feature

Dual-axis sensing; small, low profile package; 4mm x 4mm x 1.45mm LFCSP; low power; 180 μA, VS = 1.8 V (typ) single supply operation; 1.8 V to 5.25 V; 10,000 g shock survival; excellent Temperature Stability; BW Trim RoHS/WEEE Lead-Free Compatible with Single Capacitor Per Axis.

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 ADXL323 is a small, thin, low power, complete 2-axis accelerometer with signal conditioned voltage outputs, all on a single monolithic integrated circuit. The product measures acceleration with a minimum full-scale range of ±3g. It can measure static acceleration due to gravity in tilt sensing applications, as well as dynamic acceleration due to motion, shock or vibration.

The user selects the bandwidth of the accelerometer using the C and C capacitors on the X and Y pins. The bandwidth can be selected to suit the application, ranging from 0.5 Hz to 1600 Hz.

ADXL323 is available in small form factor products such as 4 mm × 4 mm × 1.45 mm, 16-lead, plastic lead frame chip scale package (LFCSP_LQ).

Absolute Maximum Ratings

Stresses listed above the Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device under the conditions described in the operating section of this specification or any other conditions above is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.

Typical performance characteristics

N>1000 unless otherwise stated.

theory of operation

It is a complete single-chip acceleration measurement system. The ADXL323 has a minimum measurement range of ±3g. 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 ADXL323 uses a single structure to detect the X and Y axes. As a result, the sensing directions of the two axes are highly orthogonal with less sensitivity on the transverse axis. Mechanical misalignment of the sensor to the package is a major source of cross-axis sensitivity. Of course, mechanical bias can be calibrated at the system level.

performance

Innovative design techniques do not use additional temperature compensation circuitry, but ensure high performance from the ADXL323. Therefore, there is neither quantization error nor non-monotonic behavior, and the temperature hysteresis is very low (typically less than 3 mg over the -25°C to +70°C temperature range).

Figure 13 and Figure 16 show the zero-G output performance of eight sections (X and Y axes) soldered to the PCB over a temperature range of 25°C to +70°C.

Figure 25 and Figure 28 show typical sensitivity versus temperature for a supply voltage of 3V. This is typically better than ±1% over the -25°C to +70°C temperature range.

application

Power decoupling

For most applications, a 0.1µF capacitor, C, placed near the ADXL323 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. Additionally, a larger overall bypass capacitor (1µF or larger) can be added in parallel with C. Make sure that the connection from the ADXL323 ground to the power supply ground is low impedance, as noise transmitted through ground has an effect similar to noise transmitted through V.

Use C, C, and C to set the Z axis of the bandwidth

The ADXL323 has provisions to limit the frequency band of the X and Y pins. Capacitors must be added at these pins for low-pass filtering to eliminate aliasing and noise. The formula for 3db bandwidth is:

F−3 dB = 1/(2π(32 kΩ) × C(X, Y, Z)) or simpler: F–3 dB = 5 μF/C(X, Y, Z)

The tolerance of the internal resistor (R) typically varies by ±15% of its nominal value (32 kΩ), and the bandwidth varies accordingly. In all cases, a minimum capacitance of 0.0047µF is recommended for C, C, and C.

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 500 mg (corresponding to 150 mV) on the X-axis and 500 mg (or 150 mV) on the Y-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 (e.g. if 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 and Y.

The output of the ADXL323 has a typical bandwidth greater than 1600 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.

ADXL323 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 ADXL323 is given by:

Determining the noise density usually requires the peak value of the noise. Peak-to-peak noise can only be estimated using statistical methods. Table 6 helps in estimating the probability of exceeding various peaks given the rms value.

Use a working voltage other than 3V

The ADXL323 is tested and specified at V=3V; however, it can be powered with voltages as low as 1.8V or as high as 5.25V. Note that some performance parameters vary with supply voltage.

The ADXL323 output is ratiometric; therefore, the output sensitivity (or scale factor) varies proportionally to the supply voltage. At V=5V, the output sensitivity is typically 550mV/g. At V=2V, the output sensitivity is typically 190mV/g.

The zero-g biased output is also a ratiometric output, so 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 while the noise voltage remains the same. When V=5v, the noise density is generally 180μg/√Hz, and when V=1.8v, the noise density is generally 360μg/√Hz.

The self-test response (g) is roughly proportional to the square of the supply voltage. However, when the sensitivity ratio measurement is related to the supply voltage, the self-test response in volts is roughly proportional to the cube of the supply voltage. For example, at V=5 V, the self-test response of the ADXL323 is approximately 700 mV on the x-axis and ±700 mV on the y-axis.

At V=1.8 V, the self-test response is approximately 40 mV for the x-axis and ±40 mV for the y-axis.

As the supply voltage decreases, the supply current decreases. Typical current consumption is 500μA at V=5V and 180μA at V=1.8V.

Dimensions