A BASIC APPLICATION
Let's consider in more detail what you can do with these things. Figure 2 shows the TC-offset versus offset characteristics of a generic Microbridge eTC divider. Offset, the deviation of the divider output voltage VIN X (R1/(R1+R2)), is measured in mV/V. TC-offset is the temperature coefficient of that divider output voltage, measured in µV per Kelvin (K) per volt of divider input voltage. Microbridge's eTC adjustment software makes it possible to pick target values for offset and TCoffset as a point within the region delineated by the parallelogram in the figure.
Consider a case in which the pre-correction divider output voltage is 5% (50 mV/V) below its designed value and in which the divider exhibits an undesired positive 75 µV/VK temperature variation. To make output temperature-stable for the nominal voltage-divider resistor values, Microbridge allows the starting point for offset and TC to be moved from the point represented by the dot in Figure 2 to the center (0, 0) of the plot.
To show how the MBT-303-A might be used, consider the passive bridge in Figure 3. Ignore, for the moment, that the voltage divider on the right consists of a pair of Rejustor resistors, and consider all of the resistors to be ideal. In that case, if the resistance ratios on either side of the bridge were matched, the voltages at the two junctions, A and B, would be equal.
Because the resistors were ideal and manifest no TC offset, the voltages at A and B would remain equal regardless of the temperature of their environment. That's the theory. In the real world, there would, of course, be some voltage offset and some TC offset, which must be addressed.
For demonstration purposes, Microbridge's founders built up two bridge circuits like the one in Figure 3 with an eTC Rejustor divider on the right-hand side. Then they measured their characteristics before and after compensation. Figure 4a presents the results, with each original sample showing an offset and one sample showing a positive TC and the other a negative TC. Figure 4 also shows measurements after compensation was applied.
For additional "fine tuning," compensation can be iterative. The "compensated" results (the more or less congruent data points in the topmost characteristics in Fig. 4a) are an improvement, but they can be made better. They represent the results of compensation targets based only on the initial, pre-compensation measurement of offset and TC-offset depicted in the lower curves.
For higher precision, Microbridge measured offset and TCoffset again to calculate new targets and repeat the calibration process. Figure 4b uses a finer scale than Figure 4a to re-plot the results of the initial compensation on one of the test samples from the "blue diamond" trace along with the results of a second compensation on the same sample (red squares).
The above example involves a sensor bridge. But the application could just as well be an operational-amplifier circuit, where trimmable elements are used to adjust amplifier gain and offset. Input offsets of a few volts and gain control better than 0.1% are readily attainable.
IN INTEGRATED CIRCUITS
Although the MBT303-A is a discrete component, Microbridge's technology can be incorporated in CMOS ICs. In terms of the fabrication process, one to three Rejustor-specific dopant implant masks may be required to tailor the resistor-poly film. Typically (but not necessarily), the functional resistance element and the heater resistor that receives the electrical adjustment signals are made in separate resistor film layers.
At the end of the fabrication process (for example, after the bond-pads are opened), the microstructures are released by a bulk-silicon etch process, leaving them suspended over a cavity. This provides the thermal isolation and low thermal mass that permits localized, controllable, and rapid thermal cycling of resistance elements embedded in the microstructures. If permanent protection is required for the microstructures, say, to protect them during plastic packaging, wafer-scale capping is applied prior to dicing.
What does Microbridge's technology do that other technologies don't? The company says existing solutions all have drawbacks, such as limited temperature range, limited maximum frequency of operation, the need for power and ground, limitations to 8- or 10- or 12- bit accuracies, the need for a skilled technician with a screwdriver or laser to perform the adjustment, and more. The range of applications includes:
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Compensating for unit-to-unit variability in other electronic components, such as actuators and active ICs in a system.
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Adjusting circuit parameters (voltage, current, offset, gain, frequency-response, etc.) as late as possible in a manufacturing flow after all of the manufacturing-induced or assembly-induced variances are already present.
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High-frequency circuits in which a digital potentiometer is impractical.
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Any time lasers are impractical, either because it's too late in the manufacturing flow or for other reasons.
Available in a 16-lead quad flat no-lead (QFN) or eight-pin small-outline IC (SOIC) package, the MBT-303-A is currently sampling. It costs $1.67 in lots of 1000.
Microbridge
www.mbridgetech.com
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