Linearity and Noise Capabilities of Ultra-HighPrecision Bulk Metal® Foil Resistors Noise is an unwanted signal inserted by the flow of electrical current in a resistor by thermal movement of electrons and by discrete nature of electrical current. Now days in electronic industry manufacturers and users are suffering from these instabilities and drifts due to noise effect. Instability is often causing due to fixed resistors. Bulk Metal® Foil resistors retain their value most of all the times, so that there is no need for costly controls and other compensating circuitry. Bulk Metal® Foil resistors possess a set of features that cannot be found (all together) in any other type of resistor: Low temperature coefficient of resistance (TCR): down to +0.2 ppm /°C and even less Resistance tolerance (down to +0.001 %) Low reactance ( that shows up in negligible distortion of pulse signals and in immunity to circuit self – oscillation ) Load life stability of +0.005 % (50 ppm) at + 70 °C, rated power for 2000 hours ESD protected at least to 25, 000 V Thermal stabilization time < 1 second (nominal value achieved within 10 ppm of steady value) Low thermal EMF of 0.1 µV/°C Low excess noise (to be explained in further detail later)
High linearity or low voltage coefficient of resistance (VCR)
Bulk Metal® Foil resistors are having high linearity specifications because of the material they made up of : a cold-rolled metal alloy foil with a thickness of several microns. As for the microstructure is concern Bulk Metal® Foil resistors are more linear as compared to the resistive cermets used in thin film or thick film or carbon composition in carbon composition resistors. Laser trimming is used in Bulk Metal® Foil resistors technology to trim resistor to its precise value which is very fine trimming which does not damage current carrying portions of resistive element. In Bulk Metal® Foil resistors there is high – quality contact between resistive element and terminals which maintain the overall functionality of performance of the resistor.
As the resistance and temperature increases Thermal noise become more active. Thermal noise can be reduced by reducing resistance, temperature or signal bandwidth. Shot noise or fluctuation noise can be avoided by increasing current or reducing bandwidth, as it is caused by the discrete nature of the charge carriers and fluctuation of their number in the unit of volume. The level of these noises does not depends on the resistor types. Resistive material used in the composition of the resistors determines the noise level of the resistor. Below are some ways with the help of that excess noise can be avoided : a) Avoiding use of the low frequency band b) Reducing current c) Increasing the volume of resistive material d) Using less noisy resistive materials Carbon composition, Thin film & Thick Film resistors are more immune to the noise .The least noisy are Bulk Metal® and metal alloys (foil, wire). This is why Bulk Metal® Foil resistors are such a good choice for low-noise applications. Low -noise differential amplifiers need tight resistance tolerance, which is an inherent characteristic of Bulk Metal® Foil resistors, which tend to suffer from common mode noise.