Alegria, F. Corrêa

Mànuel, Antoni

Panahi, S. Shariat

Revistes Catalanes amb Accés Obert

Characterization of a High Resolution Acquisition System : for Marine Geophysical Applications

Manuel Lázaro, Antonio

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Instrumentation ViewPoint.Autumn 2005551. IntroducciónThe progress of the marine instrumentation inthe last decade is continuously bringing up newgoals as higher autonomy, lower powerconsumption and better signal quality, to beachieved by the engineers.The quality of the signal is one the main issuesrelated to acquisition systems built for marinegeophysical applications where the signal levelcan be very low depending on the depth ofinstrument operation. In this field, Ocean BottomSeismometers (OBS) are widely used for thestudy of the seabed sub-layers of a certain areawhen instruments are deployed on the seabottom, at the depth of up to 6000m, where theycollect data for about 3 months. They are moreused in active seismicity where the artificialsource is a compressed air-gun dragged by theoceanographic vessel at low depth, generatingacoustic wave fronts every certain time, thattravel to the bottom of the ocean and reflectand refract by the different sub-layers beforebeing acquired by the OBS. The equipmentuses three sensors: a tri-axial geophone thatcollects seabed vibrations, a hydrophone thatcollects water pressure data and an acoustictransducer for the recovery process that startsby reception of a signal generated by the deckunit on the ship, releasing the attached weightand allowing the OBS to rise up to the surfacedue to its structural floatability.In active seismicity, the maximum operationdepth of the OBS places the equipment morethan 6000m away from the acoustic source,reducing the signal level received by theinstrument and increasing the need of a highSignal to Noise Ratio(SNR), high resolution andhigh Dynamic Range(DR) acquisition system[6]. This paper presents the characterization ofa four channel, 24 bits acquisition system,designed and built for marine geophysicalapplications, where different tests based on“IEEE-STD-1057 Standard for DigitizingWaveform Recorders” and “IEEE-STD-1241Standard for Terminology and Test Methods”have been carried out to calculate the mainparameters of the system taking into accountits final application.2. The Acquisition systemThe acquisition system is based on a 4 channel24 bits CS5372/76 analog to digital conversion(ADC) together with input signal amplification.The CS5372 are two channel high dynamicrange, fourth order •-• modulators designed forgeophysical and sonar applications. Used incombination with CS5376 digital filter, a uniquehigh resolution A/D measurement system resultswhich all together provide higher dynamic rangeof 124 dB @ 411Hz bandwidth and a TotalHarmonic Distortion(THD) of –118dB [3], whileconsuming low power per channel. Themodulators generate an oversampled serial bitstream at 512kbits per second when operatedfrom clock frequency of 2.048 MHz.The input stage of the acquisition system isbased on OP97 and OP297 low noise amplifiersin a single ended to differential signal converterconfiguration, due to the fact that the signalcoming from the geophone is single ended andthe CS5372 ADC is differential. A MC68332microcontroller is in charge of the configurationof the CS5376 digital filter selecting samplingrate, filter output and low power operations,through a QSPI (Queued Serial port interface)bus. All the necessary signals for the dataloggerto function correctly have to be generated froma single crystal and being aware of the fact thatin any marine seismic application, timing is ofgreat importance, a Vectron OC260 32.768MHzcrystal is used offering a stability of 40ppb andan electronic module is designed and builtintegrating a ICS52701 phased locked loopgenerating different signal frequencies for thesystem.Characterization of a High Resolution Acquisition Systemfor Marine Geophysical ApplicationsS. Shariat Panahi (1), F. Alegria (2), A. Manuel (1)(1) Vilanova i la Geltrú Technological Center – Technical University of Catalonia (UPC)Rambla Exposicio s/n, 08800, Vilanova I la Geltru, Spain(2) Technical University of Lisbon (IST), Av. Rovisco Pais, 1049-001, LisbonFigure 1: Ocean Bottom Seismometer (OBS)Instrumentation ViewPoint. Autumn 200556where 2 is the noise variance, yan and ybn arethe data samples of successive acquisitionsand M is the number of samples/channel. TheSave Panel Image allows to store the final resultsfor post-processing.