The industry s Lowest Noise 10 V/G Seismic IEPE Accelerometer

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1 The indutry Lowet Noie 10 V/G Seimic IEPE Accelerometer Felix A. Levinzon Endevco/Meggitt Corp Rancho Viejo Road San Juan Capitrano, CA 9675 Robert D. Drullinger Lambda Tech LLC 998 Saratoga CT, Boulder, Colo Nomenclature NIST National Intitute of Standard and Technology PE Piezoelectric IEPE Piezoelectric accelerometer with integral electronic JFET Junction Field Effect Tranitor SCC Signal conditioning circuit EMF Electromotive force a n Accelerometer overall equivalent noie acceleration pectral denity a ntr Acceleration pectral denity determined by the PE tranducer a namp Acceleration pectral denity determined by the charge amplifier a Input acceleration a n1 Intrinic noie of the enor 1 a n Intrinic noie of the enor a 1 Noie meaured by enor 1 a Noie meaured by enor a v1 Environmental vibration noie meaured by enor 1 a v Environmental vibration noie meaured by enor v PE tranducer voltage enitivity v nout Accelerometer output noie voltage Q PE tranducer charge enitivity e Signal ource C f Feedback capacitance C PE tranducer electrical capacitance C d Decoupling capacitor R b Biaing reitor R out Output Impedance Senitivity of the accelerometer S ω η R e k T 0 Reonance frequency Lo or diipation factor Active loe Boltzmann contant Abolute temperature

2 Abtract The indutry lowet noie and lowet frequency range eimic piezoelectric (PE) accelerometer with integral electronic (IEPE) ha been deigned for meauring ultra-low-level eimic event and ultra-low-frequency vibration on tructure, platform, and other object. The accelerometer incorporate an advanced ultra-low-noie hybrid JFET-input charge amplifier and a PE tranducer operated in the flexural, circular bender bimorph mode. It ha a -pin connector and cylindrical hape with a diameter of about 65 mm and a height of about 7 mm. The frequency range i from Hz to 00 Hz at the ± 3 db level. The noie floor (equivalent input noie acceleration pectral denity) i about 00, 50, 40, 10, and 4 ng/ Hz at frequencie 0.1, 0.5, 1, 10, and 100 Hz repectively. The meaured noie floor i in good agreement with that modeled from the meaured amplifier noie combined with the electrical-thermal noie of the PE tranducer. Beide ultra-low-noie and ultra-low-frequency range, the accelerometer feature two-wire output availability, very low output impedance (R out 10 Ω), protection againt hock impact (50 g pk hock limit), and operating temperature range from -10º C to 100º C. Introduction Nowaday, demand for low-noie and low-frequency piezoelectric (PE) accelerometer with integral electronic (IEPE) ha increaed ignificantly [1] [16]. The developmental tabilization platform, geological urvey indutry, atellite ytem, and pectrocopy are ome example of application for low-noie IEPE. Beide low-noie, the advantage of uch accelerometer include high enitivity, wide dynamic, frequency, and temperature range, low-output impedance, and two-wire output availability. Seimic accelerometer are the IEPE accelerometer deigned pecifically for meauring low level vibration at very low frequencie f (typically 0.01 Hz f 1000 Hz at level of +/- 3 db) [1-16]. Since ignificant progre ha been achieved in development of high-enitivity PE tranducer and low-noie FET-input amplifier, the noie floor of the IEPE eimic accelerometer ha decreaed greatly. Indeed, ome modern low-noie eimic accelerometer have a noie floor etimated at a few hundred ng/ Hz at frequencie f 1 Hz, a few dozen ng/ Hz at frequencie 1 Hz f 100 Hz and a few ng/ Hz at frequencie f 100 Hz [1, 13]. The bet exiting low-noie eimic 10 V/g accelerometer have frequency range 0.05 Hz f 500 Hz at level ± 3 db, noie floor of Hz at frequencie of 0.5, 1, 10, and 100 Hz repectively, output impedance R out about 130, 60, 10, and 