Skip to content
RFrftools.io
Sensor

Accelerometer Range & Sensitivity

Calculate accelerometer output voltage, ADC resolution in mg per LSB, and full-scale range. Design MEMS vibration and motion sensor interfaces.

Loading calculator...

Formula

Vout=Vref/2±(S×a)V_out = V_ref/2 ± (S × a)
S— Sensitivity (mV/g)
a— Acceleration (g)

How It Works

This calculator converts accelerometer sensitivity specifications to resolution and output voltage, essential for vibration analysis engineers, inertial navigation designers, and IoT motion sensing developers. MEMS accelerometers output voltage or digital counts proportional to acceleration: Vout = Vref/2 + S a, where S is sensitivity in mV/g and a is acceleration. Analog MEMS (ADXL335, MMA7361) provide 200-800 mV/g sensitivity with +/-1.5 to +/-16 g full-scale range per Analog Devices and NXP datasheets. Sensitivity scales inversely with range: wider ranges have lower mV/g (the proof mass deflects less per g). Resolution in mg/LSB depends on ADC bits and supply: resolution = (Vcc/2^N)/S 1000 mg. For ADXL335 (300 mV/g) with 12-bit ADC at 3.3V, resolution = (3.3/4096)/0.300 * 1000 = 2.7 mg/LSB. Digital accelerometers (LIS3DH, MPU6050) specify sensitivity in LSB/g: 4000-16000 LSB/g at +/-2g to +/-16g range per STMicroelectronics. IEEE 1293 defines accelerometer accuracy specifications including bias stability (0.1-10 mg), scale factor (0.1-1%), and noise density (25-300 ug/rtHz).

Worked Example

Problem

Configure an ADXL345 digital accelerometer for machine vibration monitoring. Requirements: detect 0.01 g vibrations at 200 Hz, measure up to 8 g shock events.

Solution
  1. Select +/-8g range: sensitivity = 3.9 mg/LSB (from ADXL345 datasheet)
  2. Noise density: 290 ug/rtHz at +/-8g setting
  3. Bandwidth for 200 Hz vibration: set BW_RATE = 200 Hz (ODR = 400 Hz)
  4. Noise floor: noise_rms = 290 ug/rtHz sqrt(200 Hz 1.6) = 5.2 mg rms
  5. SNR at 0.01 g: SNR = 10 mg / 5.2 mg = 1.9 (marginal, 5.6 dB)
  6. For better SNR, switch to +/-2g range: 256 LSB/g, noise = 145 ug/rtHz
  7. New noise floor: 145 * sqrt(320) = 2.6 mg rms, SNR = 10/2.6 = 3.8 (11.6 dB)
  8. Trade-off: +/-2g range saturates at 2g shock, misses 8g events
  9. Solution: use +/-2g for vibration monitoring, switch to +/-8g upon threshold trigger
Result: Use +/-2g range (3.8 mg noise floor, 2.6 SNR at 10 mg) for vibration; auto-range to +/-8g for shock events.

Practical Tips

  • ✓Mount the accelerometer rigidly to the structure; any PCB resonance appears as acceleration at the resonant frequency; use stiffening ribs or potting compound to push PCB resonance above measurement band per IEEE 1293
  • ✓For tilt sensing, use 1-10 Hz bandwidth (large filter caps); for vibration monitoring, use 100 Hz-1 kHz; for shock detection, use maximum bandwidth with no filter per ISO 16063-1 vibration calibration standard
  • ✓Calibrate at +/-1g by rotating the sensor axis through gravity; actual sensitivity and zero-g offset vary +/-10-15% from nominal; two-point calibration achieves +/-1% accuracy per manufacturer calibration procedures

Common Mistakes

  • ✗Using sensitivity spec from wrong supply voltage: ADXL335 sensitivity is 270 mV/g at 2V, 300 mV/g at 3V, 330 mV/g at 3.6V; using wrong value causes 10-20% measurement error per Analog Devices datasheet
  • ✗Confusing +/-g full-scale with peak-to-peak: +/-3g means saturation at +3g and -3g (6g total span); full-scale voltage swing is 2 S FS_range = 2 300 3 = 1800 mV for ADXL335
  • ✗Ignoring output filter bandwidth: unfiltered ADXL335 has 1.6 kHz bandwidth and 300 ug/rtHz noise; add 0.1 uF caps for 50 Hz bandwidth and 5x lower noise per datasheet filter recommendations

Frequently Asked Questions

Analog accelerometers (ADXL335, MMA7361) output continuous voltage and require external ADC; simpler for basic designs but susceptible to analog noise. Digital accelerometers (ADXL345, LIS3DH, MPU6050) contain internal sigma-delta ADC and provide I2C/SPI output with 10-16 bit resolution; they include configurable full-scale, FIFO buffers (32-1024 samples), and interrupt outputs for motion detection. Digital types cost $1-5, analog $2-4. For new designs, digital accelerometers simplify firmware and improve noise immunity per STMicroelectronics application note AN3182.
MEMS proof mass displacement for a given acceleration must stay within the mechanical structure limits (typically 1-10 um). For wider g range, the electrostatic restoring force is increased (reducing um/g displacement), which lowers voltage output per g. A +/-2g sensor has 8x higher sensitivity than +/-16g from the same die. This is fundamental to MEMS design per Analog Devices MEMS technology overview. Select the narrowest range that avoids saturation for best resolution.
Apply a high-pass filter with cutoff below signals of interest (0.1-2 Hz for vibration). In firmware, subtract the moving average (IIR: y[n] = 0.99*y[n-1] + 0.01*x[n]) or use a DC-blocking IIR filter (first-order HPF at 0.5 Hz). For tilt sensing, you need DC response; in that case, calibrate and subtract the known zero-g offset (typically +/-60 mg for MEMS per Analog Devices AN-1057). Temperature calibration may be required as offset drifts +/-0.5 mg/C.

Related Calculators

Sensor

Hall Effect Sensor

Calculate Hall voltage V_H = R_H·I·B/t, carrier density, and sensitivity for Hall effect sensors. Covers magnetic field measurement, current sensing, and position detection applications.

Sensor

Sensor Accuracy

Calculate total sensor error using RSS and worst-case methods. Analyze offset, gain, nonlinearity, resolution, and temperature drift contributions.

Sensor

Strain Gauge Bridge

Calculate bridge output voltage for strain gauges in quarter, half, and full bridge configs. Determine mV output and resistance change for load cells.

Sensor

NTC Thermistor

Calculate temperature from NTC thermistor resistance using the Steinhart-Hart beta equation. Get Kelvin and Celsius outputs for sensor circuit design.