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Zero-drift instrumentation amplifier (IA) takes the strain out of sensor measurements

Posted: 27 May 2009 ?? ?Print Version ?Bookmark and Share

Keywords:instrumentation amplifier? sensor applications? bridge circuits?

Sensor measurements typically translate physical phenomena of interest into electronic-circuit parameters such as resistance and capacitance, which can then be read with a bridge circuit. Bridge circuits produce an output voltage or current signal that is ratiometric with respect to temperature and power-supply voltages, thereby enabling the measurement system to self-compensate for these variables. Sensor examples include: thermistors for temperature sensing; resistive/capacitive strain gauges for pressure sensing; and magneto-resistive sensors for direction/position sensing.

Sensors that produce a signal voltage or current directly do not require a bridge circuit to transform the physical variables. Examples include thermocouples, ECG-based medical instrumentation, and voltage across the current-sense resistor in a power-monitoring circuit.

Today's sensor applications range from consumer electronics (thermometers, pressure scales, GPS systems), to automotive equipment (fuel sensors, knock sensors, brake-line sensors, window pinch control), to industrial and medical instrumentation (valve-position sensing, temperature-based system calibration and alarm, and ECG). Their environments are rich in EMI noise, power-supply harmonics, ground-loop currents, and ESD spikes, while the signals of interest that are to be extracted are extremely small. Thus, the analog-sensor interface becomes nontrivial, and must maintain exacting specifications while rejecting environmental phenomena. For commercial success, it must also deliver low cost, small size, and (for battery-operated meters) low supply current.

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