4 nv/ 100 Ω or R out 500 Ω, and maximum operating temperature 65º C [1, 13]. Thee enor are fragile to vibration, hock, and tranient impact (hock limit i 40 g pk) that create obtacle to their practical implementation epecially for application operating in vibration/hock rich environment. Deigner have been trying to decreae the noie floor more and more. The preent paper decribe deign of the indutry lowet noie and lowet frequency range 10 V/G eimic IEPE accelerometer. The low corner at level 3 db of it frequency range i about 17 time le compared with the bet exiting accelerometer. Noie floor of the deigned accelerometer i le than the bet exiting low-noie accelerometer mentioned above by at leat a factor of at frequencie 0.1 f 1 Hz and i about the ame at frequencie f 10 Hz. It i to be expected that at frequencie f < 0.1 Hz, noie of the deigned accelerometer i le than noie of the bet exiting accelerometer (there i no data about their noie at thee frequencie) by factor of 4. The accelerometer alo feature uch advantage a hock limit of about 50 g pk, output impedance R out 10 Ω, and temperature range up to 100º C. Configuration of the deigned accelerometer Configuration and deign of the preent eimic accelerometer i imilar to the eimic compact accelerometer decribed in author previou paper [6]. It incorporate the PE tranducer and the ultra-low noie JFET-input charge amplifier. But parameter of PE tranducer (namely, it charge and voltage enitivity) and characteritic of the charge amplifier (particularly, it gain, noie, and frequency repone) are different compared with the accelerometer preented in [6]. A a reult of that, parameter and ize of the complete accelerometer are alo different. For the reader convenience, we will how again the accelerometer configuration and give it hort decription (ee Fig. 1).

3 Fig. 1: Configuration of the deigned accelerometer The accelerometer incorporate the PE tranducer and JFET-input charge amplifier ued for the amplification of ignal from the PE tranducer. The charge amplifier i compried of two direct - coupling tage: the input JFET tage and the output bipolar tranitor tage providing low output impedance. Capacitance C f create ac feedback that correpond to the amplifier charge mode configuration. Biaing reitor R b for JFET i included in the reitive negative dc feedback circuit alo containing reitor R1 and R. The protection circuit (PC) i placed between the input of the charge amplifier and the PE tranducer and provide additional protection of JFET againt any tranient ignal coming from the PE tranducer. In Fig. 1, capacitance C i the PE tranducer electrical capacitance, e i the ignal ource electromotive force ( EMF), which correpond to the output voltage of the open-circuit PE tranducer, i.e., e = a V,. Q V = (1) C In equation (1), a i an input acceleration, V i the PE tranducer voltage enitivity, and Q i the PE tranducer charge enitivity. The ignal conditioning circuit (SCC) provide a power upply for the accelerometer charge amplifier and ubequent amplification of the meaured ignal. Uually, SCC i ditant from the accelerometer and connect to it with the help of a low-noie coaxial cable. A typical voltage upply i +4 V and can be from +4 to +30 Vdc. Current upply i contant and it i created by a contant current ource (for example, current-regulating diode). A typical value of current upply i 4 ma and can be in the range from to 10 ma. Decoupling capacitor C d eliminate influence of the accelerometer output dc bia voltage on the SCC input tage. Such a power upply arrangement make it poible to have only two wire (output and circuit ground) between the accelerometer and ignal conditioner, which carry output ignal and power upply at the ame time. Fig. how the charge amplifier hybrid ubtrate. It i built on the ceramic dik ubtrate with a diameter of about 15 mm.

