One of the most common questions asked by customers is, “Which op amp should I use with a given data converter?” The answer is enough to make a marketing engineer cringe, because it depends on the answers to many questions—way too many to be in the customer's comfort zone. Yet, the answers to these questions are not only necessary for the customer's design to be a success, they are questions that a customer must eventually answer.

It would be handy to have a table with A/D converters on one side and recommended op amps on the other. But this table will never exist—too many variables in system design affect the choice of op amp. The questions in this chapter help organize the designer's thoughts into the correct direction and define the issues. These questions are the ones that a designer should be prepared to answer, before committing to a design.

The list of questions may look daunting at the beginning, but they are divided into sections. This breaks up the system into component parts that surround the op amp and define exactly what that component needs to do. The completion of each section is a piece of the puzzle, and by the end of the process, the designer should have weeded out op amps that are unsuitable for the job.

Once the op amp has been selected, the job is not done. Some architectural considerations for the signal chain and trade-offs are associated with each one. A few of the most useful options are explored.

The overall characteristics of the system oftentimes yield valuable information. A clear understanding of the product and its function is imperative to design success:

Power supply rails can quickly rule out amplifier solutions (Figure 15.1). This is similar to clothing shopping—the style may be desirable, but if the size doesn't fit, the style is useless. So a wise shopper finds the options in the size first, before becoming attached to a style. Similarly, an op amp with fantastic specifications at ±15 V may not operate at all from a +3.3 V power supply. Power supply information is collected first, because it simply and unequivocally narrows the choices:

Understanding the input source (Figure 15.2) is key to properly designing the interface circuitry between the source and the analog to digital converter:

Now that the power supply and input signal have been defined, it is time to focus on the device that the op amp will drive, the analog to digital converter (Figure 15.3):

Although this chapter is intended to aid in the selection of the correct operational amplifier (Figure 15.4), it is possible that the nature of the system already defines some op amp characteristics:

The job is still not done. A decision must be made at this point: How will the signal chain (and interface) be implemented. The guidelines in this chapter are not meant to be device specific for every data converter or op amp, rather they are general cases that can be used as a basis for design. Let's get started.

In some cases, such as IF baseband, the best interface may well be no op amp at all but a simple transformer over a narrow frequency band, which has been employed successfully in many designs. As the title of this publication is Op Amps for Everyone, purely transformer interfaces are not explored at length, however a transformer may be employed in combination with op amps when gain is required.

Besides the transformer, the other decisions are

Some of the decisions may be premade, such as the selection of data converter or the power supplies (split versus single supply).

Figure 15.5 shows one of the simplest interface circuits. The CM pin of the data converter is used to set the common mode input of the fully differential op amp. The data converter compensation inputs are shown (R₅/C₁ and R₆/C₂). An input termination resistor, usually 50 Ω, is shown but may or may not be utilized. Gain of the op amp interface is set by R₁ through R₄, the common mode input range of the data converter includes ground, and the frequency response extends from DC to the bandwidth limit of the op amp circuit.

While this circuit is very nice, it is also not very typical of real world applications. To begin with, the op amp interface operates from split supplies, which may not be available in the system. Also, most input signals are referenced to ground, which presents its own set of challenges.

Figure 15.6 shows a more typical interface circuit. The input signal is referenced to ground, while the common mode operating point of the op amp interface is set by the data converter. The op amp interface can be run off of a single supply.

This configuration is covered in more detail in a later chapter, one devoted to application errors. Suffice it to say, a circuit such as this must be applied carefully.

There are several keys to the successful application of this circuit:

It is not absolutely necessary to utilize a fully differential op amp to drive a differential data converter. Figure 15.7 shows the preferred method to drive a fully differential op amp using single ended op amps. The input signal is referenced to ground, and decoupling from the op amp interface is accomplished by using a transformer. Each phase of the output signal is handled in a conventional inverting op amp stage, and the common mode point for both amplifiers is set by the data converter. Remember that, if termination is employed, it must be reflected through the transformer. For a 1:1 transformer, R_T1 equals R_T2.

Figure 15.8 shows the preferred way to convert single ended signals to differential without a transformer. While this circuit looks a bit unusual, the strategy is to equalize the delay for IN+ and IN– by forcing each phase of the signal to go through both op amps before being applied to the inputs. This may not be intuitive at first glance! Each amplifier, though, is in the feedback loop for the other. Think of this as an inverting op amp circuit—gain is adjusted by changing R₁ (corresponding to R_G), R₂ through R₆ are equal values (corresponding to R_F). If DC gains are taken into account, this circuit also works for DC coupled applications.