Datasheet ADXL377 (Analog Devices) - 10

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Data Sheet ADXL377 APPLICATIONS INFORMATION POWER SUPPLY DECOUPLING
Never expose the ST pin to voltages greater than VS + 0.3 V. If For most applications, a single 0.1 µF capacitor, C the system design is such that this condition cannot be guaran- DC, placed close to the ADXL377 supply pins adequately decouples the teed (for example, if multiple supply voltages are present), it is accelerometer from noise on the power supply. However, in recommended that a clamping diode with low forward voltage applications where noise is present at the 50 kHz internal clock be connected between ST and VS. frequency (or any harmonic thereof), additional care in power
SELECTING FILTER CHARACTERISTICS:
supply bypassing is required because this noise can cause errors
NOISE/BANDWIDTH TRADE-OFF
in acceleration measurement. The selected accelerometer bandwidth ultimately determines If additional decoupling is needed, a 100 Ω (or smaller) resistor the measurement resolution (smal est detectable acceleration). or ferrite bead can be inserted in the supply line. In addition, a Filtering can be used to lower the noise floor, thereby improving larger bulk bypass capacitor (1 µF or greater) can be added in the resolution of the accelerometer. Resolution is dependent on parallel to CDC. Ensure that the connection from the ADXL377 the analog filter bandwidth at XOUT, YOUT, and ZOUT. ground to the power supply ground is low impedance because The output of the ADXL377 has a typical bandwidth of 1000 Hz. noise transmitted through ground has a similar effect as noise The user must filter the signal at this point to limit aliasing errors. transmitted through VS. The analog bandwidth must be no more than half the analog-to-
SETTING THE BANDWIDTH USING CX, CY, AND CZ
digital sampling frequency to minimize aliasing. The analog The ADXL377 has provisions for band-limiting the X bandwidth can be decreased further to reduce noise and OUT, YOUT, and Z improve resolution. OUT pins. A capacitor must be added at each of these pins to implement low-pass filtering for antialiasing and noise reduction. The ADXL377 noise has the characteristics of white Gaussian The equation for the −3 dB bandwidth is noise, which contributes equal y at al frequencies and is described f in terms of µg/√Hz (that is, the noise is proportional to the square −3 dB = 1/(2π × 32 kΩ × Cx) root of the accelerometer bandwidth). Limit the bandwidth to the or more simply, lowest frequency required by the application to maximize the f−3 dB = 5 µF/Cx resolution and dynamic range of the accelerometer. The tolerance of the internal resistor (RFILT) typical y varies by as With the single-pole roll-off characteristic, the typical noise of much as ±15% of its nominal value (32 kΩ), and the bandwidth the ADXL377 is determined by varies accordingly. A minimum capacitance of 1000 pF for CX, rms Noise = Noise Density × ( BW ×1.6) CY, and CZ is recommended in all cases. It is often useful to know the peak value of the noise. Peak-to-
Table 4. Filter Capacitor Selection for CX, CY, and CZ
peak noise can only be estimated by statistical methods. Table 5
Bandwidth (Hz) Capacitor (µF)
can be used to estimate the probability of exceeding various peak 50 0.10 values, given the rms value. 100 0.05 200 0.025
Table 5. Estimation of Peak-to-Peak Noise
500 0.01
Percentage of Time That Noise Exceeds
1000 0.005
Peak-to-Peak Value Nominal Peak-to-Peak Value (%)
2 × rms 32
SELF-TEST
4 × rms 4.6 The ST pin controls the self-test feature. When this pin is set to 6 × rms 0.27 V 8 × rms 0.006 S, an electrostatic force is exerted on the accelerometer beam. The resulting movement of the beam al ows the user to test whether the accelerometer is functional. The typical change in output is −1.08 g (corresponding to −6.5 mV) for the x-axis, +1.08 g (or +6.5 mV) for the y-axis, and +1.83 g (or +11.5 mV) for the z-axis. The ST pin can be left open circuit or connected to ground (GND) in normal use. Rev. 0 | Page 9 of 12 Document Outline Features Applications General Description Functional Block Diagram Table of Contents Revision History Specifications Absolute Maximum Ratings ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Theory of Operation Mechanical Sensor Performance Applications Information Power Supply Decoupling Setting the Bandwidth Using CX, CY, and CZ Self-Test Selecting Filter Characteristics: Noise/Bandwidth Trade-Off Axes of Acceleration Sensitivity Layout and Design Recommendations Outline Dimensions Ordering Guide