4 Fig. : Photograph of the charge amplifier hybrid ubtrate. The amplifier contain both SMT and wirebondable component. The frequency range of the charge amplifier i from about Hz to about 100 khz at the 3 db level. Maximum output voltage i 5 V pk. Output bia voltage i from 9 to 13 Vdc at room temperature and from 8 to 14 Vdc over the temperature range. Output impedance i 10 Ohm. Start-up time (to 90 % of output bia voltage) i about 4 minute. The PE tranducer i compried of the PE element made of lead zirconate titanate (PZT) material and operate in the circular bender bimorph mode, which i well known for high charge enitivity and lower reonant frequency [17-18]. The reonance frequency of the PE tranducer i about 370 Hz. The accelerometer ha a -pin connector and cylindrical hape with a diameter of about 65 mm, height of about 7 mm, and weight about 770 gm (ee photograph in Fig.3) Noie floor of the deigned accelerometer Fig. 3: Photograph of the deigned accelerometer Since contruction of the deigned accelerometer i imilar to the eimic compact accelerometer decribed in [6], noie analyi of the accelerometer wa made baed on the noie analyi of the eimic compact accelerometer given in [6]. The equivalent noie circuit compriing all main noie ource of the charge amplifier and PE tranducer i the ame a that preented in [6]. A a reult of the noie analyi of thi circuit, the formula for the overall equivalent noie acceleration pectral denity a n wa derived [6, 8]. Noie of the PE tranducer i mainly determined by the electrical-thermal noie of the PE tranducer, which in turn i caued mainly by the lo factor of the PE element material [, 7]. Main noie ource of the charge amplifier are the thermal noie of the biaing reitor R b, the JFET channel thermal noie, and JFET 1/f noie [8, 19]. The noie floor of the whole accelerometer a n i etimated a a combination of both the charge amplifier noie a namp and the noie of the PE tranducer a ntr, which are uppoed to be uncorrelated with each other by definition: n namp a a + a ntr =. ()

5 In expreion (), a n, a namp, and a ntr are expreed in term of equivalent input noie acceleration pectral denitie. Uually, the amplifier noie dominate the PE tranducer noie for typical IEPE accelerometer. Therefore, the noie of the PE tranducer i often neglected. But for the deigned accelerometer which ue an ultra-low-noie amplifier, noie of the amplifier could be comparable or even le than the noie of the PE tranducer itelf at ome frequencie and, therefore, the latter can not be ignored. The ultra-low noie floor of the accelerometer make it very enitive to any environmental vibration noie and interference, which are alway preent under typical laboratory condition. Therefore, the direct meaurement of the accelerometer noie floor a n by meaurement of the accelerometer output noie voltage v nout uing the pectral analyzer and ubequent calculation of value a n according to the imple expreion: a n = v nout /S (S i a enitivity of the accelerometer, for example, in term of V/g) i practically impoible under uch condition. If you ue thi method you meaure mot likely the environmental noie intead of the intrinic noie of the accelerometer. Below, it will be hown that a direct meaurement of the noie floor can be accomplihed with a pair of enor on a pecial eimically iolated platform are ued. In order to etimate the noie floor of the accelerometer a n without uch a platform, another well known method wa ued. Thi method i compried of the determination of two item, a namp and a ntr eparately and then calculation of a n according to the expreion (). Fig. 4 how three curve of equivalent input noie acceleration pectral denity in ng/ Hz at frequencie from 0.1 to 00 Hz a a reult of uing thi method. 00 Noie acceleration pectral denity (ng/hz 1/ ) Amplifier (1) PE tranducer () Noie floor total (3) Frequency f ( Hz) Fig. 4: Curve of the deigned accelerometer equivalent noie acceleration pectral denity: 1- noie of the charge amplifier a namp, - noie of the PE tranducer a ntr, 3 - overall noie a n of the accelerometer. Curve 1 and how noie contribution of the charge amplifier a namp and the PE tranducer a ntr into the total accelerometer noie floor repectively. Curve 1 i an experimental curve obtained a a reult of the meaurement of the amplifier noie at it output uing a Hewlett Packard 356A Dynamic Spectral Analyzer. During thee meaurement the PE tranducer wa diconnected from the amplifier and the input of the charge amplifier wa connected in parallel with capacitance C = C. Curve i a theoretical curve obtained baed on calculation of the PE tranducer thermal noie caued by the lo factor of the PE element uing a formula for uch noie from [1, 10]:

