New op amps address new—and old—design challenges

Article By : Bill Schweber

The huge number of available op amps can seem overwhelming, but there’s a reason for them and why vendors keep introducing new ones.

The operational amplifier (op amp) is the fundamental building block of all types of analog circuits. There are literally thousands of different ones available from dozens of vendors, ranging from broad-line suppliers with a deep and wide catalog of devices to sources offering amps focused on a tight niche such as low-distortion audio devices, low-noise RF devices, and ultra-stable instrumentation units.

What distinguishes op amps from “higher-level” analog components is that they are single-function ICs with an internal task represented by a simple symbol (Figure 1). Unlike analog components embedded with higher levels of integration, a basic op amp IC has just that singular block but can be configured to provide or build up many other functions.

Figure 1 Don’t be misled by the simple schematic representation of the op amp, as it can be configured to implement so many vital analog functions due to surrounding arrangements with passive components. Source: The University of Liverpool, UK

Selecting the right op amp

Given the countless op amps already available, it would be somewhat logical to suppose that there really is no need for yet more of them. If fact, selecting the right op amp for a non-mundane application can be a challenge. Some engineers start with a preferred vendor and see if there is anything listed that does the job; if so, there may be no need to look further.

Others like to use an op amp which they have used previously, if it’s at least good enough. That simplifies product test and evaluation and reduces component risk and surprises. It’s not usual to see op amps introduced 10, 20, or more years ago still being made and listed as active parts suitable even for new designs. Not only does this make things easier for the design engineer, it benefits the vendor as well, who has a product with a mature manufacturing and test process. Thus, yields are high, costs are low, and deliveries are better; what’s not to like from this win-win scenario?

However, the availability of so many op amps epitomizes the core of the design engineer’s ongoing challenge. How to balance the project’s objectives and priorities? For example, you can get an op amp that excels in one or two specifications, which may be super-critical to the applications such as low bias current or thermal drift, along with good but not great specification for its other parameters. Or you can get an op amp which is pretty good in all of its specifications, but isn’t standing out in any of them.

Which one is better for the specific application depends on the project priorities, the relative weighting among them, and how badly you need or want those few truly superior specifications. Can you accept somewhat higher drift performance in order to get lower offset? The engineering dilemma is this: “how much is that better performance worth to you, and what and how much would you give up to get it?” Of course, there is no right answer; it’s determined by both a quantitative spreadsheet and qualitative experience. That’s why op amps routinely come with datasheets running 20 or 30 pages, laden with charts, tables, graphs, and more to fully present the subtleties of their capabilities.

Design case studies

A disparate pair of recently introduced op amps illustrates pushing the performance envelope along different axes. The TSV772 from STMicroelectronics is a tiny dual op amp—2.0 mm × 2.0 mm in DFN8 package—which combines high accuracy and low power consumption with a maximum input-offset voltage of 200 µV at 25°C. That low input-offset ensures accurate handling of low-amplitude signals and can eliminate the need for precision external resistors and thus avoid the need to trim or calibrate circuits in production. It’s well suited to function as a transimpedance amplifier for the low output current of photodiodes in smoke detectors and medical sensors.

Its specifications also make it a good fit for low-side load-current sensing, which is usually easier to implement than to complement high-side sensing; although in many cases, the more difficult high-side sensing must be used. In low-side sensing, sense resistor Rshunt is placed between the load and the circuit ground and the resultant voltage drop is amplified using the op amp (Figure 2).

Figure 2 Among its many functions, TSV772 is well-suited for use as low-side current-sense resistor amplifier. Source: STMicroelectronics

In the usual arrangement with resistor-pair values chosen equal:

Rf2 = Rf1 = Rf and Rg2 = Rg1 = Rg

The fairly complicated algebraic equation relating sensed current to output voltage is simplified as follows:

Vout = [Rshunt × I × (Rf/Rg)] – [Vio × (1 + Rf/Rg] + [Rf × Iio]

Where Vio and Iio are the input offset voltage and input offset current, respectively, and which clearly shows the importance of low offset values in this application. For a given current value and desired accuracy, the shunt resistor can also be chosen to have a lower value, resulting in lower power dissipation and lower voltage drop in the ground path.

Targeting a very different application, the BUF802 buffer amplifier from Texas Instruments features large-signal bandwidth of up to 3.1 GHz at 1 Vpp input (and as high as 2 GHz at 2 VPP) with fast slew rate (7000 V/µsec) and fast settling time (0.7 nsec to 1%), as shown in Figure 3. Input voltage noise is just 2.3 nV/√Hz, while input impedance—another critical parameter in these applications—is 50 GΩ in parallel with 2.4 pF.

Figure 3 The BUF802 buffer amplifier doesn’t do signal processing, but nevertheless performs a necessary interface role between a sensitive signal source and its load in the analog signal chain. Source: Texas Instruments

Operating from a ±4.5 V to ±6.5 V supply, the JFET-input BUF802 can easily drive the ubiquitous 50-Ω load. It also offers user-adjustable quiescent current for power/performance trade-off and an integrated input/output clamp with fast overdrive recovery, another concern in these applications.

Buffers play an important but perhaps underappreciated role in the signal chain. Their primary function is mostly to isolate—not galvanically—a sensitive input signal from the vagaries of the next stage. That succeeding stage load may have a reactive impedance or a varying one, which would be challenging for the source to handle. What the buffer does is not much but it’s still important to system performance.

Are there a lot of op amps out there? Absolutely. Do we need all of them? Probably not. Are there others we’d like to see? Absolutely.

What op amp parameter would you like to see improved (besides price)? What’s the trade-off you’d be willing to accept?

This article was originally published on Planet Analog.

Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical website manager for multiple EE Times sites and as both Executive Editor and Analog Editor at EDN. At Analog Devices, he was in marketing communications; as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these. Prior to the marcom role at Analog, Bill was Associate Editor of its respected technical journal, and also worked in its product marketing and applications engineering groups. Before those roles, he was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls. He has a BSEE from Columbia University and an MSEE from the University of Massachusetts, is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. He has also planned, written, and presented online courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.


Leave a comment