Accelerometer Measurements using an ADXL326 and a 12-bit ADC

The smallest increment of acceleration that can be measured by an Analog Devices ADXL326 is either determined by the noise resolution of the accelerometer or the resolution of the ADC, whichever is smaller.

The noise resolution of the accelerometer is calculated by the equation R = Noise Density * sqrt(Bandwidth * 1.6). Using the typical values from the data sheet, that works out to 0.015 g for the X- & Y-axis and 0.09 g for the Z-axis.

For the ADC, assuming its voltage rails are at 0 and 3 volts, the resolution is 0.73 mV/count (3 V/4096) if we use the typical accelerometer value of 57 mV/g that works out to 78 counts/g or 0.012 g as the smallest value measurable by the ADC. This of course doesn’t include the noise inherent to the ADC itself, but it is probably safe to assume that the ADC still has a greater resolution than the accelerometer’s noise resolution.

So, in this case, the smallest measureable increment of acceleration is limited by accelerometer’s noise resolution.  Dividing the resolution into the total range results in roughly 2133 g values for the X- and Y-axis and 355 g values for the Z–axis, assuming a range of +/-16 g.  However, this is just the best case scenario, and there are a number of things that can affect the performance of the accelerometer. Testing the accelerometer in the end product is recommended before making any hard decisions about the product’s functionality.

So for the X- and Y-axis, the smallest change that can be detected is 0.015 g.  Anything smaller is indistinguishable from noise. Smaller values can be measured by decreasing the accelerometer’s output bandwidth, but this increases the response time of the accelerometer making the whole system slower.

More on accelerometers here.

Figure 1: Simple block diagram showing signal path of measured acceleration.