Considerations in selecting precision op amps for sensor-input processing designs

As the basic building blocks used in an extensive array of consumer, industrial, scientific and other applications, op amps (operation amplifiers) are among the most widely used electronic devices, and for most low-end applications, the requirements are straightforward and the device choice is relatively easy. However, there are challenges to selecting the optimal precision op amps for implementing many higher-end sensor-input processing designs. 

The op-amp selection can be especially challenging when the types of sensors and/or the deployment environments create special demands such as ultra low power, low noise, zero drift, rail-to-rail input and output, solid thermal stability, and the repeatability to deliver consistent performance across thousands of readings and/or in harsh operating conditions. 

For precision op amps to be used in complex sensor-based applications, designers need to look at multiple aspects to get the best combination of specs and performance, while balancing cost considerations as well. In particular, chopper-stabilized op amps (zero-drift amplifiers) offer excellent solutions for ultra-low offset voltage and zero drift over time and temperature. Chopper op amps achieve high dc precision through a continuously running calibration mechanism that is implemented on-chip.

Although there is no easy “one-size-fits-all” formula, the following examples show how the op-amp selection can help achieve critical application objectives.

Weigh scales and pressure sensors

Weigh scales and pressure-sensing applications typically use a highly sensitive analog front-end sensor, such as a strain gage, that can provide very accurate measurements but output very tiny signals. For high-precision weigh scale applications, designers may use a bridge sensor network, in which individual op amps are paired with gain resistors chosen to provide common mode extraction and to deliver 10 to 20 ppm of accuracy.Such advanced “roll your own” designs require stringent performance from the op amps to extract very small signals riding on relatively large inputs.

In order to successfully amplify these small signals, the op amp must have ultra-low input offset voltage and minimal offset temperature drift, with wide gain bandwidth and rail-to-rail input/output swing. (Rail-to-rail input swing is not needed for small input signals, of course.) It is also critical for the op amp to offer very stable ultra-low-frequency noise characteristics at close to dc conditions such as 0.1 to 10 Hz. 

For high-precision weigh scale bridge network sensor applications, designers should look for a single zero-drift op amp that features very low input offset voltage and low noise with no 1/f to 1mHz.

As illustrated in Fig. 1 , a good example is the chopper-stabilized zero-drift ISL28134 op amp delivers excellent noise voltage (nV) across the range from 10 Hz down to 0.1 Hz, thus providing virtually flat noise band to dc level.  Leveraging the inherently stable chopper-based design, the ISL28134 specification actually includes a maximum noise gain of 10 ppm (seven sigma) to offer optimal performance for high-gain applications while minimizing noise gain error.

Fig. 1: ISL28134: 0.1 Hz to 10 Hz peak-to-peak noise voltage

For portable weigh scale applications where low power is also an important consideration, designers may want to consider a precision amp that combines ultra-low micropower (25 µA max) and low-voltage offset (6 µV max) characteristics with a chopper-stabilized design that delivers flat noise band to dc and near-zero drift. 

Current sensing and control applications

There are a number of different ways to sense current levels depending on the specific application requirements. These include shunt sensors using resistors, Hall-effect sensors and current transformers. In this example, we will look at op-amp requirements for use in shunt sensor applications. Today's shunt sensor techniques have evolved to provide a high level of accuracy and also offer the advantages of lower cost and applicability across a wide range of requirements and deployment scenarios. 

Basically, the shunt sense methodology places a resistor in the path of the power supply source being measured. Because the resistor drop impacts power efficiency, it is generally desirable to use the smallest resistor value possible.  Once again, this means that the current sensing application must amplify a relatively small differential power drop in resistance into a large gain. 

Therefore the op-amp circuit must offer high common mode range and high accuracy. Low power is also an important requirement, especially for current sensing in battery applications. Embedded current sensing circuits also need to be relatively inexpensive so as to not add significantly to the BOM cost of the product that is being monitored.

In addition, for many industrial, utility, and communications current sensing applications, the op amp needs to minimize drift over extremes of temperature and extended time periods. For example, current sensors deployed on top of utility poles are exposed to relatively harsh environmental swings and need to provide consistent performance over long periods of time without incurring the expense maintenance requirements.

Many shunt-based current-sensing applications are built using op amps that are chopper-based, zero-drift amplifiers that combine both low power and high accuracy in the smallest package size on the market. In addition, as illustrated in Fig. 2 , these chopper-stabilized CMOS devices provide excellent low-drift characteristics over both temperature extremes and extended time periods.

Fig. 2: Minimizing Vos Drift over temperature and time, the ISL28133 is a single chopper-stabilized op amp and the ISL28233 is a dual of the same amp.

Current sensing is already one of the most pervasive applications used across a wide range of industry segments (consumer, industrial, communications, utility, etc.) and it is only becoming more important with the proliferation of new electronic devices and the increasing emphasis on “green” power management techniques. The chopper-stabilized precision-op-amp devices described above offer very low offset voltage and offset drift, rail-to-rail input and output, and low power consumption needed to support the escalating demand for embedded current sensing applications.

Handheld toxic environment safety monitor

The final application example brings together a number of different sensor inputs within a single device and illustrates how well-designed op-amp circuitry can help to efficiently handle such a multi-sensor signal chain within a compact portable device.  Handheld devices used to monitor hazardous environments are increasingly combining multiple sensors in order to minimize size while maximizing capabilities. Such a device might combine a combustible gas sensor, oxygen sensor and catalytic heat band sensor.

Using multiple instances of an ultralow power op amp provides advantages for multi-sensor signal chains within a small handheld device.

Because these safety devices typically need to operate in an “always-on” mode, the ISL28194 ultralow micro-power profile (450 nA max and 2 nA when idle) allows for extended battery life without compromising on performance. The ISL28194 exemplifies a device designed for single-supply operation from 1.8 to 5.5 V, making it suitable for handheld devices powered by two 1.5-V alkaline batteries. In addition, multiple signal chains can feed into a single A/D converter so the overall system-level circuit complexity and parts count can be minimized. 

Because the combustible gas sensors, oxygen sensors, and heat sensors can typically take as much as 10 seconds to settle, the bandwidth of the op amps is less critical, but they need to have a constant bias on the sensors. Also, the outputs from the sensors tend to be very small signals so the op amp must provide peak-to-peak noise flatness and drift characteristics over a large gain step.

Widening range of op-amp alternatives is ready

Already among the most prolifically deployed electronic components in the world, op amps continue to increase in use.  The op-amp deployment curve is exponentially accelerating as more devices incorporate analog sensor functionality, ranging from the examples described in this article to the exploding use of millions of motion, proximity, light and other sensors in industrial and consumer devices. As with any good design practices, the first criteria always must be to achieve the system's operational objectives for accuracy and performance, so low noise, low drift, and precision in high-gain scenarios will always be critical factors for success.Fortunately, system designers are now able to choose from a widening range of precision op-amp alternatives that allow them to effectively meet even the most stringent performance and accuracy requirements while also balancing power usage, size, parts count, and overall cost.