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 also balancing cost considerations. In particular, chopper-stabilised 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 & 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 10-20 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 0.1Hz to 10Hz.
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 Figure 1, a good example is the chopper-stabilised zero-drift ISL28134 op amp delivers excellent noise voltage across the range from 10Hz down to 0.1Hz, 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 minimising noise gain error.
Figure 1: ISL28134 0.1Hz to 10Hz Peak-to-Peak Noise Voltage
For portable weigh scale applications where low-power is also an important consideration, designers may want to consider the ISL28133, which combines ultra-low micropower (25µA max) and low voltage offset (6µV max) characteristics with a chopper-stabilised design that delivers flat noise band to DC and near-zero drift. For other strain gage applications that need to use higher reference voltages, such as 10V instead of 5V, designers should also consider the ISL28217 or ISL28227.
Current Sensing & 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 minimise 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 of maintenance requirements.
Many shunt based current sensing applications are built using op amps such as the ISL28133 or ISL28233, which 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 Figure 2, these chopper-stabilised CMOS devices provide excellent low drift characteristics over both temperature extremes and extended time periods.
Figure 2: –Minimising Vos Drift over Temperature and Time, the ISL28133 is a single chopper-stabilised op amp and the ISL28233 is a dual version 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-stabilised 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 minimise size while maximising capabilities. Such a device might combine a combustible gas sensor, oxygen sensor and catalytic heat band sensor.
As illustrated by the block diagram in Figure 3, using multiple instances of an ultra-low power op amp such as the ISL28194 provides advantages for multi-sensor signal chains within a small handheld device.
Figure 3: Multi-sensor handheld toxic environment safety monitor
Because these safety devices typically need to operate in an “always-on” mode, the ISL28194 ultra-low micro-power profile (450nA max and 2nA when idle) allows for extended battery life without compromising on performance. The ISL28194 is designed for single-supply operation from 1.8V to 5.5V, making it suitable for handheld devices powered by two 1.5V alkaline batteries. In addition, because the multiple ISL28194 signal chains can feed into a single ADC (ISL26132), the overall system-level circuit complexity and parts count can be minimised.
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, as with the previous examples, 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.
Alternatives Are Ready
Already among the most prolifically deployed electronic components in the world, the usage of op amps continues to increase. 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.
Tamara Schmitz is a Senior Principal Applications Engineer and Global Technical Training Coordinator at Intersil Corporation, where she has been employed since mid 2007. Tamara holds a BSEE and MSEE in electrical engineering and Ph.D. in RF CMOS Circuit Design from Stanford University. From August 1997 until August 2002 she was a lecturer in electrical engineering at Stanford; from August 2002 until August 2007, she served as assistant professor of electrical engineering at San Jose State University.