthground #2

    CCable shield

    Signal sourceor sensor

    E

    Earthground #1

    Signal conditioning anddata acquisition system

    1

    2

    Z

    Effective capacitance

    Potential difference

    Circulatingcurrents

    Desktop ornotebook computer

    SignalprocessingAmp

    Earthground #2

    C

    Cable shield

    Signal sourceor sensor

    E

    Earthground #1

    Signal conditioning anddata acquisition system

    1

    2Z

    Effective shielded area

    Potential difference

    No path for currentto circulate

    Figure 3: Multiple Ground-Loop Paths

    Ground loop currents are generated when a difference of potential develops between the sensorground and the amplifier ground connections. More than one path exists, as shown in this example.

    Figure 4: Eliminating Ground-Loop Paths

    Removing the ground connection at the amplifier input eliminates one ground loop path, andthe other by removing the ground connection at the amplifier input terminal #2.

  • www.iotech.com IOtech, Inc. 25971 Cannon Rd Cleveland, OH 44146 (440) 439-4091 Fax (440) 439-4093 sales@iotech.com

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    however, wire resistance effects can be cancelled by using a four-wire system where one pair is the excite wires, and the otherpair is the sense wires, such as used in bridge circuits. (SeeReference 1, page 85)1.

    ThermocouplesThermocouples should be electrically isolated from the deviceunder test where possible to avoid ground-loop currents, com-mon-mode voltage problems, and induced voltage or current.However, most thermocouple instrument amplifiers provideungrounded temperature reference input terminals mountedon isothermal blocks that help minimize such noise problems.For example, Figure 5 shows the temperature compensatedisothermal block and input connections to the IOtech DBK81

    series of thermocouple amplifiers.

    When thermocouple wire leads must be shielded, use the shieldconnection provided at the input to the amplifier, and do notconnect the other end of the shield (at the device under test) toa ground. However, when the thermocouple must be grounded,use the same ground terminal to connect to the shield and do

    not use the shield connection at the amplifier end. (Remember,the shield must not be grounded at both ends.)

    Strain GagesStrain gages (and frequently, thermocouples) are fastened me-chanically and sometimes electrically to the device under test.This arrangement can set up a path for ground loop currents andcommon-mode voltage noise signals. Use non-inductive typesof strain gages and ground the shields only at the sensor end.Use the differential input terminals of the data acquisitionsystem and make certain

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