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DESIGN AND DEVELOPMENT OF A DATA ACQUISITION FOR

MEASUREMENT OF LOW LEVEL AND LOW FREQUENCY

ELECTRIC FIELDS


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Abstract:

Project work involves the measurement of the low level electric potential

from the two sensor electrodes placed at a distance of 0.5mts.

It is proposed to configure the circuit with an instrumentation amplifier, filter,

analog to digital converter and a microcontroller and develop a bread board model. The

data will be acquired for electric fields by varying the frequency and field strength. The

output of the data will be analyzed for the adequacy of the design towards detecting low

level signals.


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Electric fields in sea-an overview:

An electric field is the voltage gradient along a defined direction in a given medium,

usually expressed as volts/meter. In sea, electric fields are caused by electric current flowing thro the water,giving the voltage gradient as a result of the electric resistivity of the water. Typical electric fields range in sizefrom fraction of nanovolts/meter to millivolts/meter. Electric fields are emitted from ships, submarines Any othermetallic objects in sea. There are also naturally occurring electric fields caused by water movement andgeomagnetic activity.

Corrosion and different methods to protect from corrosion :

An electric field can be generated when certain metals immersed in sea water as a result

of electro chemical reaction between the metal and ionic species in the water. The basic model for theconsideration of the problem is the corrosion cell which consists of, the anode, cathode and the electrolyte asshown schematically in the following figure:

Direction of conventional current

E

Electrolyte

All the three items must be present for electrochemical reaction to take place. At anode, the

mostly likely reaction to occur is the dissolution of metal into metal ions, and this is termed as the anodicreaction. This is an oxidation process and is often thought of as the metal returning to its native state. Otherreactions can take place at the anode, but are generally of the less significance. To balance the anodic reaction,and use the electrons released at the anode, chemical reactions will occur at the cathode. There are a number ofpossible cathodic reactions, but the most common in sea water environment is the reduction of oxygen tohydroxide ions.

A C


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This implies therefore that electrons generated at the anode will flow through a connection tocathode, and also implies that the electrons will continue their circuit back to anode through the electrolyte. Byconvention electric current flows from the anode, through the electrolyte to the cathode, completing the circuit

through the cathode-anode connection. Since the electrolyte will have resistance, then by ohms law a potentialgradient will exit in the electrolyte.

Because one of the possible cathode reactions is the reduction of the oxygen, it follows that evenon an otherwise homogeneous material, if there is some part of the material which has restricted access for thatoxygen in the solution in then that area can become less cathodic. This means that local currents can be generated,which in case of steel, would lead to localized rusting usually evident as pitting corrosion.

A steel hulled vessel, in sea water may be considered as a large floating corrosion cell, albeitcomplex in nature because of the presence of many dissimilar metals. As an aid to protection against corrosionmost modern ships are painted, and this helps to reduce the magnitude of the problem. However, once the vesselis in commission, there are real possibilities of harming the paint film, and once the coating is breached, thecorrosion process may begin. To militate against the corrosion it is normal practice to install a cathodic protection

system on ship. The cathodic protection system deliberately produces currents in the electrolyte to act against thenormally occurring anode currents.

Two main types of cathodic protection are used, sacrificial anode and immressed current systems.For the sacrificial anode form of cathodic protection, a material which is most electronegative than the ships steelhull is chosen as the anode. This dissolves in preference to steel, and there by protects it. The anode is generallyplaced at a large number of strategic points around the hull, and because they are always active, there is nomaintenance associated with them. The following figure shows that the current will flow to both the anode and

cathode.

A C

S

A

C

SAC:

Material

more

electronegat

ive than

than native


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Currents flowing to the cathode increase the effectiveness of the cathode area. It will enable these areas toproduce larger quantities of hydroxide ions, making the surface more alkaline. It is known that as the pH on thesurface increase, metal is to more alkaline conditions, the steel enters into a passive non-coordinate state, which is

of obvious benefit to the process of cathodic protection. A second phenomenon which occurs as a result of highpH is the buildup of calcareous deposits. Rather like paint, these deposits act to insulate the surface from the

electrolyte, thus prevent the corrosion process taking place. However, they are pH dependent and with movementof the vessel it can be quite difficult to maintain a sufficiently alkaline condition for the calcareous deposit to

spread far from area immediately surrounding the anode.

