feature

Full dual-axis accelerometer system; monolithic integrated circuit; available with ±35 g/±35 g, ±50 g/±50 g, or ±70 g/±35 g outputs; full scale range; fully differential sensors and high Resistive circuit; EMI/RFI; environmentally friendly packaging; fully digital electromechanical self-test command; output ratio supply; sensitive axis on chip plane; high linearity (0.2% of full scale); frequency response down to DC; low noise; Low power consumption; close sensitivity tolerance and 0 g offset capability; maximum usable pre-filter clamping headroom; 400 Hz, 2-pole Bessel filter; single-supply operation; compatible with tin/lead and lead-free soldering processes; suitable for automotive application.

application

Vibration monitoring and control; vehicle collision sensing; shock detection.

General Instructions

The ADXL278 is a low power, full dual axis accelerometer with signal conditioned voltage output on a single monolithic integrated circuit. This product measures acceleration full-scale in (X-axis/Y-axis) ±35 g/±35 g, ±50 g/ ±50 g, or ±70 g/±35 g (min). The ADXL278 can also measure dynamic acceleration (vibration) and static acceleration (gravity).

ADXL278 is the fourth representative of surface micromachining technology ADI's iMEMS 174 ; accelerometer, higher performance and lower cost. Designed for front and side impact airbag applications, this product also provides a complete cost-effective solution for a variety of other applications.

The ADXL278 is over the automotive temperature range and has all the mechanical and electrical components of a fully exercised sensor with one digital signal applied to one pin.

The ADXL278 is available in 5mm x 5mm x 2mm in an 8-terminal ceramic LCC package.

Absolute Maximum Ratings

Stresses above the Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; equipment at these or any other conditions beyond the operating conditions is not implied by this section of this specification. Exposure to absolute long-term maximum rated conditions may affect device reliability.

theory of operation

The ADXL278 provides a fully differential sensor structure and circuit path resulting in the industry's highest immunity to EMI/RFI effects. The latest generation employs electrical feedback and zero-force feedback for increased accuracy and stability. The resonant frequency of the sensor is significantly higher than the signal bandwidth set by the on-chip filter, which avoids signal analysis problems caused by formants near the signal bandwidth.

Figure 5 is a simplified view of a differential sensor element. Each sensor includes several differential capacitance cells. Each battery consists of a fixed plate fixed on the base plate and a movable plate fixed on the frame. Displacement of the frame changes the differential capacitance measured by the on-chip circuitry.

Complementary 200khz square wave drive mounting plate. Electrical feedback adjusts the amplitude of the square wave so that the AC signal on the moving board is zero. The feedback signal is linearly proportional to the applied acceleration. This unique feedback technology ensures that the sensor is not affected by net electrostatic forces. The differential feedback control signal is also applied to the input of the filter, where it is filtered and converted to a single-ended signal.

application

Power decoupling

For most applications, a single 0.1µF capacitor C provides sufficient isolation of the accelerometer from noise on the power supply. However, in some cases, especially when noise is present at the 200 kHz internal clock frequency (or any of its harmonics), noise on the power supply can interfere with the output of the ADXL278. If additional decoupling is required, a 50Ω (or less) resistor or ferrite bead can be inserted into the power line. Additionally, larger bulk bypass capacitors (in the 1µF to 4.7µF range) can be added in parallel with C.

self test

A stationary finger in a forced battery usually remains at the same potential as the active frame. When the self-test digital input is activated, the voltage on the stationary finger on the side of the mobile board in the forced battery changes. This creates an attractive electrostatic force that moves the frame towards the fixed fingers. The entire signal channel is active; therefore, sensor displacement results in a change in V. The self-test feature of the ADXL278 is a comprehensive method to verify the operation of the accelerometer.

Since the electrostatic force is independent of the polarity of the voltage on the capacitor plates, a positive voltage is applied to one half of the forced cells and a positive voltage to the other half of the forced cells. Activating the self-test will cause a step function force to be applied to the sensor while cancelling the capacitive coupling term. The ADXL278 has improved self-test capabilities, including excellent transient response and high-speed switching capabilities. Arbitrary force waveforms can be used to measure the frequency response of the system by modulating the sensor's self-test input, such as a test signal, or even a crash signal to verify that the algorithm is within the self-test swing.

The ST pin should not be exposed to voltages greater than V+0.3 V. If this cannot be guaranteed due to system design (for example, if there are multiple supply voltages), a low-V clamp diode between ST and V is recommended.

clock frequency supply response

In any clocking system, power supply noise near the clock frequency can have an effect at other frequencies. The internal clock usually controls the sensor excitation and the signal demodulator of the micromachined accelerometer.

If the power supply contains high frequency spikes, it can be demodulated and interpreted as an acceleration signal. The signal appears as the difference between the noise frequency and the demodulator frequency. If the power supply peak is 100 Hz from the demodulator clock, then at 100 Hz. If the power clock is exactly the same frequency as the accelerometer clock, the term appears as an offset.

If the difference frequency exceeds the signal bandwidth, the filter will attenuate it. However, the power clock and accelerometer clock may vary with time or temperature, which can cause interfering signals to appear in the output filter bandwidth.

The ADXL278 solves this problem in two ways. First, the high clock frequency simplifies the task of choosing the power clock frequency so that the difference between it and the accelerometer clock is kept outside the filter bandwidth. Second, the ADXL278 is the only micromachined accelerometer with fully differential signal channels including differential sensors. Differential sensors remove most of the power supply noise before reaching the demodulator. Good high-frequency power supply bypassing, such as ceramic capacitors close to the power pins, can also minimize the amount of interference.

Clock Frequency Supply Response (CFSR) is the ratio of the response at V to the noise on the supply near the accelerometer clock frequency. A CFSR of 3 means that the signal at V is 3 times the amplitude of the excitation signal at V near the accelerometer's internal clock frequency. This is similar to the power response, except that the frequency of the stimulus and response is different. The CFSR of the ADXL278 is 10 times better than that of a typical single-ended accelerometer system.

signal distortion

Signals from collisions and other events can contain high-amplitude, high-frequency components. These elements contain very little useful information and are reduced by a 2-pole Bessel filter at the output of the accelerometer. However, if the signal saturates at any point, the accelerometer output does not look like a filtered version of the acceleration signal.

The signal may saturate anywhere before the filter. For example, if the resonant frequency of the sensor is lower, the displacement per unit of acceleration is higher. If the applied acceleration is high enough, the sensor may reach the mechanical travel limit. This can be solved by positioning the accelerometer where it cannot see high acceleration values and using a higher resonant frequency sensor such as the ADXL278.

Also, in overload conditions between the sensor output and the filter input, the electronics can saturate. Electrical saturation can be minimized by ensuring linear operation of internal circuit nodes at least several times the full-scale acceleration value. The linearity of the ADXL278 circuit is approximately 8 × full scale.

Dimensions

1. All models are on tape and reel and are RoHS compliant parts.

2. Z = RoHS compliant parts.

3, W = meet the requirements of automotive applications.

automotive products

The ADW22284, ADW22285, and ADW22286 models are available in controlled manufacturing to support the quality and reliability requirements of automotive applications. Note that specifications for these models may differ from commercial models; therefore, designers should carefully review the Specifications section of this data sheet. Only the automotive grade products shown are available for automotive applications. Please contact your local Analog Devices account representative for specific product ordering information and to obtain specific vehicle reliability reports for these models.