ADXL1005

Data Sheet

Rev. 0 | Page 12 of 14

INTERFACING ANALOG OUTPUT BEYOND 10 kHz

The ADXL1005 is a high frequency, single-axis MEMS

accelerometer that provides an output signal pass band beyond

the resonance frequency range of the sensor. Although the output

3 dB frequency response bandwidth is approximately 21 kHz

(note that this is a 3 dB response, meaning there is a gain in

sensitivity at this frequency), in some cases, it is desirable to

observe frequency beyond this range. To accommodate

frequency, the ADXL1005 output amplifier supports a 70 kHz

small signal bandwidth, which is well beyond the resonant

frequency of the sensor.

Although a mechanical interface is always important to achieve

accurate and repeatable results in MEMS applications, it is critical

when measuring greater than a few kilohertz. Typically, magnetic

and adhesive mounting are not sufficient to maintain proper

mechanical transfer of vibration through these frequencies.

Mechanical system analysis is required for these applications.

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. If any of these interferers alias in the

desired band, the aliasing cannot be removed, and observed

performance degrades. A combination of high speed sampling and

appropriate filtering is required for optimal performance.

The first consideration is the effect of the sensor resonance

frequency at 42 kHz. Approaching and above this frequency, the

output response to an input stimulus peaks, as shown in Figure 4.

When frequencies are near or above the resonance, the output

response is outside the linear response range, and the sensitivity is

different than observed at lower frequencies. In these frequency

ranges, the relative response (as opposed to absolute value) over

time is typically observed.

The ADXL1005 output amplifier small signal bandwidth is

70 kHz. The user must interface to the device with proper signal

filtering to avoid issues with out of band noise aliasing into the

desired band. The amplifier frequency response roll-off can be

modeled as a single-pole, low-pass filter, at 70 kHz. In the absence

of additional external low-pass filtering, to avoid aliasing of high

frequency noise, choose a sampling rate of at least 2× the equivalent

noise bandwidth (ENBW) for a single-pole, low-pass filter, as

follows:

ENBW = (π/2) × 70 kHz ≈ 110 kHz

The sample rate must be at least 220 kHz. This sample rate

reduces broadband noise due to the amplifier from folding back

(aliasing) in-band, but does not prevent out of band signals

from aliasing in-band. To prevent out of band responses,

additional external low-pass filtering is required.

Another artifact that must be addressed is the coupling of the

internal clock signal at 200 kHz onto the output signal. This

clock spur must be filtered by analog or digital filtering so as

not to affect the analysis of results.

To achieve the lowest rms noise and noise density for extended

bandwidth applications, it is recommended to use at least a

multiple order low-pass filter at the output of the ADXL1005 and

a digitization sample rate of at least 4× the desired bandwidth,

assuming there is sufficient filtering of the 200 kHz internal clock

signal. Use an ADC sample rate of 1 MSPS or greater along with

digital low-pass filtering to achieve similar performance.

OVERRANGE

The ADXL1005 has an output (OR pin) to signal when an

overrange event (when acceleration is greater than 2× the full-scale

range) occurs. Built in overrange detection circuitry provides an

alert to indicate a significant overrange event occurred that is

larger than approximately 2× the specified g range. When an

overrange is detected, the internal clock is disabled to the sensor

for 200 μs to maximize protection of the sensor element during an

overrange event. If a sustained overrange event is encountered, the

overrange detection circuitry triggers periodically, approximately

every 500 μs (see Figure 18).

MECHANICAL CONSIDERATIONS FOR MOUNTING

Mount the ADXL1005 on the PCB in a location close to a hard

mounting point of the PCB. Mounting the ADXL1005 at an

unsupported PCB location, as shown in Figure 24, may result

in large, apparent measurement errors due to undamped PCB

vibration. Placing the accelerometer near a hard mounting point

ensures that any PCB vibration at the accelerometer is above the

mechanical sensor resonant frequency of the accelerometer and

effectively invisible to the accelerometer. Multiple mounting

points, close to the sensor, and a thicker PCB help reduce the

effect of system resonance on the performance of the sensor.

MOUNTING POINTS

PCB

ACCELEROMETERS

Figure 24. Incorrectly Placed Accelerometers