The operating principle of an immersed current system, to be deliberately introduce a current intothe electrolyte from the electrode in the opposite direction to the natural anode current, thus suppressing the

electrochemical reaction and preventing corrosion. Because of the direction of anode, this electrode is calledimmersed current anode , and is generally of a material which will not dissolve at any great rate, whilstprotecting the hull vessels hull. There are generally a small number of anodes situated at critical points of thevessels hull. The power unit which supplies current to the anodes is generally controlled by monitoring referenceelectrode in order to limit the impressed current in a vessel which will sufficiently depress the hulls natural

potential so as to prevent free corrosion, but not sufficiently high to cause damage to any paint film, or to alter thechemical structure of the steel.

Sources of electric fields and approximate magnitudes:

Electric fields sources of interest cover a wide range of magnitudes and frequencies. There arestatic electric fields often referred to as UEP or SE fields and alternating electric fields or AE fields.Under water electric potential (UEP) is a unfortunate misnomer but it seems to have become acceptedterminology. Electric potential is the voltage of a point referred to another usually infinitely distant point.

It is the electric field gradient or just electric fields which is measured in practical systems ie., thedifference between two closely spaced points.

Electric field emissions from ships and submarines have similar sources, but with differentmagnitudes and frequencies. The emitted electric fields are of great interest, as they may be used to detectand characterize a ship or submarine.

Static electric fields or SE:

These fields are caused by the sum of the all static dipole sources on ship or submarine. The dccurrent flowing through the seawater, difference between parts of a vessel give raise to electric fields. The

largest sources of the dc electric current flow are the cathodic protection anodes, whether of sacrificial orimmersed current type. Many tens of the amperes of the electric current flow between these and thepropeller as well as the currents flowing to other parts of the hull. The resultant static emitted electricfields look like a single dipole when measured at a distance. The emitted signal strength is measured in

ampere meters and the product of the effective total current multiplied by effective dipole length.

The static electric emission may be measured as the DC field if the ship and the sensor arestationary or may appear as a varying field when the ship moves past a static sensor. The frequency range


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of the SE fields is generally specified as the few milliHz to few Hzs which really encompasses some ofthe AE frequency band. Static electric fields propagate a very long distances and are detectable overuseful distance with the latest generation of high sensitivity sensor, especially in costal water depths.

Electric field sensor and sensitivities:

A simple electric field sensor consists of two electrical contact points, in sea connected to ameasuring device. The key to making a successful low noise sensor is in the design of contact points or electrodes

as they are called must not generate random voltages themselves, when they are in contact with the sea. Thismeans that if two electrodes are placed in a non metallic container of the sea, where there is no electric fields, theymust not show a measureable voltage difference voltage difference voltmeter or other measurable devices. Alltypes of sensing electrodes will give a randomly varying electric signal in zero field and this is called sensorelectrode noise. SILVER/SILVER CHLORIDE electrodes are used as a sensors which have self noise levels in

the nanovolt or sub nanovolt region.

NOISE IN SENSORS AND ITS UNITS:

Noise can be expressed as the power available in unit band width or more usually as the RMSvoltage in square root of the unit bandwidth. Eg: nv/Hz. The reason for this strange unit is that noise isbandwidth dependent. This choice of unit allows different amplifier and sensor systems to be compared. Thenoise level is expressed at a given frequency particularly at 1Hz and below.

SENSORS & SENSITIVITIES:

The sensitivity of a sensor is given by the electrode total noise in nanovolts/root hertz, divided bythe electrode spacing in meters. To this is added the amplifier noise. The total sensitivity is then expressed in

nv/m/Hz

Instrumentation amplifier:

An instrumentation amplifier (IA) has two inputs and one output. It is distinguished from anoperational amplifier by its finite gain (which is usually no more than 100) and the availability of bothinputs for connecting to the signal sources. The latter feature means that all necessary feedbackcomponents are connected to other parts of the instrumentation amplifier, rather than to its noninvertingand inverting inputs. The main function of the IA is to produce an output signal which is proportional tothe difference in voltages between its two inputs:

,

Vout =A(V+ V)=AV

where V+ and V are the input voltages at noninverting and inverting inputs, respectively, and A is the gain. Aninstrumentation amplifier can be either built from an OPAM, in a monolithic or hybrid form. It is important toassure high input resistances for both inputs, so that the amplifier can be used in a true differential form. Adifferential input of the amplifier is very important for rejection of common-mode interferences having anadditive nature. An example of a high-quality monolithic instrumentation amplifier is INA118 from Burr-


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Brown/Texas Instruments (www.ti.com). It offers a low offset voltage of 50 V and a high common -moderejection ratio (110 dB). The gain is programmed by a single resistor.