- Crosstalk test [1][2]: The calculation of crosstalklevel of a multi-channel acquisition system is agood evaluation of the PCB design of themodule, as this parameter is result ofElectromagnetic Interference (EMI) betweendifferent parts of the board as well as the groundand power supply plane problems. A full-scalesignal is applied to the channel under test whileall the other channels are shielded and a 4096samples/channel data acquisition is carried outand stored in a data file. The Crosstalk Analysisoption of the designed software reads the datafile stored previously and calculates the crosstalklevel related to every channel using the followingexpression:4. ResultsThe need of a high-resolution system with ahigh Signal-to-Noise Ratio (SNR) to collect lowlevel signals has taken us to use low levelamplitude signals using a DS630 ultra-lowfunction generator together with batteries forpower supply of this module to reduce noisefrom external elements. The bandwidth of thesystem is defined by the selected samplingfrequency as in CS5372/76, this is given by40% of the sampling frequency, in this case100Hz.The 50 Hz line coupling through the input signalis corrected by eliminating the samplescorresponding to this frequency and itssuccessive harmonics, and replacing theeliminated samples by the last value before the50Hz interference peak set of DFT points. Thenumber of DFT points to be eliminated at 50 Hzhas to be adjusted by the user before obtainingthe final result of the test. For low amplitudesignals, the frequency of the input signal has tobe introduced by the user as LabVIEWfundamental frequency detection functions donot operate below a certain signal amplitudelevel.The characterization tests have been carriedout over the system bandwidth (100Hz) and theresults of different parameters have beengathered in graphs. Figures 2 and 3 show theDynamic Range (DR) and crosstalk resultsobtained during the implemented tests:3. Characterization testsThe designed test programming is based onthe graphical language LabVIEW 7.1, where afrequency analysis of the acquired data isperformed. In order to characterize theacquisition system built, the following testsbased on the IEEE-1057 and IEEE-1241standards have been carried out:- Discrete Fourier Transform (DFT) test- Random noise test- Crosstalk testTaking into account that the noise analysis isof great importance in any acquisition systemaimed for geophysical applications and 24 bitsresolution of the system under test, an ultra-low noise and distortion Stanford ResearchSystem (SRS) DS630 function generator isused to generate the input signals and AA typebatteries are used to supply the acquisitionsystem. The DS630 function generator presentsa –110dB Total Harmonic Distortion (THD) in5kHz bandwidth and minimum 10uVppamplitude allowing to characterize the systemusing low input amplitudes as in real conditions.- DFT test [1][2]: A sine wave signal with knownamplitude and frequency is applied at the inputof the system and 4096 samples/channel witha sampling frequency of 250sps are acquiredand transferred to a PC, where the test programin LabVIEW stores the data in a data file. TheDFT Analysis option of the test program readsthe data file stored previously, calculating theDFT of the acquired data and the desiredparameters from the DFT data for every channelseparately. The Save Panel Image option allowsthe user to store the front panel data into a filefor post-processing. As signal level received bythe geophone can be very small due to the factthat the instrument can be deployed at morethan 6000m away from the artificial source, thetotal noise performance of the system resultsin a key feature of the overall equipment,therefore parameters as Signal-to-Noise Ratio(SNR), Dynamic Range (DR) and EffectiveNumber of Bits (ENOB) have been calculated.In addition, to characterize the system non-linearities, the calculation of parameters basedon harmonic distortion has been included.