6 a ntr ηc 4kT ωq = η 4kT ωc V = (3) In equation (3), ηi a lo or diipation factor of the PE element material, η =, where R e reflect active loe in the PE tranducer electrical capacitance C [, 7]. We aumed that the lo factor η i contant overall the frequency range. Curve 3 how the total noie floor of the deigned accelerometer a n determined according to (). You can ee from plot 3 that the noie floor of the deigned accelerometer i about 00, 50, 40, 10, and 4 ng/ Hz at frequencie 0.1, 0.5, 1, 10, and 100 Hz repectively. From Fig. 4 we can ee that at frequencie above about 0.4 Hz the PE tranducer noie predominate over or i comparable with the amplifier noie. At low frequencie below about 0.4 Hz the overall accelerometer noie i determined mainly by the amplifier noie. Among the charge amplifier noie ource, the contribution of the thermal noie of biaing reitor R and the 1/f noie of the JFET to the overall amplifier noie predominate over other noie ource at b low frequencie f 1Hz. At frequencie f 10 Hz, overall noie of the JFET amplifier i determined mainly by the JFET channel thermal noie. At frequencie 1 Hz f 10 Hz all noie ource hould be taken into account. Meaurement of the accelerometer noie One of the popular application for the deigned accelerometer i the meaurement of environmental vibration noie applied to tabilization platform that carry different device or intrument ued for preciion meaurement. In thee application vibration meaured by accelerometer play the role of undeirable interference for the tabilization platform. Specifically, the deigned accelerometer wa ued for meaurement of vibration noie applied to a pecial very quiet vibration tabilization platform at National Intitute of Standard and Technology (NIST). The intended purpoe of thi platform i to carry the world mot frequency table laer ytem ued for the atomic clock reearch conducted by NIST [0]. Thi i a paive vibration iolation platform, which i actually a heavy teel table upended by 3 m long rubber band. It iolation corner frequency i about 0.3 Hz. It i extremely effective at removing mot of the building and air born noie which i concentrated above 10 Hz. The ame platform wa ued for the meaurement of noie of the deigned accelerometer. Thi meaurement wa made uing the following method. Two the ame enor bolted baeplate-to-baeplate were mounted on the platform. The enor-pair wa placed on a mall lead-foam pad. Signal from each enor were amplified by additional low-noie charge amplifier having voltage gain about 50 and flat frequency repone at the enor frequency band. Two amplifier were very well matched in gain. Output ignal from thoe amplifier were further ubtracted one from other. A a reult, the noie ource related to the reidual environmental vibration noie were eentially common mode and, to that extent, eliminated in the ubtraction. At the ame time, the intrinic noie of the enor can not be compenated ince thoe noie are uncorrelated with each other. Indeed, the enor 1 and meaure two ignal, or rather noie a 1 and a : ωc 1 R e a 1 (t) = a v1 (t) + a n1 (t), (4) a (t) = a v (t) + a n (t). In the equation (4), a n1 and a n are intrinic noie of the enor 1 and enor repectively uncorrelated with each other. a v1 and a v are environmental vibration noie correlated with each other: a v1 (t) = a v (t) = a v (t). (5) A a reult of ubtraction, we will obtain only the two intrinic noie of thoe enor, which, ince they are uncorrelated with each other, can be preented by the following expreion: If the enor are identical a 1 (t) - a (t) = a n1 (t) - a n (t). (6) a n1 n Then difference (6) will give u value of a n for the ingle enor: n a a, (7)