Instrumentation amplifier with three operational amplifiers and matched resistors.

Although several monolithic instrumentation amplifiers are presently available, quiteoften discrete component circuits prove to be more cost-effective. Basic circuit of an IA is shown inabove fig. The voltage acrossRa is forced to become equal to the input voltage difference V. This setsthe current through that resistor equal to i =V/Ra. The output voltages from the U1 and U2 OPAMs areequal to one another in amplitudes and opposite in the phases. Hence, the front stage (U1 and U2) has adifferential input and a differential output configuration. The second stage (U3) converts the differentialoutput into a unipolar output and provides an additional gain. The overall gain of the IA is

=1 +

23

2The common-mode rejection ratio (CMRR) depends on matching of resistors within the corresponding

group (R,R2, andR3). As a rule of thumb, 1% resistors yield CMRRs no better than 100, whereas for 0.1%, theCMRR is no better than 1000. A good and cost-effective instrumentation amplifier can be built of two identicaloperational amplifiers and several precision resistors (below figure). The circuit uses the FET-input OPAMs to


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which for values indicated in Fig. B gives a gain of about 50. The connections and values of the externalcomponents are different for different types of the operational amplifier. In addition, not all OPAMs can be usedin such an unusual circuit.

The important instrumentation amplifier is INA103which is having low noise and low distortion. Thedetailed study of this amplifier is as below.

INA 103

FEATURES:

1. LOW NOISE: 1nV/Hz2. LOW THD+N: 0.0009% at 1kHz, G = 1003. HIGH GBW: 100MHz at G = 1000

4. WIDE SUPPLY RANGE: 9V to 25V5. HIGH CMRR: >100dB6. BUILT-IN GAIN SETTING RESISTORS: G = 1, 100

7. UPGRADES AD625

Description:

The INA103 is a very low noise, low distortion monolithic instrumentation amplifier. Its current feedbackcircuitry achieves very wide bandwidth and excellent dynamic response. It is ideal for low-level audio

signals such as balanced low-impedance microphones. The INA103 provides near-theoretical limit noiseperformance for 200W source impedances. Many industrial applications also benefit from its low noise andwide bandwidth. Unique distortion cancellation circuitry reduces distortion to extremely low levels, even in highgain. Its balanced input, low noise and low distortion provide superior performance compared to transformer-coupled microphone amplifiers used in professional audio equipment. The INA103s wide supply voltage(9 to 25V) and high output current drive allow its use in high-level audio stages as well. A copper lead frame in

the plastic DIP assures excellent thermal performance. The INA103 is available in 16-pin plastic DIP and SOL-16surface-mount packages. Commercial and Industrial temperature range models are available.


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The pin configuration of this amplifier is as follows:

ELECTROSTATIC DISCHARGE SENSITIVITY

Any integrated circuit can be damaged by ESD. Burr-Brown recommends that all integrated circuits be handledwith appropriate precautions. Failure to observe proper handling and installation procedures can cause damage.ESD damage can range from subtle performance degradation to complete device failure. Precision integratedcircuits may be more susceptible to damage because very small parametric changes could cause the device not tomeet published specifications.


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APPLICATIONS INFORMATIONFigure 1 shows the basic connections required for operation. Power supplies should be bypassed with 1Ftantalum capacitors near the device pins. The output Sense (pin 11) and output Reference (pin 7) should be lowimpedance connections. Resistance of a few ohms in series with these connections will degrade the common-mode rejection of the amplifier. To avoid oscillations, make short, direct connection to the gain set resistor and

gain sense connections. Avoid running output signals near these sensitive input nodes.

INPUT CONSIDERATIONSCertain source impedances can cause the INA103 to oscillate. This depends on circuit layout and

source or cable characteristics connected to the input. An input network consisting of a small inductor andresistor (Figure 2) can greatly reduce the tendancy to oscillate. This is especially useful if various inputsources are connected to the INA103. Although not shown in other figures, this network can be used, ifneeded, with all applications shown.

