Sensitivity, Resolution and Threshold

Sensitivity   

It is the ratio of magnitude of output signal to the magnitude of input signal. In other words it defines how large the response of the sensor is to a small change in input. For a better reading, Sensitivity of the sensor must be high.

For sensitivity to be high, the sensor should not have range greatly exceeding the value to be measured, barring the margin for accidental overloads.
Its unit depends upon the type of input and output. For example, for a proximity sensor, sensitivity can be expressed as V/cm or mV/cm.

Sensitivity = Change in Output 
                    Change in Input

Resolution

 It is the minimum detectable change in the parameter which is being monitored by the sensor. A change in the parameter should be greater than a certain value to be detected by the sensor. This value is known as resolution. For example, if a voltmeter has a sensitivity of 1mV, then a change lesser than 1mV will not be detected by it. i.e., if it reads 9.999 V, then it will become 10.000V only if a change of 1mV is made and not less than that.
         This is analogous to the resolution of an image. Higher the resolution, more clearer is the image, or more smaller pixels can be viewed. Similarly a sensor having higher resolution will be able to read smaller readings or detect a smaller change.

Threshold

 It is the minimum value of input below which the sensor gives no output. When we gradually increment the value of input from zero, we get no output at first. As soon as the input reaches above the value of sensor's threshold, it starts giving output.
         Threshold of a process is the minimum requirement of that process. Consider, for example, a vehicle running on gas or some fuel. Whenever the tank is completely empty, the fuel scale shows empty. When we start filling fuel, the fuel scale does not rise immediately. We need to fill some fuel until the scale starts rising. This is the threshold of the fuel required to detect its amount, and consecutively, to start the car. We can even consider charging of our cellphone battery for an example. The cellphone does not start until certain amount of battery is charged. Same concept of threshold applies for all sensors and transducers. They require a specific minimum amount of the entity under measurement, to start sensing it. This minimum amount is known as threshold. Sometimes this threshold is desirable, while for many applications, it is not.