Increased demand toughens resistors

Over the past few years, resistor manufacturers have seen a dramatic increase in demand for precision and ultraprecision discrete resistors and resistor networks, particularly for instrumentation, industrial, medical and telecommunications applications.

These precision networks go into any applications where precision analog circuits are present, said Jerry Seams, applications engineering manager for TT electronics' IRC Advanced Film Division. One example is medical equipment, where small changes in some physical parameters have to be measured, requiring extreme accuracy in the front end of the circuits, he explained.

The need for more precise measurements has translated into a greater requirement for higher-precision resistive devices with tighter tolerances, tighter temperature coefficients of resistance (TCRs) and higher stabilities. In general, precision resistors offer tolerances tighter than 0.5 percent and TCRs tighter than ±50ppm/°C. However, some resistor makers identify 1 percent and 0.5 percent tolerances as precision ratings for thick-film parts.

Seams said stability has become a much more important parameter with the growth of digital electronics. "It's getting to be as important as initial tolerance," he said.

"A lot of circuit boards have digital electronics combined with analog front-ends, so what becomes a lot more important than initial tolerances is how stable the part is once it's on the board," Seams said.

Typically, precision developments have been the domain of thin-film technology, but now thick-film materials are able to achieve tighter tolerances. For example, you can now find thick-film chip arrays with a 1 percent or even 0.5 percent tolerance. The key reason for the development is cost; thick-film devices have always been much lower-priced than thin-film parts. Nonetheless, you'll find that most high-precision devices still fall into the thin-film category.

Vishay Intertechnology Inc. has been one of the biggest manufacturers of ultrahigh-precision networks. Most recently, the electronic components giant introduced its SMNZ high-precision Z-Foil four-resistor network, which provides a low absolute TCR of ±0.05ppm/°C from 0°C to 60°C and ±0.2ppm/°C from 55°C to 125°C. In addition, the part is said to offer excellent power coefficient of resistance tracking, of 5ppm at rated power, and a tolerance match of 0.01 percent.

The SMNZ devices can be used independently or as divider pairs. Applications include instrumentation and differential amplifiers, bridge networks, radio arms in bridge circuits, medical and test equipment, and military and aerospace systems.

Vishay also offers a surface-mount power metal strip resistor that it says offers improved resistance stability during operation, with a maximum resistance change of 0.5 percent through a 2,000hr workload. In contrast, a typical current sensing resistor offers a resistance stability of The Vishay WSLS2512 device features specially selected materials that allow for current sensing in high-temperature (125°C), tight-stability applications that require precision current monitoring of sensitive circuits such as automotive electronic controls, including engine, transmission and pollution controls. The devices, with a resistance value range of 0.01Ω to 0.1Ω, offer a tight tolerance down to ±0.5 percent, along with TCRs as low as ±30ppm/°C.

High precision, stability
TT electronics' IRC Advanced Film Division's latest precision product introduction is its PFC Divider series of precision chip voltage dividers, which are available in either tantalum nitride (TaNFilm) technology, for precision performance, or IRC's Tantalum Ultride technology, for ultraprecision and high-stability circuits. Both versions of the PFC Divider are offered in a 1206 package.

The TaNFilm resistive elements are said to provide excellent environmental performance and precision, as well as tight TCRs, tolerance and stability specifications. The TaNFilm PFC voltage divider features a resistance range from 10Ω to 150kΩ with a package power rating of 250mW. The divider offers a maximum voltage rating of 100V, absolute TCRs to ±25ppm/°C and TCR tracking to ±5ppm/°C. The TaNFilm voltage divider is suitable for battery monitoring, voltage references, voltage conversion, voltage regulators, logic-level conversion, DC/DC converters and power supplies, as well as COTS applications.

Figure 1: BI Technologies offers its RoHS-compliant SOT-23 precision voltage-divider network in a sample kit for designers.

Boasting greater performance, the Tantalum Ultride voltage divider, with a resistance range from 1kΩ to 50kΩ and a power rating of 50mW, delivers an absolute TCR of ±10ppm/°C, with TCR tracking to ±1ppm/°C and absolute tolerance to ±0.02 percent. Ratio stability at 70°C ambient is less than 50ppm over 1,000hrs at rated power, and absolute stability under same conditions is less than 0.02 percent.

The Tantalum Ultride voltage divider is specified for high-stability applications including medical equipment, petrochemical equipment, instrumentation and test devices, military avionics, and missile guidance.

Monolithic design
Another development from IRC is the use of a single-sided monolithic design for its precision resistor networks to eliminate parasitic effects due to wraparound terminations. The devices in the company's CHC-Precision series are housed in a ceramic BGA package that features all circuits on the bottom side of the device to eliminate wraparound terminations.

The BGA devices feature a resistance range from 10Ω to 100kΩ, TCR tracking to ±5ppm/°C, ratio tolerances to ±0.05 percent, absolute tolerances to ±0.1 percent and absolute TCRs to ±25ppm/°C. Applications include industrial gear, process control and monitoring, medical equipment, and instrumentation and metering.

TT electronics' BI Technologies Electronic Components Division released the RoHS-compliant SOT-23 precision voltage divider network this year. Designated the SS103KITLF, the network is available in a sample kit that includes three dividers:

SS103VD01DQLF7, which offers 12.4kΩ/20kΩ resistance values (0.62:1), absolute tolerance to 0.5 percent, ratio tolerance to 0.1 percent, TCRs of ±25ppm/°C and maximum TCR tracking of 5ppm/°C;

SS103VD06CBQLF7, with 11.3kΩ/101.7kΩ resistance values (1:9), absolute tolerance to 0.25 percent, ratio tolerance to 0.1 percent, TCRs of ±25ppm/°C and maximum TCR tracking of 5ppm/°C;

SS103VD07FAQLF7, with 10kΩ/20KΩ resistance values (1:2), absolute tolerance to 1 percent, ratio tolerance to 0.05 percent, TCRs of ±25ppm/°C and maximum TCR tracking of 5ppm/°C.

The SOT-23 voltage divider networks are being specified for gain circuits in military, aerospace, industrial and medical applications.

Figure 2: IRC Advanced Film Division offers a precision chip voltage divider with tantalum nitride technology.

There have also been improvements in the chip resistor arena. Stackpole Electronics Inc., for example, has extended its RNC series of precision Nichrome thin-film chip resistors with an extended resistance range at 0.01 percent tolerance and TCR values of 5ppm and 10ppm. The strength of Nichrome as a thin-film resistor element has always been the wide resistance range offered for 0.1 percent tolerances and 25ppm TCR requirements, said Stackpole. What has changed is the ability to achieve high levels of stability using the relatively inexpensive Nichrome thin-film technology, the company said, IRC has expanded its PCF series of precision thin-film chip resistors with extended resistance ranges and tolerance values in eight package sizes: 0201, 0402, 0603, 0805, 1206, 1210, 2010 and 2512. Typical applications include precision circuits in instrumentation, industrial applications, medical equipment, computers, power supplies and telecommunications.

The PCF chip resistors feature a wide resistance range, from 1Ω to 2MΩ, with power ratings of 50mW, 62.5mW, 100mW, 125mW, 250mW and 500mW.

Maximum working voltage ranges from 25V to 150V, with rated tolerances to ±0.01 percent and TCRs to ±5ppm/°C.

- Gina Roos
eeProductCenter