Pushing the limits of precision – understanding the precision challenges of op amps

Summary

During precision measurements, the first challenge for system engineers is how to choose the best performance op amp and other components to install around it. This work is important. In some space-constrained applications, engineers often seek the smallest package, but this small package has certain advantages but does not provide the desired accuracy. This article discusses some of the techniques that IC manufacturers use to overcome precision challenges, and provides readers with a better understanding of the various methods used to achieve optimal performance, even in the smallest package, before and after packaging.

High Accuracy Analog Definition

Engineers define the accuracy of operational amplifiers (op amps) differently, depending on the application. When faced with tens of thousands of amplifiers from more than a dozen manufacturers, engineers are often faced with the challenge of selecting the best performing amplifier. That is, the best price/performance—assuming the system requires other components. For example, vibration analysis applications for oil exploration or seismic research require amplifiers to have very low input offset voltages and very little offset drift over extended periods of use and temperature changes. Only in this way can the impact on the digitized signal be minimized. In other words, low-noise, high-precision op amps do not seriously compromise the performance of high-resolution data converters, providing greater accuracy. Conversely, blood glucose monitors typically have much lower requirements for offset and temperature offset drift.

Pushing the limits of precision – understanding the precision challenges of op amps

picture1 Typical block diagram of a portable vibrometer

Pushing the limits of precision – understanding the precision challenges of op amps

picture2 Typical block diagram of a blood glucose monitor

Most semiconductor companies agree on a defining term for op amp accuracy. Actually, they will group it. In general, op amps are grouped by accuracy if their initial offset voltage is less than 1mV and the unity-gain bandwidth is less than 50 MHz. However, this accuracy is process technology dependent, even within the same Device. Depending on the package, it is not uncommon for two different production lines to produce the same amplifier. This is because smaller packages are more susceptible to the stress of the package molding that squeezes the die.

In the past, bipolar input devices led the way in accuracy. While some argue that these devices are still the best option (and in many ways they are), some recent CMOS and JFET designs have made huge strides. An example of a JFET input amplifier is the OPA140, which has a maximum offset voltage of 120 uV and an offset drift of only 1 uV/°C over the wider industrial temperature range.

High Accuracy Without Chopping

Like system engineers, IC designers use a variety of IC-level techniques to achieve high precision. One way for IC designers to achieve this accuracy is to use chopper stabilization, or auto-zero either individually or in combination. While these techniques are very effective, they have some drawbacks that make the amplifier unsatisfactory in some applications. To address this problem, many manufacturers offer some IC-level trimming methods to obtain lower offset voltages. This approach in turn improves the temperature variation bias drift performance. However, not all trimming methods have this advantage. Some trimming methods may not be suitable for designs for cost-sensitive applications. Typically, once the product has been defined and the target application identified, the method to achieve high accuracy can be selected.

To trim or not to trim

One of the oldest trimming methods is “Zener-zapping”. In the past, many precision amplifiers have used this method. In general, this approach is used for large size processors and is less cost effective for smaller processors such as CMOS. “Zener Removal” is an on-chip processing technique. Although very high accuracy can be achieved, it usually requires a larger die area, which makes it difficult to fit into small packages.

Laser trimming is a common method used in precision devices and has many advantages, such as fewer solder joints for testing and lower cost of trimming connections. This method is widely used in differential and measurement amplifiers to improve resistor matching and provide the necessary common-mode rejection ratio (CMRR). However, this method lacks the ability to trim after assembly.

EEPROM is another on-chip method that we can use, but it is rarely used for stand-alone amplifiers because this method usually requires more pins and shielding.

Due to the ever-increasing need for precision, many manufacturers now offer post-assembly trimming capabilities. This polysilicon fuse blowing technique does not require additional pins or test pads, and offers significant cost savings over package trimming methods. This is a true technological breakthrough, as many CMOS amplifiers can now achieve unprecedented levels of DC precision, namely less than one hundred microvolts of initial offset and less than one microvolt of offset drift. The OPA376, a CMOS input amplifier with a guaranteed offset voltage of 25 µV, can also benefit from these DC parameters. Post-assembly trimming allows IC designers and layout engineers to overcome the mechanical stress that occurs in small packages, resulting in superior precision in miniaturization. In addition to cost savings, CMOS uses this approach to enable the use of lower voltages. Lower voltage power supplies allow users to have longer battery runtimes (a fundamental requirement for portable applications) and help save power on high-density boards, while also providing a simple interface to logic devices and microcontrollers.

surface1 The various trimming methods are outlined and categorized by technology and manufacturer by pre-assembly and post-assembly.

surface1Summary of trimming methods

Choose the precision according to the trimming method

The accuracy of choosing an amplifier based on the trimming method is somewhat misleading. Occasionally, we trim after assembly at a specific trim point. To keep offset and temperature drift to a minimum, the design may require the use of more complex circuitry, which adds a lot of real estate to the chip. Look beyond the first page of the datasheet spec sheet for the actual value of the offset and its common mode change, don’t rely on the trimming algorithm.

Some manufacturers have taken advantage of the success of precision devices to introduce non-trimmed versions that can be used in different applications. This approach benefits both IC manufacturers and customers as costs are easily reduced.

references

· To download the OPA376 product manual, please visit: http://www.ti.com.cn/product/cn/opa376.

· To learn more about TI’s op amps, please visit: http://www.ti.com.cn/lsds/ti_en/analog/amplifiersandlinears/amplifiersandlinears.page.

About the Author

Soufiane Bendaoud is a business development manager for TI’s Precision Analog Products Group. iSoufiane graduated from San Francisco State University in California with a Bachelor of Science degree in Electrical Engineering, and then graduated from the University of San Francisco with an MBA. .

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