Re-adjustable resistor maintains high accuracy over wide temperature range
Many precision analog, sensor and power supply circuits need some kind of temperature compensation and calibration to maintain performance and accuracy. Toward that goal, Microbridge Technologies has developed Rejustor, a CMOS and MEMS-based electrically re-adjustable polysilicon resistor. The developer claims it is the world's first passive electronic temperature compensation (eTC) component. The operating temperature range for the Rejustor is -50 °C to +150 °C.
The first in this family of resistor dividers and networks employing eTC technology, the MBT-303-A is a high accuracy dual 30 k divider (two resistors in series). Each resistor can be set to any value between 21 k and 30 k with an accuracy of 0.01%. Having two Rejustor elements in the same package makes it easier to implement divider networks where resistors must be equally matched (Figure 1). The MBT-303-A Rejustor enables automated, independent adjustment of resistance and the temperature coefficient of resistance (TCR). Control of TCR, which is independent of Ohmic value, enables the MBT-303-A to not only provide one to two orders of magnitude performance improvement for analog designs but also maintain consistency of resistance over an extended temperature range for circuits that require set-on-test calibration and compensation, according to the manufacturer.
Employingt electric current, independent adjustment of resistance and TCR for each resistor in the divider is accomplished by heating and cooling the resistive polysilicon element in a closed-loop system under control of Rejustor calibration tools and Rejust-it software. The output voltage adjustment range for the divider is ±100 mV/V, with a temperature coefficient output adjustment range of ±150 ppm/K. Maximum power rating is 2 mW at 70 °C.
Rejustors shift compensation in temperature-sensitive applications from an active process to a passive one. Traditionally, engineers have had to compensate for changes in temperature using complex digital circuitry comprised of a microprocessor and digital-to-analog converters switching out fixed resistances. This approach is limited in accuracy (about 0.1% for a compensation circuit employing 10 switches) and consumes a notable amount of power. The additional circuitry involved adds further uncertainties that reduce accuracy. Rejustors eliminate many of the detrimental side effects associated with digital compensation to achieve accurate and cost-effective temperature compensation. In-circuit adjustability of Ohmic resistance and TCR, as well as support for positive and negative temperature coefficients, enables developers to apply compensation after devices such as sensors are fully manufactured and hermetically sealed. Rejustors also eliminate the need for thin and thick film laser trimming used in IC manufacturing. The MBT-303-A requires no power source during operation, does not suffer from wiper resistance, and no external temperature sensor is required since the Rejustor is its own temperature sensor as well as adjustment controller. Rejustors are capable of replacing complex digital potentiometers and can be used to raise the performance of lower-cost analog circuits to the level of higher-precision circuits, stated Microbridge. Figure 2 depicts the performance of the Rejustor divider in comparison to an uncompensated resistor network. Available in a 16-lead QFN package or 8-pin SOIC package, the MBT-303-A passive Rejustor is sampling and costs $1.67 each in quantities of 1,000. The Rejustor calibration tool MBK-408A, which includes Rejust-it software, costs $400.
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