Alegria, F. Corrêa

Cruz Serra, A.

Manuel Lázaro, Antonio

Panahi, S. Shariat

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Instrumentation 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)equal to the noise of the real ADC under test. Itis calculated from the estimated random noisestandard deviation usingwhere nb is the number of bits of the ADC andQ is the ideal quantization width. The quantizationnoise of an ideal ADC is  [1].Specifying these parameters is important, forinstance, in the comparison of different acquisitionsystems or in evaluating the influence of differentfactors in the performance of a given system.One should however be concerned with theuncertainty of the estimates obtained. In anymeasurement made, whatever the domain, thequality of the results should be stated clearly. Thegenerally accepted way to do it is the onepresented in the Guide for the Expression ofUncertainty in Measurement (GUM) [2].To make this possible in the area of ADC testing,the Instrumentation Group at Instituto deTelecomunicações in Lisbon, Portugal, has beenworking in the precision of several ADC testmethods. The conclusions arrived, concerningthe uncertainty of the estimated random noisestandard deviation obtained with the RandomNoise Test of ADCs, was published in [3]. Herewe will extend that study to the uncertainty of theENOB. This will be used to present the results ofthe test made on a module containing a 24-bitADC used in the OBS.2. Uncertainty of the ENOBThe following expression for the uncertainty ofthe random noise standard deviation waspresented in [3] and thoroughly justified,mathematically and experimentally:Since ENOB is a function of Or we can derive teuncertainty of ENOB from the uncertainty  of Or:Deriving  in order to  leads toInserting into  (4) givesInstrumentation ViewPoint. Autumn 200558Figure 1. Representation of the ENOB as afunction of the number of active channels. Thevertical bars represent the confidence intervalfor a confidence level of 95%.(2)(3)(4)Using (3) and considering that the test is unbiased we finally obtainWe can se that the uncertainty of the estimatedENOB does not depend on the ADC parameters,namely, on the Number of Bits or the Full ScaleRange. For large values of noise standarddeviation it depends only on the number ofsamples chosen.3. Practical ApplicationThe expression derived here is used now topresent the results of a study made on theacquisition module of the OBS. We analyzed theeffect of number of active channels on the randomnoise present on one channel.Since we acquired 4096 samples in each set thestandard uncertainty of the estimated ENOB is,from , equal to 0.016. Using a coverage of factorof 2 (Ku) we obtain an expanded uncertainty of0.032.The ENOB as a function of the number of activechannels is represented in Figure 1. The verticalbars, representing the confidence interval allowsthe conclusion, from observation of the chart, thatthe number of active channels has indeed aninfluence on the ENOB and that the differentvalues of ENOB are not just due to uncertaintyin the measurement.4. ConclusionsWe maintain that it is important to translate thequality of a measurement through the specificationof a confidence interval stating its uncertainty.We derived the expression for the determinationof the uncertainty of the ENOB of an ADC testedwith the Random Noise Test and applied it to apractical example of data acquisition moduletesting used in marine instrumentation.5. AcknowledgmentsThis work was sponsored by the Portuguesenational research project entitled “New errorcorrection techniques for digital measurementinstruments”, POCTI/ESE/46995/2002, whosesupport the authors gratefully acknowledges.(5)(6)(7)Instrumentation ViewPoint.Autumn 200559Using the parallel port for data acquisition.A. Pazos (1), G. Alguacil (2), J. M. Davila (1)(1) Real Instituto y Observatorio de la Armada. Cecilio Pujazón s/n. 11.100 San Fernando.Tel.- 956 599285   pazos@roa.es(2) Instituto Andaluz de Geofísica, Observatorio de la Cartuja, Universidad de Granada, 18.071Granada1. IntroductionA PC based acquisition system normally usesserial ports to acquire digital data from an A/Dconverter. Also, serial ports are used forcommunication with GPS card (to get GPSmessages), communication with data Centresby phone modem, and with others local PC’s.Therefore, a large number of serial ports areneeded, but this is no a problem in modernsPC that usually have enough USB and serialsports.We have developed, in the GeophysicalDepartment of the Royal Naval Observatory(ROA), an acquisition system based on a Linuxembedded PC using the parallel port for collectdigital data from a high resolution A/D converter.Serial ports are reserved to GPS and datatransmission communications.In this work an overview of the overall systemis presented, focus on the parallel port function.2. The PC based acquisition system.The overall system is shown in figure 1. We usea 1Hz Mark L4C sensor with the band extended[1] using a simple capacitor. The amplifier hasa non-inverter configuration in order to obtaina high input impedance.The designed A/D card includes the A/Dconverter and the serial to parallel interface.We have used a CS5323/CS5322 highresolution 24 bits A/D converter that providesa high dynamic range (> 130 dB at 125 sps).This converter uses the oversampling technique,so analogical antialiasing filters are not needed.Tree 74HC595 is used for the serial to parallelconversion and to carry out the byte transfer bythe parallel port of the PC. A completeexplanation of this card can be found in [2].The system is synchronized using an ONCOREUTC GPS. The pps signal is connected to theIRQ11 (pin 4 of the PC104 connector) and theGPS messages are received by COM1. A PLLsoftware, running in kernel space, was developedby the ROA Time department. It acts over thePC clock system and in the worse case themaximum observed differences was better than700 microseconds.The parallel port has 4 control line (output), 5state lines (input) and 8 data lines (input/output).It can operate in several mode but we haveconfigured it in the Enahanced Parallel Port(EPP) mode [3]. This mode permits abidirectional data transfer, having a rate between500K to 2 M bytes per second, and the I/Ocycles are hardware controlled. Figure 2 showsthe read cycle handshake.Figure 1. A PC based acquisition system scheme.Figure 2. A EPP mode read cycle handshake.Figure 3. Data acquisition process.6. References[1] IEEE-STD-1057 Standard for DigitizingWaveform Recorders.[2] BIPM, IEC, IFCC, ISO, IUPAC, IUPAP, OIML, “Guide to the Expression of Uncertainty inMeasurement”, ISO, 1995.[3] F. Alegria, A. Cruz Serra, “Uncertainty of ADCRandom Noise Estimates Obtained With the IEEE1057 Standard Test”, IEEE Transactions onInstrumentation and Measurement, vol. 54. nº 1,pp. 110-116, February 2005.

Determination of the Confidence Interval for the ENOB of and ADC : tested with the IEEE 1057 Random Noise Test

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