Abstract
The first block in a precision signal chain after the sensor is often an amplifier circuit, which must amplify the signal of interest while maintaining signal fidelity. This article will discuss how to select the appropriate precision amplifier circuit topology for a sensor application, with a specific focus on the operational amplifier, difference amplifier, current-sense amplifier, instrumentation amplifier, and fully differential amplifier.
Introduction
Precision sensors have long been key to measuring much of the physical world. As the variety and quantity of precision-measurement applications increases, engineers are called upon to design systems that can sense ever-smaller amplitude signals in the presence of interferers, while maintaining signal fidelity. This entails not only the selection of the appropriate sensor, but also careful design of the complete signal path—from the sensor through to the data converter—to digitize the analog measurements. So which amplifier topology is best for a given sensor application? The choice requires consideration of the final system objectives and the design priorities for the amplifier circuit.
The first consideration in choosing an amplifier topology is generally to determine if the sensor output (and therefore amplifier input) signal is single-ended or differential. Some topology configurations accept a single-ended input signal, and some accept a differential signal. The selection of the best amplifier topology will also depend on if we want the amplifier to output a single-ended or differential signal.
Single-Ended Operational Amplifier Topology
The operational amplifier (op amp) is an extremely versatile amplifier that can be configured for many different operations, hence the name. In the case of an amplifier circuit with a single-ended input and output, a simple op amp circuit with appropriate feedback from a resistor pair would be used. When using a single-ended op amp circuit to amplify a high precision signal, the choice of which topology to use is usually between a non-inverting amplifier and an invert- ing amplifier (Figure 1). Both circuits utilize a single feedback resistor and single gain resistor.
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