7 a 1 (t) - a (t) = n1 n n a + a = a. (8) The ue of two enor bolted baeplate-to-baeplate gave high common-mode rejection for the reidual eimic noie on the platform. The ue of the platform and it urrounding quiet houe jut reduced the amount of noie that needed to be rejected by thi technique. In practice, correlation relation between thoe noie and equality of the enor are not o precie a wa mentioned above. Neverthele, the decribed method of meaurement of the enor noie in combination with the condition of the quiet NIST platform provided uppreion of the environmental vibration noie to a coniderable extend allowing meaurement of ultra-low value of the intrinic noie of the deigned accelerometer at frequencie from 0.0 Hz to 100 Hz Noie acceleration pectral denity (ng/hz 1/ ) Experiment (1) Specification () Frequency f ( Hz) Fig. 5: Experimental (1) and pecification () curve of the noie floor of the deigned accelerometer. Fig. 5 how the experimental curve (curve1) and pecification curve (curve ) of noie floor of the accelerometer in term of input acceleration pectral denity. Curve 1 i drawn a a reult of the meaurement mentioned above. Curve how the accelerometer noie floor obtained a a reult of noie etimation made above according to expreion () baed on the meaurement of the amplifier noie and calculation of the PE tranducer noie. You can ee from Fig. 5 that experimental noie data (curve 1) have good correlation with pecification noie data (curve ). Concluion The indutry lowet noie 10 V/g eimic accelerometer ha been deigned, fabricated, and teted. The accelerometer ha a -pin connector and cylindrical hape with a diameter of about 65 mm, height of about 7, and weight about 770 gm. The frequency range i from Hz to 00 Hz at the ± 3 db level. The noie floor (equivalent input noie acceleration pectral denity) i about 00, 50, 40, 10, and 4 ng Hz at frequencie 0.1, 0.5, 1, 10, and 100 Hz repectively. The meaured noie floor i in good agreement with that modeled from the meaured amplifier noie combined with the electrical-thermal noie of the PE tranducer. In addition to ultralow-noie and ultra-low-frequency range, the accelerometer feature two-wire output availability, very low output impedance (R out 10 Ω), protection againt hock impact (50 g pk hock limit), and operating temperature range from -10º C to 100º C.

8 Acknowledgement The author thank the Endevco/Meggitt Corporation for upport. Reference [1] Walter P. L. The handbook of dynamic force, preure and acceleration meaurement, 001. []. Schlo F., Accelerometer noie, Sound and Vibration, pp. 3, Mar [3] Gabrielon T.B., Modeling and meauring elf-noie in velocity and acceleration enor, in Pro. AIP Conference, vol. 368, 1995, pp [4] Gabrielon T. B., Mechanical-thermal noie in micromachined acoutic and vibration enor, IEEE Tranaction on Electron Device, vol. 40, no. 5, pp , May [5] Wlodkowki P. A. et al. The development of high-enitivity, low-noie accelerometer utilizing ingle crytal piezoelectric material, Senor and Actuator, vol. 90, pp , April 001. [6] Levinzon F. A. Ultra-low-noie 10 v/g compact IEPE eimic accelerometer, IMAC-6, 008. [7] Levinzon F.A., Fundamental noie limit of piezoelectric accelerometer, IEEE Senor Journal, vol. 4, no. 1, pp , February 004. [8] Levinzon F.A., Noie of piezoelectric accelerometer with integral FET amplifier, IEEE Senor Journal, vol. 5, no. 6, pp , December 005. [9] Boer B.E., and Howe R.T., Surface micromachined accelerometer, IEEE Journal of Solid-tate Circuit, vol.31, pp , March, [10] Dynamic Tet Handbook, Endevco Corp. [11] Judd J.E., Ultra high-temperature enor developed for NASA, Tet Engineering & Management, vol. 60, no. 5, p.9, October/November [1] Seimic Accelerometer, Model 731A, Specification Sheet, Wilcoxon Reearch, 05/004. [13] Seimic Accelerometer, Model 393B31, Specification Sheet, PCB Piezotronic. [14] Seimic Accelerometer, Model , Specification Sheet, Wilcoxon Reearch, 01/001. [15] Seimic Accelerometer, Model 393B1, Specification Sheet, PCB Piezotronic. [16] Senor for acceleration, hock, and vibration. In Shock and vibration catalog, PCB [17] Germano C. P., Flexure mode piezoelectric tranducer, Technical publication TP-18, Morgan Electro Ceramic, 005. [18] Cantilever mounted PZT 5A bimorph, Technical publication PT-45, Morgan Electro Ceramic, 005., [19] Levinzon F. A., Noie of the JFET amplifier, IEEE Tranaction on Circuit and Sytem, vol. 47, no. 7, pp , July 000. [0] Young B. C. et al. Viible laer with ubhertz linewidth, Phyical Review Letter, vol. 8, no. 19, pp , May 1999.