Selecting a Precision Op Amp

Tamara Schmitz, Intersil Corporation


For sensor-input processing designs

Click image to enlarge

Figure 1 - ISL28134: 0.1Hz to 10Hz Peak-to-Peak Noise Voltage

Choosing an op amp for high end sensor input projects can be especially challenging if the sensors or their design environments require ultra low-power, low-noise, zero-drift, rail-to-rail input and output, and the repeatability to deliver consistent performance in punishing environments or across many, many readings. 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. System designers can 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. To make these choices, designers need to look at multiple aspects to get the best combination of specs and performance, without going wildly over budget. Chopper-stabilized op amps are one place to start. Also known as Zero Drift Amplifiers, they offer excellent solutions for ultralow offset voltage and zero drift over time and temperature. Chopper op amps achieve maximum DC precision using a continuously running, on-chip calibration scheme. Here are some examples of 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 custom' designs require stringent performance from the op amps to extract very small signals riding on relatively large inputs. To successfully amplify these small signals, the op amp must have ultralow input offset voltage and minimal offset temperature drift, with wide gain bandwidth and rail-to-rail input/output swing. It is also critical for the op amp to offer very stable ultralow 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 shown in Figure 1, the ISL28134 op amp delivers excellent noise voltage (nV) 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 minimizing noise gain error. For portable weigh scale applications where low-power is also an important consideration, designers may want to consider the ISL28133, which combines ultralow micropower (25μA max) and low voltage offset (6μV max) characteristics with a chopper-stabilized 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 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 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, these chopper-stabilized CMOS devices provide excellent low drift characteristics over both temperature extremes and extended time periods. 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 Devices in Rough Environments Another application example combines various different sensor inputs one device and to show how op amp circuitry can 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. As illustrated by the block diagram in Figure 2, using multiple instances of an ultralow power op amp such as the ISL28194 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 (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 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, as with the previous examples, the outputs from the sensors tend to be very small signals so the op amp must provide excellent rail-to-rail noise flatness and drift characteristics over a large gain step peak-to-peak noise flatness and drift characteristics over a large gain step. The use 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.