feature

3-axis sensing; small, low-profile package; 4mm x 4mm x 1.45mm LFCSP; low power consumption: 350 μA typical; single supply operation: 1.8 V to 3.6 V; 10000 g shock survival; excellent temperature stability; single Shaft capacitor bandwidth adjustment; RoHS/WEEE lead-free compliant.

application

Cost-sensitive, low-power, motion and tilt sensing applications; mobile devices; gaming systems; disk drive protection; image stabilization; sports and wellness devices.

General Instructions

The ADXL326 is a small, low power, complete triaxial. Accelerometer with signal conditioned voltage output. This product measures acceleration with a minimum full-scale range. It can measure the static acceleration of gravity in tilt sensing applications, as well as the dynamic acceleration from motion, shock or vibration.

The user uses the CX, CY and CZ capacitors on the XOUT, YOUT and ZOUT pins. Bandwidths can be selected to suit the application ranging from 0.5 Hz to 1600 Hz for the X and Y axes and 0.5 Hz to 550 Hz for the Z axis.

The ADXL326 is available in a small, low profile, 4 mm 4 mm x 1.45 mm, 16-lead, plastic lead frame chip scale package (LFCSP_LQ).

Typical performance characteristics

N>1000 unless otherwise stated.

theory of operation

The ADXL326 is a complete three-axis acceleration measurement system. The ADXL326 has a minimum measurement range of ±16 g. It contains polysilicon surface micromechanical sensors and signal conditioning circuitry to implement an open-loop acceleration measurement architecture. The output signal is an analog voltage proportional to acceleration. Accelerometers can measure static acceleration due to gravity in tilt sensing applications, as well as dynamic acceleration due to motion, shock or vibration.

The sensor is a polysilicon surface micromechanical structure built on a silicon wafer. Polysilicon springs suspend the structure from the wafer surface and provide resistance to acceleration forces. The deflection of the structure is measured with a differential capacitor consisting of separate stationary plates and plates attached to the moving mass. The stationary plate is driven by a 180° out-of-phase square wave. The acceleration deflects the moving mass and unbalances the differential capacitor, resulting in a sensor output whose amplitude is proportional to the acceleration. Phase-sensitive demodulation techniques are then used to determine the magnitude and direction of the acceleration.

The output of the demodulator is amplified and taken away from the chip through a 32 kΩ resistor. The user then sets the signal bandwidth of the device by adding capacitors. This filtering increases measurement resolution and helps prevent aliasing.

mechanical sensor

The ADXL326 uses a single structure to sense the X, Y, and Z axes. As a result, the three-axis sensing direction is very orthogonal to the quadrature-axis sensitivity. Mechanical misalignment of sensor die and package is the main source of transverse axis sensitivity. Of course, mechanical bias can be calibrated at the system level.

performance

Rather than using additional temperature compensation circuitry, innovative design techniques ensure high performance built into the ADXL326. Therefore, there is neither quantization error nor non-monotonic behavior, and the temperature hysteresis is very low (typically <3 mg over the -25°C to +70°C temperature range).

application information

Power decoupling

In most applications, a 0.1-µF capacitor, C, placed near the ADXL326 power supply pins is sufficient to isolate the accelerometer from noise on the power supply. However, in applications where there is noise at the 50 kHz internal clock frequency (or any of its harmonics), extra attention is needed to supply bypassing, as this noise can cause errors in the acceleration measurement. If additional decoupling is required, a 100Ω (or less) resistor or ferrite bead can be inserted on the power line. Also, a larger bulk bypass capacitor (1µF or larger) can be added in parallel with C. Make sure that the connection from the ADXL326 ground to the power supply ground is low impedance, as noise transmitted through ground has a similar effect as noise transmitted through V.

Set bandwidth with CX, CY and CZ

The ADXL326 has provisions to limit the frequency band of the X, Y, and Z pins. Capacitors must be added at these pins for low-pass filtering to eliminate aliasing and noise. The 3db bandwidth equation is:

or simpler

self-test

The ST pin controls the self-test function. When this pin is set to V, an electrostatic force is exerted on the accelerometer beam. The resulting beam movement allows the user to test that the accelerometer is working properly. A typical change in output is 1.08 g on the x-axis (corresponding to 62 mV), +g on the y-axis (+62 mV), and +1.83 g (+105 mV) on the z-axis. In normal use, this ST pin can be left open or connected to common (COM).

Never expose the ST pin to voltages greater than V+0.3 V. If this cannot be guaranteed due to system design (for example, there are multiple supply voltages), a low-V clamp diode between ST and V is recommended.

Design Tradeoffs in Choosing Filter Characteristics: Noise/BW Tradeoffs

The chosen accelerometer bandwidth ultimately determines the measurement resolution (minimum detectable acceleration). Filtering reduces the noise floor and improves the resolution of the accelerometer. The resolution depends on the analog filter bandwidth at X, Y and Z.

The output of the ADXL326 has a typical bandwidth greater than 500 Hz. The user must filter the signal at this point to limit aliasing errors. The analog bandwidth must not exceed half the analog-to-digital sampling frequency to minimize aliasing. The analog bandwidth can be further reduced to reduce noise and improve resolution.

ADXL326 noise has the characteristics of white Gaussian noise, which contributes equally at all frequencies and is described in μg/√Hz (noise is proportional to the square root of the accelerometer bandwidth). The user should limit the bandwidth to the lowest frequency required by the application to maximize the resolution and dynamic range of the accelerometer.

Using the unipolar roll-off feature, the typical noise of the ADXL326 is given by:

Noise peaks are usually required. Peak-to-peak noise can only be estimated using statistical methods. Table 5 helps in estimating the probability of exceeding various peaks given the rms value.

Use a working voltage other than 3V

The ADXL326 is tested and specified at V=3V; however, it can be powered with voltages as low as 1.8V or as high as 3.6V. Note that some performance parameters vary with supply voltage.

The ADXL326 output is ratiometric; therefore, the output sensitivity (or scale factor) varies proportionally to the supply voltage. At V=3.6V, the output sensitivity is typically 68mV/g. At V=2V, the output sensitivity is typically 38mV/g.

The zero-g biased output is also a ratiometric output; therefore, the zero-g output is nominally equal to V/2 at all supply voltages.

Output noise is not a ratio measurement, but an absolute value in volts; therefore, noise density decreases as supply voltage increases. This is because the scale factor (mV/g) increases when the noise voltage remains constant. At V=3.6 V, the X and YAXIS noise densities are typically 120 μg/Hz, and at V=2 V, the X- and Y-axis noise densities are typically 270 μg/Hz.

Self-test response (g) vs. supply voltage. However, when the ratio of sensitivity is measured in relation to supply voltage, the self-test response (in volts) is roughly proportional to the cube of the supply voltage.

For example, at VS=3.6V, the x-axis of the ADXL326 is approximately 107 mV and +mV is 107 mV. For the y-axis, the z-axis is +181 mV. When VS=2v, the self-test response is approximately 18 mV on the x-axis, +18 mV. For the y-axis, the z-axis is 31 mV. As the supply voltage decreases, the supply current decreases. Typical current consumption is 375μA at VS=3.6V and 200μA at VS=2V.

Layout and Design Recommendations

Recommended welding profiles are shown in Figure 25, followed by descriptions of profile characteristics in Table 6. The recommended PCB layout or solder ring diagram is shown in Figure 26.

Dimensions