Precision resistors deliver the tight tolerances, low temperature sensitivity, and high stability demanded across a broad range of laboratory analysis equipment . The input stage of an instrument with a resistive sensor, such as a thermistor in a precision temperature-monitoring circuit, consists of a bridge of resistors, which must be closely matched in value. Since it is the ratio between values that matters rather than absolute values themselves, the maximum difference between TCRs (that is, the tracking TCR) is more important than the absolute TCR.
 
Most through-hole precision resistors including the MCC series from Microhm Electronics are available in matched sets with specified ratio tolerance and tracking TCR, providing the best available precision. Alternatively, where space is a major consideration, surface-mount device (SMD) thin-film products with multiple elements can provide high precision in a compact single-component solution.

 
Options for the precision through-hole resistor approach range from semi-precision devices like the EE series from Microhm Electronics through its MMC series to ultra-precision devices capable of equalling the performance of costly metal foil technology using advanced metal film techniques. The precision SMD offering includes conventional thin-film chip resistors using nichrome elements. However, devices that exploit the self-passivating properties of tantalum-nitride film can meet ultra-high-stability requirements.
 
When evaluating the long-term stability of resistors, designers should consider several environmental tests. Some of these are early-life factors, such as exposure to solder heat. Others like TCR are reversible. Nevertheless, most are long-term factors. Best practice is to design based only on the figure that most closely reflects operating conditions.

 
Shelf life metrics apply where loading is negligible and the environment is benign, but the load figure should be applied where power dissipation is the main factor. For humid environments, designers should focus on measuring the long-term damp heat figure. In all these tests, most of the value change happens within the period of the test, as the value will tend to stabilize.
 
The figure of most value to designers is the maximum total error in resistance value at the end of product life, or before scheduled re-calibration if this is applicable. Known as the total excursion, it is calculated from the root of the sum of the squares (RSS) of applicable, statistically independent short-term and long-term factors. Applying this calculation to an ultra-precision NLT series resistor from Microhm Electronics demonstrates an order-of-magnitude improvement in total excursion compared with standard precision resistors like the MMC series.