FAQ

Devices Defined

What is a MEMS accelerometer?

MEMS accelerometers are microelectromechanical systems that measure the static or dynamic force of acceleration.  Static force refers to the earth’s gravitational pull.  On the surface of the Earth, all objects fall with an acceleration defined as one "g", which is approximately equal to 9.81 m/s² (32.17 ft/s²).  Quite often, "g" is used as a unit of acceleration rather than expressing acceleration in m/s² or ft/s².  Dynamic forces result when movement or vibration is applied to the accelerometer itself.  Accelerometers enable such functionality as screen orientation, hard-disk-drive protection, gesture recognition for interactive games, activity monitoring for power management, and much more. 

What is a MEMS gyroscope?

MEMS gyroscopes are microelectromechanical systems that detect rate of rotation in degrees per second around the three axes (pitch, yaw and roll).  MEMS gyroscopes are used currently in hand-held electronics for image stabilization, GPS assist, and user interface, and the world-wide market for these devices is expanding rapidly.

What is a magnetometer?

Magnetometers are sensors that detect the earth’s magnetic field along multiple axes.  They are the only sensors that provide absolute heading (direction) information.  Used in conjunction with accelerometers and gyroscopes, magnetic sensors augment GPS for location-based services.

What type of motion sensor do I need?

Please see the table below for an overview of what each type of motion sensor measures, as well as the limitations of each type of motion sensor.

Motion Sensing 101

 

Measures

Limitations

Accelerometer
  • Vector sum of gravity and linear acceleration(meters/second2)
  • Provides gravity vector for calibration of pitch and roll gyroscopes
  • Noisy in dynamic conditions
  • Can filter for less noisy output at the cost of slow response
Compass
  • Raw sensors measure x,y,z magnetic field strength
  • Without local interference, in-plane (earth x-y coordinates) vector sum points toward magnetic north (absolute directional ref)
  • Used to calibrate Yaw gyroscope
  • Very susceptible to local magnetic fieldsTypically slow response
  • Has limitations on placement on PCB
Gyroscope
  • Rate of angular rotation (degrees/second)
  • Response is fast and smooth
  • Allows system to be operated correctly in the presence of magnetic fields
  • Data must be integrated at high rate in regular time intervals to calculate orientation vector
  • No absolute reference
  • Susceptible to drift
  • High current draw
Altimeter
  • Altitude/Atmospheric pressure (kPa)
  • Augments GPS altitude
  • Environmental affects
  • Low resolution
What development tools does Kionix offer?

Kionix offers development kits and boards, as well as evaluation boards, to provide customers with simple environments in which they can begin the development of applications, firmware, and prototype work with Kionix accelerometers.  Kionix currently offers the Kionix Accelerometer Application and Firmware Development Kit, the EZ430-C9 and EZ430-F2013 development boards and tools, and evaluation boards for all Kionix accelerometers.

What is pin compatibility?

Many Kionix parts are pin compatible with some of our competitors’ product offerings.  Pin compatibility means that similar functions are assigned to each of the corresponding pins on both products.  In addition, both parts also have the same package size and pin layout.  This allows Kionix parts to be used as drop-in replacements for our competitors’ parts.

Why do Kionix parts have different pin configurations and package sizes?

Kionix parts come in a variety of package sizes and pin configurations.  Pin count and configuration are governed by the feature set of the ASIC.  For example, the number of pins can be affected by the number of bias pins, the number of interrupt pins, whether the part is digitally interfaced, and which protocol the part uses (I2C or SPI, or both).  Some parts are purely analog while others can be interfaced through SPI and/or I2C communication protocols.  Also, some parts have embedded engines such as data ready, Tap/Double-tap, free-fall, motion, orientation, etc. that are supported by either one or two interrupt pins.  Lastly, some of our products are pin compatible to our competition.

Package sizes, on the other hand, vary due to the size of the sense element and the ASIC.  Those two pieces of silicon govern how small the final package can be.  Furthermore, we are often required by customers to meet a predefined footprint and pin configuration.

Please see below for a list of our accelerometers and their corresponding pin count:

10-pins:  KXCJ9, KXTJ9, KXTI9, KXTIA, KXTF9, KXTE9, KXSD9, KXUD9, KXTH9, KXTC9
12-pins:  KXTJ2
14-pins:  KXSS5, KXTH5, KXSC4, KXSB5, KXR94, KXRB5, KXD94
16-pins:  KXTIK, KXTC8

Sensor Fusion

What is sensor fusion?

