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Ratiometricity, signal conditioning enable high-res, low-noise smart sensors

Posted: 26 Jan 2012 ?? ?Print Version ?Bookmark and Share

Keywords:ratiometricity? signal conditioning? smart sensors?

Nowadays, customers for sensors and sensor systems expect to see improvements to performance parameters like module size, operating complexity, price, and energy consumption, as well as lower overall costs. The generally ever-growing need for information and performance leads to constantly increasing demands for both consumer and industrial applications when determining environmental conditions, such as pressure, temperature, weight, flow rate, torque, vibration, tension, and strain.

These requirements result in higher demands on sensor sensitivity, resolution, interference immunity, and precision. Within this context, the concept of a "smart sensor" system with a direct bus connection has continued to gain widespread acceptance in recent years. This system approach usually comprises the following functional elements: Sensor, analog signal conditioning (such as amplification, offset correction), analog-to-digital conversion, digital signal correction, bus interface, and digital analysis.

While smart sensors are now considered the de facto standard for new products launched on the market, particularly when it comes to high-precision sensor applications, one still finds extremely varied levels of performance as far as the actual signal conditioning and processing is concerned. For example, companies often advertise and offer an interface or signal conditioning IC with 16bit signal resolution, although ultimately the resulting measurements may exhibit noise of up to several tenths of a percent of the full signal range. In these cases, the user only sees the desired performance in virtual form, since the low signal quality of the resulting measurements means that, for example, only 10 to 12 bits of effective resolution is actually available from the original range.

Thus, in addition to system concepts, the elimination, compensation, or at least minimization of circuit-specific analog interference still is, and, in the transition to smaller technologies, repeatedly becomes a major task.

Luckily enough, circuit topologies and approaches exist which remain valid and particularly effectiveirrespective of the underlying technologyfor the implementation of high-resolution, energy-efficient, low-noise smart sensors.

Figure: The ratio of the measured voltages V1 and V2 to the resistances R1 and R2 is independent of the absolute value of the supply voltage VDD.

Ratiometricity
The ratiometric measurement principle is an often-used concept that eliminates interference in the power supply. In ratiometric measurements, the measured quantity sought after is the ratio of two quantities that typically exhibit interference. In this context, however, it is crucial that the interference does not impact the actual measurement. A ratiometric value is independent of the supply voltage, for example.

The figure shows that the ratio of the measured voltages V1 and V2 to the resistances R1 and R2 is independent of the absolute value of the supply voltage VDD. As a result, when the value of R1 is known, one can determine the resistances R2 by means of measuring the voltages' ratio and using the formula: R2 = R1�V2/V1.

In a system-integration approach, this principle can be extended for the use in complex sensor interface and sensor signal conditioning (SSC) integrated circuits. A ratiometric topology allows for nearly noise-free applications that are essentially immune to supply voltage interference and have an effective signal resolution of 16bit.

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