- Random Noise test [1][2]: In order to calculatethe noise generated by the acquisition systemcircuitry, in this case by the input amplifier stageof the module, a random noise test has beenimplemented. All the channels under test aregrounded and two separate acquisitions of 4096samples/channel at 250sps sampling frequencyhave been carried out and stored in two datafiles. The Random Noise Analysis option of thedesigned software reads the data files previouslystored and by subtracting the data samples,eliminates the ADC fixed-pattern errors [4] thatoccur in the same location in successive recordsand calculates the noise variance.Figure 2: Dynamic Range (DR) results, samplingfrequency: 250sps, number of samples/channel: 4096Instrumentation ViewPoint.Autumn 200557Figure 3: Crosstalk results, sampling frequency:250sps, number of samples: 4096Determination of the Confidence Interval for the ENOB of and ADCtested with the IEEE 1057 Random Noise TestF. Corrêa Alegria(1), S. Shariat Panahi(2), A. Manuel(2), A. Cruz Serra(1)(1) Instituto de Telecomunicações, Av. Rovisco Pais 1, 1049-001 Lisboa, PortugalTelefone: +351-218418485, Fax: +351-218417672, e-mail: falegria@lx.it.pt(2) Vilanova i la Geltrú Technological Center – Technical University of Catalonia (UPC)Rambla Exposicio s/n, 08800, Vilanova I la Geltru, Spain.1. IntroductionAnalog to Digital (ADC) converters are nowadaysextensively used in marine instrumentation. Itbrings several benefits to a data acquisitionsystem like an efficient storage and processingof the gathered information. In certainappl icat ions, l ike the Ocean BottomSeismometer (OBS) developed in Vilanova i laGeltrú Technological Center where the signalsto be measured are very small, the importancein reducing noise in the acquisition system isparamount. One of the components thatintroduce noise is precisely the ADC.Several tests exist to determine the noiseproduce by an ADC. One of them, described inthe 1057 IEEE Waveform Digitizers Standard[1], is used to measure random noise andconsists in shorting the ADC input and acquiringtwo sets of M samples each (ya and yb) andsubtracting, point by point the values of thesamples in each record. The noise standarddeviation, expressed in least significant bit units(LSB), is then calculated from the mean squarevalues:If the amount of random noise present is smallcompared with the quantization step, this methodcan not be used because the sample values willalways be the same and the result will be 0. Tocircumvent this problem, a small triangularstimulus signal should be applied to the input[1]. The start of the acquisition of the two setsshould be triggered on the same level.The amount of random noise may also beexpressed in a parameter called EffectiveNumber of Bits (ENOB). It states the number ofbits of an ideal ADC with a quantization noiseThese results show a 120dB dynamic rangeover the system bandwidth, which means areduction of 10dB of the CS5372/76 dynamicrange, while the crosstalk figures obtained inthe test are 25dB above the modulatorspecification value for below 10Hz input signalfrequency and about 40dB above the samevalue for over 90Hz input signal frequency. Thenoise analysis of the acquisition system hasshown an Effective Number of Bits (ENOB) ofabout 18 bits that can be further optimised byusing fully differential amplifiers as CS3301-2with a noise figure of only 8.5nV/Hz. The useof fully differential amplifiers together withimprovement of ground and power supply planeswill improve the crosstalk results considerably[5].5. ConclusionsThe need of a high resolution and high signal-to-noise ratio acquisition system in order to beused in deep ocean seismometers has takenus to design and implement a datalogger whosecharacterization tests have been explained inthis paper. The implemented test results haveshown a dynamic range of about 120dB forbelow 80Hz input signal frequency at 250Hzsampling rate, leading to a 18bits of ENOB,while similar equipment available in the marketoffer 78dB of dynamic range and 14 bits ofENOB at 200Hz sampling rate.However, noise features of the system can befurther optimized by using CS3301-2 low noisefully differential amplifiers, that also improve thechannel crosstalk levels detected during thetests.6. References[1] IEEE-STD-1057 Standard for DigitizingWaveform Recorders.[2] IEEE-STD-1241 Standard for Terminologyand Test Methods for Analog- to-DigitalConverters.[3] Cirrus Logic, “CS5371/72 low-Power highperformance •-• modulators, 2001.[4] Kent, H Lundberg . “Analog to Digitalconverter testing”, 2002.[5] Dallas-Maxim, “Effects of digital crosstalk indata converters”, 2002.[6] J. Basilio Simoes, Custodio Loureiro, JorgeLandeck, Carlos Correia, “Testing high resolutiondigitizers using conventional signal sources”,Instrumentation Centre of the PhysicsDepartment, University of Coimbra, 1997.(1)

Characterization of a High Resolution Acquisition System : for Marine Geophysical Applications

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