Sensor fusion is software that intelligently combines data from several sensors for the purpose of improving application or system performance. Combining data from multiple sensors corrects for the deficiencies of the individual sensors to calculate accurate position and orientation information.

Please visit our Sensor Fusion page for more information.

How does sensor fusion function across different operating systems?

Sensor fusion operates across multiple operating systems, including Google's Android OS, Apple's iOS, and Microsoft's Windows OS.  However, each platform carries its own unique set of sensor integration challenges that must be overcome in order to maximize sensor functionality and user experience.

Because these platforms are all quite diverse, early access to development hardware and software is extremely important for sensor suppliers.

Sensor suppliers are often tasked to overcome these challenges in cooperation with their technology partners and their mutual customers.

Sensor Manager Architecture

Bandwidth

What is bandwidth?

Bandwidth is the frequency range in which the accelerometer or gyroscope operates.  Kionix sensors respond from 0 Hz to a user-definable upper cutoff frequency.

Electrical

Is the center pad of Kionix's DFN package supposed to be grounded?

Yes, the center pad of Kionix's DFN package should be grounded. This will provide the lowest noise performance. Soldering the center pad to the printed circuit board (PCB) is recommended in general because it helps provide a strong mechanical coupling between the sensor and the PCB.

How do you calculate acceleration from the digital output of an accelerometer?

Kionix has several digital tri-axis accelerometer products. All of these products provide digital outputs proportional to the linear acceleration in each axis. Per standard convention, the zero acceleration (or zero-g offset) is usually defined as an output equal to half the maximum output value (4096 for a 12-bit output, 1024 for a 10-bit output, etc.). For an accelerometer providing 12-bit output, this would be a zero-g offset equal to 2048. Outputs above 2048 indicate a positive acceleration. Outputs below 2048 indicate a negative acceleration. The magnitude of the acceleration is typically expressed in units of g (1g = 9.8m/s2 = the Earth’s acceleration). It is calculated by finding the difference between the measured output and the zero-g offset then dividing by the accelerometer’s sensitivity (expressed in counts/g or LSB/g). For a 2g accelerometer with 12-bit digital outputs, the sensitivity is 819 counts/g or 819 LSB/g. The acceleration would be equal to: a = (Aout - 2048)/(819 counts/g) in units of g.

What is aliasing?

Aliasing is a distortion of the output due to sampling a continuous signal at too low a rate. To avoid aliasing you need to sample at a minimum rate of 2 times the low pass filter cutoff frequency.  (Nyquist–Shannon sampling theorem)

What is span?

Span, in accelerometers, is the output voltage or digital count value relative to zero-g output for full scale ± input acceleration at nominal Vdd and temperature. For gyroscopes, span is the output voltage or digital count value relative to zero-rate output for full scale ± input rotation at nominal Vdd and temperature.

What is zero-rate output?

Zero-rate output refers, in gyroscopes, to the output voltage or digital count for zero rate input rotation at nominal Vdd and temperature. 

What is zero-g offset?

Zero-g offset refers, in accelerometers, to the output voltage or digital count value for zero-g input acceleration at nominal Vdd and temperature.

How do you calculate acceleration from the analog output of an accelerometer?

Kionix has several analog tri-axis accelerometer products. All of these products provide an output voltage proportional to the linear acceleration in each axis. Per standard convention, the zero acceleration (or zero-g offset) is usually defined as an output voltage equal to half the supply voltage (Vdd/2). For an accelerometer supplied using 3.3V, this would be a zero-g offset equal to 1.65V. Voltages above 1.65V indicate a positive acceleration. Voltages below 1.65V indicate a negative acceleration. The magnitude of the acceleration is typically expressed in units of g (1g = 9.8m/s2 = the Earth’s acceleration). It is calculated by finding the difference between the measured output and the zero-g offset then dividing by the accelerometer’s sensitivity (expressed in V/g or mV/g). For a 2g accelerometer operating at 3.3V, the sensitivity is 660mV/g or 0.660V/g. The acceleration would be equal to: a = (Vout - 1.65V)/(0.660V/g) in units of g.

Multi-Plexing

What do I do if I'm not using the built-in multiplexer?

