Page 3 ©Endevco. Proprietary. 6 August 2009. Practical considerations of accelerometer noise By:...

Page 3 ©Endevco. Proprietary. 6 August 2009

Practical considerations of accelerometer noiseBy: Bruce LentIMAC 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


Importance

Determine the minimum discernible signal with reasonable resolution.

Low level signals

Wide dynamic range

High level shock testing


Low – level signals 3 X the noise level


High level signals – shock pulse


Noise terminology

Resolution:Broadband noise over a specified bandwidth in terms of mg rmsor 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 rmsThreshold = maximum noise/sensitivity

Spectral Noise: Noise level over a limited portion of the totalbandwidth expressed in V/√Hz (may be volts or g)


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.


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


Capacitance Sources

The AccelerometerThis value is measured during calibration and listed on the data sheet

The CableCapacitance 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 ofClear finger NailPolish 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.


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


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 0.008 pC rms per 1000 pF of source capacitance. Source capacitance includes the total cable capacitance and the accelerometer’s capacitance.


Cable Noise Calculation

Given:Cable length = 500 feetCable capacitance = 30 pF/foot

Accelerometer capacitance = 350pFAccelerometer sensitivity = 40 pC/g

In pC rms = 0.03 [amplifier residual noise] +(0.008 x 15.35)= 0.1528 pC rmsSince noise is random, multiply the above rms value by the crest factor of three to obtain the peak value.Pc peak noise= 3 x 0.1528pC rms = 0.4584 pC peakEquivalent noise = 0.4584/40 pC/g = 0.0115gSince a 3 to 1 signal to noise ratio is necessary for a measurement, multiply 0.0115g x 3 = 0.344g to obtain the minimum measurable acceleration.


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



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 2401-2420), the Export Administration Regulations promulgated thereunder (15 CFR 730-774), and the International Traffic in Arms Regulations (22 CFR 120-130). 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.'

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