In some applications the noise generated when current flows through a thick film resistor can cause serious system errors. In this post, we consider the source of resistor noise and how it can be minimised.
What Is Electrical Noise
Electrical noise is an electrical signal that distorts a desired electrical signal and causes an error. The impact depends on the comparative amplitude of the noise and the desired electrical signal. Noise can overwhelm the small electrical signals in sensitive equipment.
Noise tends to be random; it can be temporary or constant. Temporary effects are often generated by spikes in voltage or current during switching operations. Constant noise may be white noise (described below) or generated by adjacent electronic components or systems. Noise can extend in its various forms across the frequency spectrum.
Noise Generated By Resistor Devices
Noise generated by a typical resistor is made up of two elements. Thermal issues are the main source of noise. The level of noise generated for a given resistor is dependent on three variables: resistance, temperature and frequency. Hence for a given resistance and frequency temperature is the driving factor.
Any electrical current is a movement of electrons. That movement is generated by a potential difference (voltage) across two points. However, there is also a random element to the movement of electrons. This noise level (white noise) is generated whenever current flows through a resistor.
Depending on materials and construction some resistor technologies have better noise characteristics than others. The final choice of a resistor is always a compromise depending on performance (including noise), cost, precision, power rating, size, robustness, stability and frequency response.
Noise In Thick Film Resistors
We have discussed the thick film manufacturing process elsewhere on this blog
The resistive paste consists of a mixture of metal oxides that combine during the manufacturing process. The conduction path is through the touching oxide particles
In thick film resistors, the junction between oxide particles is a source for the random movement of electrons when a current flows (white noise). Increasing temperature can increase the noise level.
Noise only tends to be a problem in more sensitive applications such as measurement equipment. With higher resistive inks the level of noise is reduced. In many applications, such as high voltage applications, other performance issues (see above) are of greater concern than noise.
Although noise generated by thick film resistors cannot be eliminated it can be reduced. Specialist thick film resistor manufacturers can eliminate many imperfections in the design of the resistor. Modifications to the shape of the resistor can have a positive impact as can changes in the manufacturing process and the choice of materials.