When not using the built-in multiplexer, make the following connections: S0 and S1 - ground or Vdd. Vmux: leave floating.

How do I use the built-in multiplexer?

Connect the Vmux pin of the accelerometer to an Analog to Digital (A/D) input port of your microprocessor, and connect S0 and S1 of the accelerometer to digital outputs of your microprocessor. Select the desired multiplexer output by toggling S0 and S1 to the appropriate values. Once the desired multiplexer output has been selected, wait 5 microseconds then begin an Analog to Digital conversion. Refer to the product specification for an output select table that defines the S0 and S1 states that correspond to the Vmux output options. Application note AN003 also describes how to use the built-in multiplexer as well as an external multiplexer.

Why is there a built-in multiplexer on some of your accelerometers?

The built-in multiplexer allows the user to measure X, Y, and Z axis acceleration with only one Analog to Digital (A/D) port and two digital output ports on their main microprocessor. This can be advantageous when the number of available A/D ports is limited. The KXPA4, KXPB5, KXP94, and KXR94 feature a built-in multiplexer.

Noise

How do I calculate noise from noise density?

Since the noise output of a sensor is highly dependent on the output filter settings, the noise is reported as a noise density (ND). The noise density is defined as the noise per unit of square root bandwidth.  The units for noise density are typically ug/sqrt(Hz) for an accelerometer and deg/s/sqrt(Hz) for a gyroscope. To determine the expected noise for a given application first determine the equivalent noise bandwidth, B, of your output filter. The equivalent noise bandwidth of a filter is the -3 dB bandwidth (f-3dB) multiplied by a coefficient which is dependent on the order of the filter according to the following table.

B = 1.57 * f-3dB Hz for a 1st order filter

B = 1.11 * f-3dB Hz for a 2nd order filter

B = 1.05 * f-3dB Hz for a 3rd order filter

B = 1.025 * f-3dB Hz for a 4th order filter

In the case of an accelerometer, if you apply a 50 Hz first order low pass filter to the outputs of a KXR94 accelerometer (ND = 40 ug/sqrt(Hz)) the expected noise would be (40 ug/sqrt(Hz)) * sqrt(1.57 *50 Hz) = 354 ug. This is the RMS sensor dependent noise expected on the outputs of the KXR94. The actual noise seen on the sensor outputs may be larger than this reported error due to environmental noise (thermal, Vdd regulation, mechanical accelerations) on the sensor.

What is noise density?

When multiplied by the square root of the measurement bandwidth, this value will give the rms acceleration noise for accelerometers and the rms rotation noise for gyroscopes at nominal Vdd and temperature.  Accelerations and rotations below this value will not be resolvable. 

Non-Linearity

What is non-linearity?

Sensors do not demonstrate a perfectly linear relationship between input acceleration (accelerometers) or rotation (gyroscopes) and output voltage or digital count.  This non-linearity is the maximum deviation of output voltage or digital count from the “best fit line,” the straight line defined by sensitivity.  Non-linearity is typically expressed as a percentage of Full-Scale Output (FSO), i.e., the ratio of the maximum output deviation divided by the full-scale output, specified as a percentage.

The method for calculating non-linearity of an analog accelerometer is shown below:

What is non-linearity?

Range

What happens if the product g-range is programmed higher than the specifications state?

The sensitivity of the product will decrease, and it will exhibit non-linearity. Non-Linearity is where sensors do not demonstrate a perfectly linear relationship between input acceleration and output voltage.

What is range?

For accelerometers, range is the input acceleration that causes the output to reach span voltage or digital count.  For gyroscopes, it is the input rotation that causes the output to reach span voltage or digital count.

Ratiometricity

Are Kionix accelerometers ratiometric?

Ideally, the sensor is ratiometric—the output scales by the same ratio that Vdd increases or decreases. For example, a 5% increase in Vdd results in a 5% increase in 0g offset.

What is ratiometric error?

Ideally, an analog sensor is ratiometric—the output scales by the same ratio that Vdd increases or decreases.  For example, a 5% increase in Vdd results in a 5% increase g offset.

Ratiometric error is defined as the difference between actual change in the offset or sensitivity and the ideal or expected change in offset or sensitivity.  It is calculated differently for analog and digital sensors. 

