ADXL1004 Data Sheet
Rev. 0 | Page 12 of 14
INTERFACING ANALOG OUTPUT BEYOND 10 kHz
The ADXL1004 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 24 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 this, the
ADXL1004 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 in cases 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 ADXL1004 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, it 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 45 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 ADXL1004 output amplifier small signal bandwidth is
70 kHz. The user must properly 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 2× the equivalent noise bandwidth (ENBW) for a
single-pole, low-pass filter, as follows:
ENBW = (π/2) × 70 kHz ≈ 110 kHz
The sampling rate must be at least 220 kHz. This sample rate
addresses reducing 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 ADXL1004 and
a digitization sample rate of at least 4× the desired bandwidth,
assuming 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 ADXL1004 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 ADXL1004 on the PCB in a location close to a hard
mounting point of the PCB. Mounting the ADXL1004 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
16508-012
Figure 24. Incorrectly Placed Accelerometers
