The system’s two ADXL210 accelerometers measure three-dimensional acceleration. Each accelerometer generates two axes of PWM outputs that are measured by the LPC2138’s timer input captures. I used ADXL210EB evaluation boards for convenience, so it was necessary to supply only a pair of filter capacitors and a timebase resistor.

The ADXL210 is a complete two-axis acceleration measurement system on a single IC. It contains micro-machined sensor and signal conditioning circuitry for acceleration measurement. The output signals are digital PWMs proportional to acceleration. The ADXL210 can measure both positive and negative accelerations to at least ±10 g.

After reading over Analog’s application notes, I chose a filter bandwidth of 50 Hz (0.1 µF). It seems that a living organism shouldn’t vibrate more than 50 times per second, and this bandwidth provided a relatively low noise figure. Operational testing should reveal if a higher bandwidth is required (see Figure 2).

I used a sample period of 1 ms (124 kW). The sampling bandwidth would need to change by more then an order of magnitude before this sample period would be unacceptable. The LPC2138’s 15-MHz peripheral clock can easily measure small acceleration changes with this 1-ms pulse.