Electronic – Calculate G’s of an Accelerometer


Pat asks:

Accelerometer Breakout Board – ADXL193 +/-250g

could you tell me if this is correct.

Looking at the data sheet the mid sensitivity is about 8mv/G @ 5volts. If I use a 10bit adc @ 5volts then 5/1024=4.9 mV, so 1g=8mV so in bits 8/4.9=1.63 bits/g. Then if I have a zero adc reading of say 500 and I get a reading of 530, then

530-500- 30/1.63=18 g's.

Is that the way to calculate G's from this Breakout Board.

  • the data sheet says that voltage as low as 3.5 could be used and I was thinking of trying both 3.7v and see if 3.3v will work. Any idea what this will do for the calculations and that maximum range. I know the sensitivity is ratiometric so that will drop based on a lower input voltage.

Best Answer

If you use a 10 bit a/d converter to digitise a voltage in the range 0 to 5V then each count will represent 5/1024 = 4.88mV

The sensor data sheet gives an output of 8mV per g acceleration.

In the case that you are stating then the offset from the zero reading is 30 counts. This equates to 30*4.88 = 146.5

at 8mV per g this is 18.3g

From a system level I would have a number of concerns (and these really depend upon the end application for which the device is being used)

  1. +/-250g is one hell of an acceleration. This sort of acceleration can only be seen for a very short time. In order to capture this you would have to have a fast A/D converter, but then the sensor has a 400Hz filter on the output which will remove the fast transients from the output.

  2. The resolution of the A/D only allows the determination of the acceleration to steps of 0.61g. Is this resolution sufficient.

  3. Any noise on the A/D input can give a 1 or 2 count error in the A/D conversion resulting in a large error in the measured value.

If you scale/amplify the output from the accelerometer and limit its value to to lie in the A/D converter input range then you can, at the cost of a reduced dynamic range, improve the sensitivity of yor device without having to increase the resolution and cost of your converter.