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SENSORS & SENSING SYSTEMS n
: UNDERSTANDING N MEMS SENSORS
FEATURE
range) and the ADXL380 (up to ±16g range) have the same survivability rating as a ±400g range sensor like the ADXL373, which is 10,000kg peak amplitude, 0.1ms pulse width half-sine wave shock profile. However, the ADXL373 has a much higher vibration mechanical headroom. The shock tolerance rating can be seen as a system- level test. Not only is the MEMS sensor put under test, but also the integrity of wire bonds, die attach, package, and even solder joints need to withstand this test. These MEMS sensors from ADI might be able to survive repeated shocks even higher than 10,000g without structural failure, but it might not be the case for other parts of the system that form part of the device.
SHOCK TESTING Replicating standardised shock tests in-house is often challenging due to the need for specialised equipment – such as drop towers or shock tables – that can be several meters tall and precisely controlled. As a result, engineers frequently seek practical alternatives. One common approach is to reduce the peak acceleration while increasing the pulse duration, maintaining equivalent shock energy. This is based on the principle that the area under the acceleration- time curve (velocity) is a key indicator of shock, as it correlates with the impact energy. For a half-sine shock pulse severity, the impact velocity can be estimated using Equation 1:
Figure 2. ADXL357B mechanical headroom vs. frequency. The mechanical headroom decreases near resonance, highlighting the importance of
damping in sensor design. The ADXL357B is rated to 70g peak amplitude continuous random vibration with frequency content from 0Hz to 2kHz, based on MIL-STD-883 Method 2007, Test Condition C specifications
experience continuous random vibration in the tens of g’s (or even over 100g) peak amplitude and require high tilt precision, high temperature stability and repeatability. An accelerometer like the ADXL357B is the
perfect candidate in terms of performance. Even though its full-scale range is limited to ±40g, it can withstand larger vibrations. The vibration safe zone is highly dependent on the sensor’s mechanical design, including its resonant frequency, damping, and the acceleration input required to hit the mechanical stoppers (called mechanical headroom). To illustrate the vibration safe zone, we can examine the mechanical headroom vs. frequency, as shown in Figure 2. This helps engineers understand how
much margin exists before the proof mass hits the stoppers, and how the sensor resonant frequency and quality factor play a role in it. As the input vibration gets mechanically amplified by the quality factor, the closer the vibration frequency is to the sensor resonance, effectively reducing the mechanical headroom.
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ELECTRICAL BANDWIDTH AND MECHANICAL LIMITS Accelerometers usually have analogue and digital filters on their built-in signal chain, and newer sensors, like the ADXL380, even have a digital equaliser filter, effectively extending its bandwidth flatness up to 4kHz. This is useful for applications like road noise cancellation (RNC), where accurate detection of broadband vibrations is critical for generating effective anti-noise signals. However, it’s important to note that electrical
filtering or equalisation does not eliminate the physical excitation of the MEMS structure. The sensor still experiences mechanical stress, and operating beyond its mechanical headroom can result in stiction, fatigue, or structural degradation. Designers must ensure that vibration amplitudes remain within safe mechanical limits, even if the electrical output appears linear across the extended bandwidth.
SHOCK VS SENSOR FULL-SCALE RANGE It is worth noting that the ADXL357B (±40g
where V is velocity in m/s, A is peak acceleration in m/s², and D is pulse duration in milliseconds. For example, both a 200g shock lasting 3ms and a 3000g shock lasting 0.2ms result in an impact velocity of approximately 3.8m/s, as specified in IEC 60068-2-27. This equivalence allows for more accessible test setups while still approximating the mechanical energy of a standard shock event.
CONCLUSION Shock survivability and vibration tolerance are often misunderstood as interchangeable specifications, yet they represent fundamentally different stress profiles and failure mechanisms in MEMS accelerometers. Shock events are rare but intense, requiring robust mechanical design to prevent catastrophic damage, while vibration is persistent and insidious, demanding long-term reliability and resistance to wear. Understanding these distinctions is critical
for engineers selecting sensors for harsh environments. By considering factors such as mechanical headroom, damping, and system-level robustness, designers can ensure that the chosen sensor not only survives but performs reliably over time. Ultimately, matching the sensor’s
specifications to the application’s mechanical stress profile is essential for achieving both durability and precision. With the right sensor and a clea r understanding of its limitations and strengths, engineers can confidently deploy MEMS technology in even the most extreme conditions.
Analog Devices
www.analog.com
MAY 2026 DESIGN SOLUTIONS 29
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