The resistor manufacturer selects and combines chips of the appropriate resistance values in hermetic or non-hermetic packages.  Trimmed and/or matched, individual chip-resistor assemblies reside in a plastic or hermetic enclosure. These devices allow for resistance variation from circuit to circuit without the non-recurring engineering cost (NRE) of specialized networks. When using foil chips, it’s important to keep in mind that all foil resistors have very low absolute TCR.  The TCR of Microhm Electronics metal foil resistors MVR series is as low as 10ppm. This ensures excellent TCR tracking, even among randomly selected chips.
Foil resistors provide stabilities well under the maximum allowable drift necessary to withstand thousands of hours of operation under harsh conditions. Examples include the extreme temperatures and radiation-rich environments of down-hole oil-well logging applications, in the frigid arctic, under the sea, or in deep space. The devices receive stabilization processing, such as repetitive short-term power overloads, to ensure reliable service through the unpredictable stresses of extreme operation.
 
New foil resistors have a thermal EMF of <0.1 µV /°C, which would result in an output change of only 0.05 µV, or an error of only 0.001%—80 times less than the typical resistor. Whatever system is being fed, this divider’s output would start off with 80 times more error using typical resistors instead of foil resistors. Say the typical divider for an op amp requires ratio matches of 0.005% to 0.01%. The thermal EMF error would then amount to 10 times the design ratio using a typical resistor, versus only one-tenth the design ratio using a foil resistor.

 
Designers can avoid costly redesigns by implementing new-generation foil resistors into circuits. Significant performance differences exist among the families of products categorized as precision resistors. Resistors built on foil technology offer several advantages over thin and thick film, including absolute TCR, stability, thermal EMF, current noise, ESD immunity, rise time and others. In critical applications that have high-temperature, high-reliability, or stringent military, medical and space requirements, foil resistors have become the device of choice. Metal foil resistors are an important part of Microhm Electronics lineup, including MVR2321-4, MVR3825-4, MVR4618-4, NMS2818, NMS4527 and MPR series.