For analog sensors, it is the ratio that 0g offset or sensitivity changed and the ratio that Vdd changed, expressed as a percentage. For our specifications, ratiometric error is typically calculated for a +/- 5% change in Vdd from nominal.  For example, the offset ratiometric error at Vdd = 3.3V +/- 5% is defined as the maximum absolute value of:

Ratiometric Error

Ratiometric Error

For digital sensors, it is the ratio that 0g offset or sensitivity changed, expressed as a percentage. For our specifications, ratiometric error is typically calculated on a +/- 5% change in Vdd from nominal for parts without internal voltage regulators, and on a change of approximately ½ the range of Vdd for parts with a voltage regulator.  Offset ratiometric error measured for Vdd = 3.3V +/- 5% is defined as the maximum absolute value of:

Ratiometric Error

Similarly, the sensitivity ratiometric error for Vdd = 3.3V +/- 5% is defined as the maximum absolute value of:

Ratiometric Error

Resolution

What is resolution?

Resolution is the minimum detectible change in acceleration for accelerometers and rotation for gyroscopes.  To be detectible, the signal must be greater than the noise of the sensor.

The resolution of our analog parts can be determined with the following equation: RES = ND * sqrt(B)

Since digital parts convert an analog signal to a digital output, the resolution of digital parts additionally depends on the conversion resolution.  The conversion resolution can be determined using the following equation: Digital Resolution = 1/(sensitivity).  The resolution of digital parts is, therefore, the larger of the Analog Resolution and the Digital Resolution.

Self Test

If I'm not using self test, can the pin be left as a no connect (floating)?

No, the pin should not be left as an open circuit. Depending on the surrounding conditions, the voltage on the pin may float high enough to inadvertently trigger a self test. Please connect the self test pin to GND for normal operation.

What does self test do?

This test allows the user to verify the functionality at any point of both the mechanical structure, and the ASIC. When the self test feature is enabled, the ASIC electrostatically actuates (excites) the mechanical structure causing a shift and thus a resulting change in acceleration output. This shift is different for each accelerometer product that Kionix provides, and thus one will need to refer to its specification sheet to see what this change in acceleration is.

Sensitivity

How do you determine that a +/-2g accelerometer will have a sensitivity of 660mV/g or 819counts/g?

First, for an analog part, it is important to know what supply voltage the part will be operating at. For this example we'll use the very common supply voltage (Vdd) of 3.3V. The zero-g offset is most often Vdd/2 which in this case would be 1.65V. Output voltages above Vdd/2 indicate positive accelerations, and output voltages below Vdd/2 indicate negative accelerations. Obviously, one would assume that 0V would be equal to -2g and Vdd (3.3V, for example) would be equal to +2g. Therefore, a +/-2g part should have a sensitivity of (Vdd/2) / g-range, which in the example would be (3.3V/2) / (2g) = 0.825V/g = 825mV/g. There are a couple problems if you use this methodology to make a +/-2g part. First, the zero-g offset isn't always exactly Vdd/2. Because of offset trimming tolerances, misalignment errors, and temperature effects, the zero-g offset has a tolerance. The zero-g offset might be 1.65 +/- 0.10 V. If your part happens to have a zero-g offset of 1.70V, then at 825mV/g your part would measure a maximum of +1.94g and a minimum of -2.06g. You wouldn't be able to measure exactly +2g of acceleration and your part wouldn't truly be a +/-2g part. The second problem with using this methodology is that an output reading of 0V or Vdd (3.3V) would be indeterminate. Any acceleration of +2g or higher would give an output reading of Vdd, and any acceleration of -2g or lower would give an output reading of 0V. Thus, when the accelerometer gave an output reading of 3.3V, for example, you wouldn't know if that were +2g, +3g, or +5g. To prevent the problems described above, we choose to make 0.1*Vdd equal to -2g and 0.9*Vdd equal to +2g. This gives enough margin from 0V and Vdd to account for the zero-g offset tolerances and temperature variation. It also guarantees that +/-2g will always be measurable quantities. The sensitivity calculation in this methodology is: (0.9*Vdd - 0.1*Vdd) / (+g-range - -g-range). For our example, (0.9*3.3V - 0.1*3.3V) / (+2g - -2g) = (2.64V / 4g) = 0.66V/g = 660mV/g. For a digital part, the methodology is similar but dependent on the number of output bits. For example, a 12-bit digital part would give readings from 0 to 4095. We substitute the maximum output reading (4095) in place of Vdd in the above equations. Sensitivity is then: (0.9*4095 - 0.1*4095) / (+2g - -2g) = (3276 / 4g) = 819 counts/g.