GAIN SELECTION

Gains of 1 or 100V/V can be set without external resistors. For G = 1V/V (unity gain) leave pin14 open (no connection)see Figure 4. For G = 100V/V, connect pin 14 to pin 6see Figure 5. Gaincan also be accurately set with a single external resistor as shown in Figure 1. The two internal feedbackresistors are laser-trimmed to 3kW within approximately 0.1%. The temperature coefficient of theseresistors is approximately 50ppm/C. Gain using an external RG resistor is

= 1 + ( )


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Accuracy and TCR of the external RG will also contribute to gain error andtemperature drift. These effects can be directly inferred from the gain equation.Connections available on A1 and A2 allow external resistors to be substituted for theinternal 3kW feedback resistors. A precision resistor network can be used for veryaccurate and stable gains. To preserve the low noise of the INA103, the value of externalfeedback resistors should be kept low. Increasing the feedback resistors to 20kW wouldincrease noise of the INA103 to approximately 1.5nV/Hz. Due to the current-feedbackinput circuitry, bandwidth would also be reduced.

NOISE PERFORMANCE:

The INA103 provides very low noise with low source impedance. Its 1nV/Hz voltagenoise delivers near theoretical noise performance with a source impedance of 200W. Relativelyhigh input stage current is used to achieve this low noise. This results in relatively high input biascurrent and input current noise. As a result, the INA103 may not provide best noise performancewith source impedances greater than 10kW. For source impedance greater than 10kW, considerthe INA114 (excellent for precise DC applications), or the INA111 FET-input IA for high speedapplications.

OFFSET ADJUSTMENT:

Offset voltage of the INA103 has two components: input stage offset voltage is produced by A1and A2; and, output stage offset is produced by A3. Both input and output stage offset are lasertrimmed and may not need adjustment in many applications.

Offset voltage can be trimmed with the optional circuit shown in Figure 3. This offset trim circuitprimarily adjusts the output stage offset, but also has a small effect on input stage offset. For a 1mVadjustment of the output voltage, the input stage offset is adjusted approximately 1V. Use thisadjustment to null the INA103s offset voltage with zero differential input voltage. Do not use thisadjustment to null offset produced by a sensor, or offset produced by subsequent stages, since this will


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increase temperature drift. To offset the output voltage without affecting drift, use the circuit shown inFigure 4. The voltage applied to pin 7 is summed at the output. The op amp connected as a bufferprovides a low impedance at pin 7 to assure good commonmode rejection. Figure 5 shows a method totrim offset voltage in ACcoupled applications. A nearly constant and equal input bias current ofapproximately 2.5A flows into both input terminals. A variable input trim voltage is created by

adjusting the balance of the two input bias return resistances throughwhich the input bias currents must flow.

Figure shows an active control loop that adjusts the output offset voltage to zero. A2, R, andC form an integrator that produces an offsetting voltage applied to one input of the INA103. This produces a6dB/octave low frequency rollofflike the capacitor input coupling

COMMON-MODE INPUT RANGEFor proper operation, the combined differential input signal and common-mode inputvoltage must not cause the input amplifiers to exceed their output swing limits.

OUTPUT SENSE

An output sense terminal allows greater gain accuracy in driving the load. Byconnecting the sense connection at the load, IR voltage loss to the load is included insidethe feedback loop. Current drive can be increased by connecting a current booster insidethe feedback loop .


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APPLICATIONS

1 HIGH QUALITY MICROPHONE PREAMPS(REPLACES TRANSFORMERS)2 MOVING-COIL PREAMPLIFIERS3 DIFFERENTIAL RECEIVERS4 AMPLIFICATION OF SIGNALS FROM: Strain Gages (Weigh Scale Applications) Thermocouples BridgeTransducers.

The other type of instrumentation amplifier is AD620:

EASY TO USE

Gain Set with One External Resistor (Gain Range 1 to 1000)Wide Power Supply Range (62.3 V to 618 V)Higher Performance than Three Op Amp IA Designs

Available in 8-Lead DIP and SOIC Packaging

Low Power, 1.3 mA max Supply Current

EXCELLENT DC PERFORMANCE (B GRADE)50 mV max, Input Offset Voltage0.6 mV/8C max, Input Offset Drift1.0 nA max, Input Bias Current

100 dB min Common-Mode Rejection Ratio (G = 10)

LOW NOISE9 nV/Hz, @ 1 kHz, Input Voltage Noise0.28 mV p-p Noise (0.1 Hz to 10 Hz)EXCELLENT AC SPECIFICATIONS120 kHz Bandwidth (G = 100)15 ms Settling Time to 0.01%APPLICATIONSWeigh Scales

