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Practical considerations of accelerometer noise By: Bruce Lent IMAC XXVIII The information contained in this document is the property of [name of Meggitt company] and is proprietary and/or copyright material. This information and this document may not be used without the express authorization of [name of Meggitt company]. Any unauthorized use or disclosure may be unlawful. Information contained in this document is subject to U.S. Export Control regulations, specifically the (choose as appropriate) International Traffic in Arms Regulations and / or Export Administration Regulations. Each recipient of this document is responsible for ensuring that transfer or use of any information contained in this document complies with all relevant (choose as appropriate) International Traffic in Arms Regulations and / or Export Administration Regulations.

Page 3 ©Endevco. Proprietary. 6 August 2009 Topics covered Importance of noise level Charge mode accelerometers Voltage mode accelerometers Piezoresistive Important terms

Page 4 © Endevco. Proprietary. 9 January 2009 Voltage (IEPE) /Charge Mode Comparison) Integrated accelerometer/charge Amplifier (IEPE) Accelerometer – cable – remote charge amplifier

Page 3 ©Endevco. Proprietary. 6 August 2009 Importance Determine the minimum discernible signal with reasonable resolution. Low level signals Wide dynamic range High level shock testing

Page 3 ©Endevco. Proprietary. 6 August 2009 Low – level signals 3 X the noise level

Page 3 ©Endevco. Proprietary. 6 August 2009 High level signals – shock pulse

Page 3 ©Endevco. Proprietary. 6 August 2009 Noise terminology Resolution: Broadband noise over a specified bandwidth in terms of mg rms or µg rms Residual noise: Is the same as resolution but usually used for electronic amplifiers Threshold: Noise expressed in terms of mg rms or µg rms Threshold = maximum noise/sensitivity Spectral Noise: Noise level over a limited portion of the total bandwidth expressed in V/Hz (may be volts or g)

Page 3 ©Endevco. Proprietary. 6 August 2009 Spectral noise

Charge mode noise sources The information contained in this document is the property of [name of Meggitt company] and is proprietary and/or copyright material. This information and this document may not be used without the express authorization of [name of Meggitt company]. Any unauthorized use or disclosure may be unlawful. Information contained in this document is subject to U.S. Export Control regulations, specifically the (choose as appropriate) International Traffic in Arms Regulations and / or Export Administration Regulations. Each recipient of this document is responsible for ensuring that transfer or use of any information contained in this document complies with all relevant (choose as appropriate) International Traffic in Arms Regulations and / or Export Administration Regulations.

Page 3 ©Endevco. Proprietary. 6 August 2009 Charge mode noise sources 1/F noise is produced by impurities in the PE material Mechanical and thermal noise Loss factor of the PE material Long cable length adds capacitance Triboelectric noise – low noise cable required Electronic amplifiers are the largest noise contributors

Page 3 ©Endevco. Proprietary. 6 August 2009 Capacitance Sources The Accelerometer This value is measured during calibration and listed on the data sheet The Cable Capacitance is specified in pC/foot (meters)

Page 13 © EndevcoProprietary. 7 August 2009 Routing the cabling First we will discuss TRIBOELECTRIC EFFECT: This is the self generation of electrical noise from within the cable due to the flexing of the cable itself. LOW NOISE treatment is and has been the solution to minimizing this effect Mounting Solution Below Then Tie Down Every 6 to 12 Put a Dab of Clear finger Nail Polish at the connection

Basic Low Noise Cable

Low Noise application Wrapped – Teflon tape coated with carbon material on both sides. Dipped (Smaller gauge wire) – Wire is passed through a reservoir containing the low noise dispersion material. The low noise film is then fused to the cable by heat.

Page 3 ©Endevco. Proprietary. 6 August 2009 Cable Length Myth: Charge mode accelerometer cables must be short Fact: This is dependent on resolution requirements and ambient EMI conditions. EMI problems are usually resolved using differential accelerometers. In the following example, we will see how good performance can be obtained with a 500 foot cable

Page 3 ©Endevco. Proprietary. 6 August 2009 Cable Length Facts As the input shunt capacitance of a charge amplifier is increased the input noise is increased. As cable length is increased, the shunt capacitance is typically increased by 30pF/foot (check cable specifications for exact amount). Rule of Thumb: Input noise increases by pC rms per 1000 pF of source capacitance. Source capacitance includes the total cable capacitance and the accelerometers capacitance.

Page 3 ©Endevco. Proprietary. 6 August 2009 Cable Noise Calculation Given: Cable length = 500 feet Cable capacitance = 30 pF/foot Accelerometer capacitance = 350pF Accelerometer sensitivity = 40 pC/g In pC rms = 0.03 [amplifier residual noise] +(0.008 x 15.35) = pC rms Since noise is random, multiply the above rms value by the crest factor of three to obtain the peak value. Pc peak noise= 3 x pC rms = pC peak Equivalent noise = /40 pC/g = g Since a 3 to 1 signal to noise ratio is necessary for a measurement, multiply g x 3 = 0.344g to obtain the minimum measurable acceleration.

Page 3 ©Endevco. Proprietary. 6 August 2009 Piezoresistive accelerometers Generally low noise 1 – 5 µV Often not specified because of their high level applications Electronic amplifiers are the major contributor Low impedance – cable noise not a problem

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The information contained in this document is the property of Endevco and is proprietary and/or copyright material. This information and this document may not be used or disclosed without the express authorization of Endevco]. Any unauthorized use or disclosure may be unlawful. The information contained in this document may be subject to the provisions of the Export Administration Act of 1979 (50 USC ), the Export Administration Regulations promulgated thereunder (15 CFR ), and the International Traffic in Arms Regulations (22 CFR ). The recipient acknowledges that these statutes and regulations impose restrictions on import, export, re-export and transfer to third countries of certain categories of data, technical services and information, and that licenses from the US Department of State and/or the US Department of Commerce may be required before such data, technical services and information can be disclosed. By accepting this document, the recipient agrees to comply with all applicable governmental regulations as they relate to the import, export and re-export of information.'