Data Sheet

ADXL1005

Rev. 0 | Page 9 of 14

THEORY OF OPERATION

The ADXL1005 is a low noise, single-axis, MEMS accelerometer,

with a 42 kHz resonant frequency that provides an analog output

proportional to mechanical vibration. The ADXL1005 has a high

g range of ±100 g, suitable for vibration measurements in high

bandwidth applications. Such applications include vibration

analysis systems for monitoring and diagnosing machines or

system health.

The low noise and high frequency bandwidth allows the

measurement of vibration patterns caused by small moving

components, such as internal bearings. The high g range

provides the dynamic range necessary for high vibration

environments such as heating, ventilation, and air conditioning

(HVAC) and heavy machine equipment. To achieve proper

performance, be aware of system noise, mounting, and signal

conditioning.

System noise is affected by supply voltage noise. The analog

output of the ADXL1005 is a ratiometric output. Therefore,

supply voltage modulation affects the output. Use a properly

decoupled, stable supply voltage to power the ADXL1005 and to

provide a reference voltage for the digitizing system.

The output signal is impacted by an overrange stimulus. An

overload indicator output feature indicates a condition that is

critical for an intelligent measurement system. For more infor-

mation about the overrange features, see the Overrange section.

Proper mounting ensures full mechanical transfer of vibration

to accurately measure the desired vibration rather than vibration

of the measurement system, including the sensor. A common

technique for high frequency mechanical coupling is to use a

sensor stud mount system while considering the mechanical

interface of fixing the ADXL1005 in the stud. For lower frequencies

(below the full capable bandwidth of the sensor), it may be possible

to use magnetic or adhesive mounting. Proper mounting technique

ensures proper and repeatable results that are not influenced by

measurement system mechanical resonances and/or damping at

the desired frequency, and represents an efficient and proper

mechanical transfer to the system being monitored.

Proper application specific signal conditioning is required to

achieve optimal results. Understanding the measurement

frequency range and managing overload conditions is

important to achieve accurate results. The electrical output

signal of the ADXL1005 requires some band limiting and a

proper digitization bandwidth. See the Interfacing Analog

Output Below 10 kHz section and the Interfacing Analog

Output Beyond 10 kHz section for more information.

MECHANICAL DEVICE OPERATION

The moving component of the sensor is a polysilicon surface-

micromachined structure built on top of a silicon wafer. Polysilicon

springs suspend the structure over the surface of the wafer and

provide a resistance against acceleration forces.

Differential capacitors that consist of independent fixed plates

and plates attached to the moving mass measure the deflection

of the structure. Acceleration deflects the structure and unbalances

the differential capacitor, resulting in a sensor output with amp-

litude proportional to acceleration. Phase sensitive demodulation

determines the magnitude and polarity of the acceleration.

OPERATING MODES

The ADXL1005 has two operating modes: measure mode and

standby mode. Measure mode provides a continuous analog

output for active monitoring. Standby mode is a nonoperational,

low power mode.

Measure Mode

Measure mode is the normal operating mode of the ADXL1005.

In this mode, the accelerometer actively measures acceleration

along the axis of sensitivity and consumes 1.0 mA (typical)

using a 5.0 V supply.

Standby Mode

Placing the ADXL1005 in standby mode suspends the measure-

ment and reduces the internal current consumption to 225 μA

(typical for the 5.0 V supply). The transition time from standby

to measurement mode is <50 μs. Figure 16 shows the transition

from standby to measure mode.

BANDWIDTH

The ADXL1005 circuitry supports an output signal bandwidth

beyond the resonant frequency of the sensor, measuring accel-

eration over a bandwidth comparable to the resonant frequency

of the sensor. The output response is a combination of the sensor

response and the output amplifier response. Therefore, external

band limiting or filtering is required. See the Interfacing Analog

Output Below 10 kHz section and the Interfacing Analog

Output Beyond 10 kHz section for more information.

When using the ADXL1005 beyond 10 kHz, consider the

nonlinearity due to the resonance frequency of the sensor, the

additional noise due to the wideband output of the amplifier,

and the discrete frequency spurious tone due to coupling of the

internal 200 kHz clock. Aliased interferers in the desired band

cannot be removed, and observed performance degrades. A

combination of high speed sampling and appropriate band

limiting filtering is required for optimal performance.