ECG and Medical Instrumentation


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Transducer Interface

Data Acquisition Systems

Industrial Process ControlsBattery Powered and Portable Equipment

PRODUCT DESCRIPTIONThe AD620 is a low cost, high accuracy instrumentation amplifier that requires only one

external resistor to set gains of 1 to 1000. Furthermore, the AD620 features 8-lead SOIC and DIPpackaging that is smaller than discrete designs, and offers lower power (only 1.3 mA max supply current),making it a good fit for battery powered, portable (or remote) applications. The AD620, with its highaccuracy of 40 ppm maximum nonlinearity, low offset voltage of 50 V max and offset drift of 0.6 V/Cmax, is ideal for use in precision data acquisition systems, such as weigh scales and transducer interfaces.Furthermore, the low noise, low input bias current and low power of the AD620 make it well suited formedical applications such as ECG and noninvasive blood pressure monitors. The low input bias current of1.0 nA max is made possible with the use of Super beta processing in the input stage. The AD620 workswell as a preamplifier due to its low input voltage noise of 9 nV/Hzat 1 kHz, 0.28 V p-p in the 0.1 Hz

to 10 Hz band, 0.1 pA/Hzinput current noises. Also, the AD620 is well suited for multiplexedapplications with its settling time of 15 s to0.01% and its cost is low enough to enable designs with onein amp per channel.

Pin diagram:

ABSOLUTE MAXIMUM RATINGS1

Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 VInternal Power Dissipation2 . . . . . . . . . . . . . . . . . . . . . 650 mWInput Voltage (Common Mode) . . . . . . . . . . . . . . . . . . . . VSDifferential Input Voltage . . . . . . . . . . . . . . . . . . . . . . . .25 VOutput Short Circuit Duration . . . . . . . . . . . . . . . . . IndefiniteStorage Temperature Range (Q) . . . . . . . . . .65C to +150CStorage Temperature Range (N, R) . . . . . . . .65C to +125C


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Operating Temperature RangeAD620 (A, B) . . . . . . . . . . . . . . . . . . . . . .40C to +85CAD620 (S) . . . . . . . . . . . . . . . . . . . . . . . . 55C to +125C

Lead Temperature Range(Soldering 10 seconds) . . . . . . . . . . . . . . . . . . . . . . . +300C

THEORY OF OPERATIONThe AD620 is a monolithic instrumentation amplifier based on a modification of the

classic three op amp approach. Absolute value trimming allows the user to program gainaccurately (to 0.15% at G = 100) with only one resistor. Monolithic construction and laser wafertrimming allow the tight matching and tracking of circuit components, thus ensuring the highlevel of performance inherent in this circuit. The input transistors Q1 and Q2 provide a singledifferential pair bipolar input for high precision , yet offer 10x lower Input Bias Current thanks toSuper beta processing. Feedback through the Q1-A1-R1 loop and the Q2-A2-R2 loop maintainsconstant collector current of the input devices Q1, Q2 thereby impressing the input voltage acrossthe external gain setting resistor RG. This creates a differential gain from the inputs to the A1/A2outputs given by G = (R1 + R2)/RG + 1. The unity-gain subtracter A3 removes any common-mode signal, yielding a single-ended output referred to the REF pin potential. The value of RGalso determines the transconductance of the preamp stage. As RG is reduced for larger gains, thetransconductance increases asymptotically to that of the input transistors. This has three importantadvantages: (a) Open-loop gain is boosted for increasing programmed gain, thus reducing gainrelated errors. (b) The gain-bandwidth product (determined by C1, C2 and the preamptransconductance) increases with programmed gain, thus optimizing frequency response. (c) Theinput voltage noise is reduced to a value of 9 nV/Hz, determined mainly by the collector currentand base resistance of the input devices. The internal gain resistors, R1 and R2, are trimmed to anabsolute value of 24.7 kW, allowing the gain to be programmed accurately with a single externalresistor.

The gain equation is :

=49.4

+ 1

Different applications of AD620:

Pressure Measurement

Although useful in many bridge applications such as weigh scales, the AD620 isespecially suitable for higher resistance pressure sensors powered at lower voltageswhere small size and low power become more significant. Below Figure shows a 3 kW

pressure transducer bridge powered from +5 V. In such a circuit, the bridge consumesonly 1.7 mA. Adding the AD620 and a buffered voltage divider allows the signal to beconditioned for only 3.8 mA of total supply current. Small size and low cost make theAD620 especially attractive for voltage output pressure transducers. Since it delivers lownoise and drift, it will also serve applications such as diagnostic noninvasive bloodpressure measurement


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