Data Sheet

ADXL1005

Rev. 0 | Page 11 of 14

INTERFACING ANALOG OUTPUT BELOW 10 kHz

The ADXL1005 senses mechanical motion along a single axis and

produces a voltage output. The system performance depends on

the output response resulting from sense mechanical vibration and

signal processing of the electrical output.

The sensor must be effectively mechanically coupled. Mechanical

coupling can be a complex integration of multiple components,

typically unique for each application. Consideration must be

made for all mechanical interfaces including the mounting of

the MEMS to the PCB (the location on the PCB as well as the

solder chemistry), the size of the PCB (both thickness and active

surface area), and the mounting of the PCB to the system being

monitored (either in a module or directly mounted).

In general, the following guidelines for effective mechanical

interface must be used to support up to 10 kHz bandwidth:



Keep the ADXL1005 near a stable mechanical mounting on

the PCB.



Provide multiple hard mounting points.



Keep the PCB thick and avoid a large surface area PCB that

induces higher magnitude and lower frequency resonances.



Ensure the mechanical connection is sufficiently stiff to

transfer mechanical forces up to the desired frequency.

Below 10 kHz, magnetic and adhesive mounting is possible

with proper attention. The EVAL-ADXL1005Z evaluation

boards can be used as a reference.

The ADXL1005 electrical output supports a bandwidth beyond the

resonance of the sensor. The small signal bandwidth of the output

amplifier in the ADXL1005 is 70 kHz. During the digitization

process, aliasing (which is the folding of higher frequency noise

and signals into the desired band) can occur. To avoid aliasing

noise from the amplifier and other internal circuits (for example,

coupling of the internal 200 kHz clock), it is recommended that

an external filter be implemented at the desired bandwidth and

the chosen analog-to-digital converter (ADC) sampling rate be

faster than the amplifier bandwidth.

The output amplifier is ratiometric to the supply voltage, and

there are two distinct cases regarding digital conversion, as

follows:



The user has an ADC downstream of the accelerometer

voltage supply tolerance and voltage temperature

coefficient (commonly associated with external regulators)

tracks between the sensor and the ADC. Therefore, the

supply and reference voltage induced error cancels out.

This design approach is recommended.



If the ADC cannot reference the same 5 V supply as the

sensor for any reason, the sensitivity of the digitized sensor

output reflects the regulator tolerance and temperature

coefficient.

The ADXL1005 output amplifier is stable while driving capacitive

loads of up to 100 pF directly without a series resistor. At loads

greater than 100 pF, an 8 kΩ or greater series resistor must be used.

See Figure 23 for an example of the interface, including compo-

nents when measuring mechanical vibration from 0 kHz to

5 kHz. For a 5 kHz pass band, a single-pole resistor capacitor

(RC) filter is acceptable. However, in some applications, use of a

more aggressive filter and lower sample ADC sample rate is

possible. The following components are recommended to form

a 5 kHz low-pass RC filter at the output of the ADXL1005 when

interfacing to an ADC, such as the ADAQ7980: R1 = 91 kΩ, C1 =

330 pF, R2 = 0 Ω, and C2 is not required. A minimum ADC

sample rate of 16 kHz is recommended to avoid aliasing. When

using sampling rates less than the resonance frequency (typically

42 kHz), be aware and account for the effective gain at the output of

the sensor due to the resonance to ensure out of band signals

are properly attenuated and do not alias into the band.

See Figure 23 for an example of the interface, including compo-

nents when measuring mechanical vibration from 0 kHz to 10 kHz.

The following components are recommended to form a two-pole

RC filter at the output of the ADXL1005: R1 = 500 Ω, C1 =

10,000 pF, R2 = 1 kΩ, and C2 = 10,000 pF. A minimum ADC

sample rate of 200 kHz is recommended to avoid aliasing.

REF

GND

IN+

IN–

VDD

ADAQ7980

3.0V TO 5.1V*

*3.0V LIMITED BY ADXL1005; 5.1V LIMITED BY ADAQ7980.

0.1µF

(+1µF, OPTIONAL)

10µF

R1

C1

R2

C2

ADXL1005

Figure 23. Application Circuit for the ADXL1005