What is cross-axis sensitivity?

Cross-axis sensitivity is the output induced on a sense axis from the application of acceleration for accelerometers and rotation for gyroscopes on a perpendicular axis, expressed as a percentage of the sensitivity.  There are multiple cross-axis sensitivities: Sxy, Sxz, Syx, Syz, Szx, and Szy, where the first subscript is the sense axis and the second subscript is the off-axis direction.  For example, the calculation of the cross-axis sensitivity for the x-axis sensor of a tri-axis accelerometer or a tri-axis gyroscope is shown below:

Cross-axis Sensitivity

What is sensitivity?

Sensitivity, in accelerometers, is the output voltage or digital count change per unit of input acceleration at nominal Vdd and temperature, measured in mV/g or counts/g.  For gyroscopes, it is the output voltage or digital count change per unit of input rotation at nominal Vdd and temperature, measured in mV/deg/sec or counts/deg/sec.

Shock

What is mechanical shock?

Mechanical shock is characterized by a sudden and/or severe motion or applied force that causes significant displacement in the sensor system during the pulse.  The mechanical shock specification in a Kionix Product Specification is the maximum mechanical shock applied in any direction at which the accelerometer or gyroscope will remain within specification when nominal Vdd is applied to the device.

What is ESD tolerance?

The accelerometer or gyroscope will continue to meet specifications after an electrostatic shock that is less than or equal to the ESD Tolerance.  The Human Body Model (HBM), where an ESD pulse similar to that produced by a person who is electrically charged, is specified.

Temperature

What is zero-rate output vs. temperature?

This refers, in gyroscopes, to the maximum change in the nominal zero rate output over the full operating temperature range.

Does internal temperature compensation preclude an end user temperature calibration and compensation?

No. Any end user compensation or calibration can be done on top of the internal compensation. Cost saving can be realized by requesting a part specification with global compensation which does not require additional thermal testing on each part.

What is sensitivity variation from RT over temperature?

Sensitivity variation from RT over temperature is the change in the sensitivity from the room temperature sensitivity as the temperature changes. The variation is measured at Kionix by placing the accelerometer in a thermal chamber. First, the sensitivity is measured at 25C. Then the chamber is heated to a high temperature (usually 85C), and the sensitivity is measured a second time. Finally, the chamber is cooled to a low temperature (usually -40C), and the sensitivity is measured a third time. After the testing is complete, the data is analyzed. The sensitivity at 25C is subtracted from each of the measurements. The resulting change in sensitivity is divided by the accelerometer's sensitivity at 25C to express the change in output in terms of percentage change. The three data points (high temp., 25C, and low temp.) are plotted on a graph, and a least squares linear fit is done. The resulting slope (expressed in %/C) gives the variation of the sensitivity from room temperature (25C) over temperature.

What is zero-g offset variation from RT over temperature?

Zero-g offset variation from RT over temperature is the change in the zero-g offset from the room temperature zero-g offset as the temperature changes. The variation is measured at Kionix by placing the accelerometer in a thermal chamber. First, the zero-g offset voltage is measured at 25C. Then the chamber is heated to a high temperature (usually 85C), and the zero-g offset voltage is measured a second time. Finally, the chamber is cooled to a low temperature (usually -40C), and the zero-g offset voltage is measured a third time. After the testing is complete, the data is analyzed. The zero-g offset voltage at 25C is subtracted from each of the measurements. The resulting change in voltage is divided by the accelerometer's sensitivity to express the change in output in terms of acceleration (g or mg). The three data points (high temp., 25C, and low temp.) are plotted on a graph, and a least squares linear fit is done. The resulting slope (expressed in mg/C) gives the variation of the zero-g offset from room temperature (25C) over temperature.

What is offset vs. temperature?

Offset vs. temperature refers, in accelerometers, to the maximum change in the nominal zero-g output over the full operating temperature range.

What is storage temperature?

Storage temperature is the temperature at which the accelerometer or gyroscope can be stored unpowered and still meet performance specifications when powered within the operating temperature.

What is operating temperature?

Operating temperature is the temperature range over which the accelerometer or gyroscope will meet performance specifications