利用零漂移仪表放大器(IA)应对传感器测量的设计

利用零漂移仪表放大器(IA)应对传感器测量的设计挑战

Abstract: This article examines the use of instrumentation amplifiers (IAs) for sensor applications. It highlights system challenges and implementation choices, describes new architectures for integrated-circuit IAs, and outlines applications such as ratiometric bridges and low-side current sensing.

A similar article appeared in February 28, 2008 edition of Electronic Design.

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
• 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.

To Amplify or Not to Amplify

System designers like to keep analog chains short in the hope of improving the signal's immunity to external noise phenomena. (Digital circuitry is generally immune to noise, but not always.) In the past, lengthy analog chains tackled a given signal-processing task in sequential stages. One stage, for example, provided differential gain without common-mode rejection, and another provided common-mode rejection without differential gain. Dual and high-voltage supply rails also helped relax the signal-to-noise constraints on analog circuits. The requirements for shorter analog chains and single-supply, low-voltage analog power-supply rails have forced the evolution of innovative architectures to meet these challenges.

One decision that arises early in a system design is whether or not the ADC and sensor can interface directly. Such direct connections can save both space and power in some applications. High-resistance ratiometric bridges, for instance, can use the rudimentary internal reference present in many ADCs thus eliminating the need for an external reference.

On the other hand, a substantial case can be made for the use of an instrumentation amplifier (IA) to interface the sensor to an ADC:

• Amplifying small analog signals at their source improves the overall signal-to-noise ratio in certain applications, especially if the sensor is located at some distance from the ADC.
• Many high-Performance ADCs do not have high-impedance inputs, and must therefore be driven by an amplifier of low sourc