# Noise

- Related Links

Noise can be added to all sensor types, technologies, and material types provided in the web pages for the Enose Toolbox. Noise is provided by the function file addnoise.m and is a percentage of the baseline value of the sensor; it is assumed to be normally distributed with a variance (root mean square) value that is extracted from empirical (experimental) data and reflects noise sources due to electronic factors (such as thermal noise) and transduction factors.

All noise values can be accessed and computed with the function file
addnoise.m using the following syntax:
*
output_noise = addnoise (type, technology, material_type)
where type is the type of sensor (such as "chemiresistor")
and technology is the subset of sensor (such as "comp_polymer")
and material_type is the sensor film (such as "poly1a")
*

The output of the addnoise function is a percentage change with respect to the baseline condition of the sensor.
For example:

*output_noise = addnoise ('chemiresistor', 'comp_polymer', 'poly1a')*

gives a randomly selected value from a normal distribution of noise associated with the poly1a film when it is used as a chemiresistor. This randomly selected value is a percentage of change from a baseline condition. For instance, if a value of 0.01 is returned as the output_noise variable, for a baseline resistance of 20,000 ohms, the noise-induced variation in the output of the sensor will be (0.01/100)*20,000 or 2 ohms which is then added to the sensor output at a desired concentration (by the toolbox user)

Once the overall noise is calculated, known sources of noise can be subtracted from the total to isolate lesser understood causes of variation in sensor output. For example, in a chemiresistor, the electronic noise (thermal) can be subtracted from the overall noise, assuming a 100MHz bandwidth (the data were collected using a standard benchtop multimeter) and using the familiar thermal noise expression where the equivalent voltage fluctuation across the resistor due to thermal noise is the square root of 4*Kb*T*R*bandwdith, where T is the temeprature in degrees Kelvin, and Kb is Boltzmann's constant. Thermal noise can also be expressed as equivalent power or current, depending on the way the resistance is inserted into the user's simulation.

**Current Status**: Accurate noise values are provided for composite polymer chemiresistor films poly1a, poly2a, poly3a, poly6a, poly111a, poly13a, poly15a; average noise (over all composite polymer films) is used for all remaining composite polmer chemiresistor films; Noise values of 0 are used for noise in remaining sensors. Noise information will be updated regularly as sufficient empirical data become available.