Low TCR Calibration Resistor

DN Staff

July 17, 2006

3 Min Read
Low TCR Calibration Resistor

Precision systems require a minimum impact of temperature variations on offset and gain adjustments. Unfortunately, the resistors used in these and other voltage or current calibration techniques have a Temperature Coefficient of Resistance (TCR) that is typically not measured or trimmed. Traditional trim approaches at the wafer or chip level use thin-film laser trimming or blow fuseable links. Hybrid- and board-level products use thick-film laser trimming, trim pots and digital potentiometers. A Micro-Electromechanical (MEMS)-based precision resistor provides a highly stable approach for trimming high-precision systems.

Microbridge Technologies' Rejustor micro-resistor consists of a micromembrane (or membranes), suspended over a cavity etched in a silicon substrate with an electrical micro-heater fabricated on the membrane using a conductive, heat-dissipating polysilicon layer. Suspending the micro-heater over an air cavity provides high thermal isolation for the micro-structure and allows the device to reach very high temperatures, such as 800 to 1000C, at very low power without affecting the surrounding circuitry. The small structure's very low thermal inertia, combined with the isolation allows rapid heating.

Fabricated above an etched cavity, the resistors and the polysilicon resistive heating elements provide a highly stable, operating temperature insensitive system. A pair of rejustors and their seperate trim inputs can be packaged in a high-volume, 8-pin SOIC package.

A specially developed hardware-and-software technique provides high-precision electrical programming to within 5 to 20 parts per million or better of the resistive elements in the suspended microstructure. Typical industry production test equipment performs the trim operation for the Rejustor at wafer probe, final packaged test, and/or during in-circuit board-level testing. Trimming at the system level or during production testing requires custom designed calibration circuitry. Both techniques avoid the expense of laser trimming equipment.

Used in a calibration circuit, the Rejustor's TCR is 0 plus or minus 100 ppm/K for trimmed and untrimmed units and the change in temperature coefficient is within plus or minus 30 ppm/K. The design provides constant TCR across the trim range for resistance values from approximately 200 to 1 MO with a trim range 30 percent down from the "as manufactured" value. Units have adjustment resolution to 0.01 percent.


Tim Warland, applications engineer, Microbridge Technologies ([email protected])

For more information on Microbridge Technologies' Rejustor, visit http://rbi.ims.ims.ca/4928-500.

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