Sensors - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 14

2.2.8 Resolution

The resolution of a sensor is the minimum fluctuation of the input signal that can
be detected.

2.2.9 Bandwidth

After any change in the input parameter, a certain amount of time is required before
the resulting change in the output parameter can be detected and measured. This
time is called response time. Some sensors also many sensors have decay times,
which would represent the time after a step change in physical signal for the sensor
output to decay to its original value. Values reciprocal to the response time and the
decay time are called lower and upper cutoff frequencies. The bandwidth of a sensor
is the frequency range between these two frequencies. A sensor with high band-
width can change its output to follow fast (high-frequency) variations in the input.
For measures that change slowly, such as the temperature of a large liquid tank,
bandwidth may not be important. For other applications such as scanning shape or
vibration sensors, bandwidth may be the most important figure of merit.

2.3 Classifications

There are two basic ways to categorize sensors. The first is based on the principal
by which they function, and the second is based on the function the sensor performs.
Most sensors act like passive devices (i.e., capacitors or resistors). These sensors
require external circuitry for biasing and amplification of the output signal.
Resistive sensors are devices whose resistance changes with the value of input
signal being measured. These sensors can be used in a simple voltage-divider con-
figuration (Fig. 2.1 ). For more precise measurements a variety of configurations
can be used (e.g., the Whetstone bridge circuit).

+ V -

I

R1 R2

V1 V2

Fig. 2.1 Voltage divider. Legend: R1 - Resistive sensor, R2 - Reference resistor, V1 - Voltage
on the resistive sensor, V2 - Voltage on the reference resistor, V - reference voltage