Is An Accelerometer Digital or Analog?

To put it simply, an accelerometer is an electromechanical device that measures, the change in velocity or the force of acceleration caused by static forces like gravity or movement over time.

Accelerometers basically have a myriad of applications and it they are quite a sophisticated piece of technology.

One of the questions I came across in one online tech forum is ” is an accelerometer digital or analog?”

An accelerometer can either be one with digital or analog outputs. Digital output accelerometers use pulse width modulation which means that there is a square wave at a certain frequency.

In which case, the time period of high voltage is proportional to the amount of acceleration. On the other hand, Analog output accelerometers have an output that is a continuous voltage proportional to the acceleration.

Acceleration

There are basically two types of acceleration forces: static forces and dynamic forces. 

Static forces are those that are constantly being applied to the object such as friction or gravity.

Dynamic forces are described as “moving” forces applied to the object at various rates an example of such forces is vibration, or the force exerted on a cue ball in the game of pool.

It is for this reason that accelerometers are used in collision safety systems in vehicles.

A good example is basically when a car is acted on by a powerful dynamic force, the accelerometer in which case senses a rapid deceleration ..sending an electronic signal to the cars onboard computer which in turn deploys the airbags to keep the people in vehicle safe.

Analog and Digital Output Accelerometers In Use

If you’re using a microcontroller with purely digital inputs, you will most likely need to go for a digital output accelerometer.

The disadvantage here is that it requires you to use the timing resources of the microcontroller to measure the duty cycle, as well as performing a computationally intensive division operation.

If you are using a PIC/AVR/OOPIC/Javelin with analog inputs, or a completely analog based circuit, analog is is the way to go.

Some things to consider when buying an Accelerometer

The Number of axes 

For most projects, two should be enough. However, if you want to attempt 3D positioning, you’re better off using a  3 axis accelerometer, or two 2 axis ones mounted at the right angles.

The Maximum swing 

If you only care about measuring tilt using gravity, a ±1.5g accelerometer will work just fine.

If you are going to use the accelerometer to measure the motion of a car, plane or robot, ±2g should give you enough headroom to work with.

For a project that experiences very sudden starts or stops, you will need one that can handle ±5g or more.

Sensitivity

When it comes to accelerometers, the more sensitivity the better.

….what this means is that, for a given change in acceleration, there will be a larger change in signal.

Since larger signal changes are easier to measure, you will get more accurate and precise readings.

Bandwidth 

Bandwidth refers to the amount of times per second you can take a reliable acceleration reading.

For slow moving tilt sensing applications, a bandwidth of 50Hz will most likely work.

If you intend to carry out vibration measurement, or control a fast moving machine, you will atleast need a bandwidth of several hundred Hz.

Impedance/buffering issues 

This is the most common source of problems in projects involving analog accelerometers, because few people thoroughly read the required documentation.

 For A-D conversion to work properly, the connected device must have an output impedance under 10kΩ.

Unfortunately, Analog Devices’ analog accelerometers have an output impedance of 32kΩ.

The solution to this is to use a low input offset rail to rail op amp as a buffer to lower the output impedance. The DE-ACCM  is a good solution that can takes care of this problem for you.

Types of Accelerometers

Piezoelectric

A piezoelectric accelerometer uses what we call the piezoelectric effect ( means, piezoelectric materials produce electricity when put under physical stress like being bent) in order to detect change in acceleration.

 This type of accelerometer is mostly used in vibration and shock measurement.

Piezoresistance

Piezoresistance accelerometers are less sensitive compared to piezoelectric accelerometers, and they are better suited to vehicle crash testing.

A piezoresistance accelerometer increases its resistance in proportion with the overall amount of pressure applied to it.

Capacitive

Capacitive accelerometers use the change in electrical capacitance in order to determine an object’s acceleration.

When the sensor undergoes acceleration, the distance between its capacitor plates changes as the diaphragm of the sensor moves.