Electronic Design

Sensor-Conditioning Amps Use Rejustors For Precision Compensation

Microbridge Technologies’ MBSTC-02 compensation amplifier is the first IC to marry the company’s Rejustor technology with a CMOS analog process (see the figure). Rejustors are trimmable resistors that have marked advantages over older technologies. Notably, they can be trimmed iteratively to a high degree (0.01%) of precision, while their temperature coefficient (TC) can be adjusted independently.

Once set, Rejustors maintain their resistance and temperature coefficient of resistance (TCR) indefinitely, but they can be re-adjusted anytime as required. They also can be independently trimmed to any value between 21 and 30 kO with 0.01% precision (see “Thermal Trimming Revolutionizes The Resistor” at www.electronicdesign.com, ED Online 16230). Each Rejustor has a thermally isolated poly film resistor and an adjacent power resistor.

For trimming, the power resistor is pulsed in a controlled fashion, briefly raising the temperature of the adjacent Rejustor resistor. The result is an annealing that changes the requestor’s resistance in a controlled and predictable manner. The annealing takes place at temperatures far above normal operating temperatures.

The MBSTC-02 is a single-chip compensation amplifier for conditioning the signal from Wheatstone-bridge sensors that exhibit negative TC-sensitivity. Four Rejustors parallel the bridge elements to balance offset and temperature- induced offset drift. Other Rejustors compensate amplifier gain and offset. Setting these characteristics for a specific sensor is a one-time, in-circuit process.

The adjustment process is fully automated. First, the Rejustors that parallel the four sensor-bridge resistors adjust offset and TC-offset. Typically, offset is first reduced by at least an order of magnitude for a given set of output measurements at two temperatures. Even finer precision of offset adjustment can usually be obtained by making another set of output versus temperature measurements. These adjustments are done in-circuit, using the output of the chip for feedback to the adjustment software.

That takes care of compensating the bridge. Inside the MBSTC-02, the first gain stage is a low-noise amplifier that provides a fixed positive TC-gain for the coarse compensation of bridge sensors. Fine-tuning takes place in the next gain stage, which uses a pair of Rejustors to compensate for the residual TCsensitivity.

The third amplification stage employs a pair of low-TCR Rejustors to adjust overall gain. Because the TCs were already adjusted out, the final gain adjustment can be accomplished via single-temperature output-voltage versus sensor-input measurements. The fourth stage provides differential output, if necessary.

Microbridge has made those adjustments easier than they sound. Rejustors are adjusted using LabVIEW-based software from Microbridge that can run on the company’s MBK-408 adjustment kit, more elaborate instrumentation from test equipment makers such as National Instruments, or custom hardware. In any of these cases, the Rejust-it software simply operates in a closed loop to adjust the Rejustors to achieve the desired output condition while monitoring the input.

The MBSTC-02 provides signal conditioning and amplification for Wheatstone- bridge sensors with bridge resistance from 2.5 k to 6.5 kO and a full-scale bridge output of 20 to 100 mV. Two versions are available. The MBSTC- 02A is recommended for sensors with a nominal TC-sensitivity of –1400 ppm/K. The MBSTC-02B suits sensors with a nominal TC-sensitivity of –2200 ppm/K.

Limited quantities of the MBSTC-02 are sampling now. Mass-sample quantities will be available in the second quarter of 2008, with high-volume production slated for the third quarter. In thousand-piece quantities, the unit price is $1.99.

Microbridge Technologies
www.mbridgetech.com

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