System components

The term “system components” covers a broad universe of semiconductor devices, ranging from the simply functional to the extremely complex. Given the variety in this class of components, there is one common denominator in that all of them provide solutions to specific problems. In this chapter, we will provide examples of three categories of components: integrated solutions, line drivers and basic functions.

In previous chapters, you learned how to use the device data sheet to evaluate performance and use conditions, so we will not provide a detailed technical discussion here.

Integrated solutions

Figure 1's depiction of a general system block diagram could probably represent your senior project as well. Previous chapters have described how to evaluate devices that make up such a general system: op amps for the creation of filters, control systems or input and output signal-conditioning systems, microcontrollers or DSPs that process data captured by the system, power-management solutions, wireless solutions, and interface options.

Figure 1

By necessity, most of the information in this book is based on basic circuit elements – an op amp, a low-dropout (LDO) regulator or a specific microcontroller – that represent the system building block under discussion. But what about real-world applications, which hopefully your senior projects are tackling? Since the invention of the integrated circuit by Jack Kilby in 1958, the semiconductor industry has continually integrated more and more into its products. From Bell Lab’s single transistor to Kilby’s integrated circuit, from Texas Instruments' single-chip DSP to today’s embedded system engines (powering smartphones, automobiles, washing machines and practically everything else), the inexorable technological march to more complex integrated solutions continues.

You might be asking yourself, “Why shouldn’t my senior project benefit from more complex solutions?” The question can also be reworded as, “What if I took advantage of application-specific solutions used by industry design teams?” The answer to both questions is that it depends. It depends on the application you are addressing and the maturity of the solution you are looking for. If a device you are interested in provides available samples on its www.ti.com page, the technology is mature enough for your senior project.

It also depends on your team’s ability to understand and manage the functionality so that it can be used properly in your system. You should also consider the trade-offs in time and effort to use an integrated solution. A simple example would be in power management. The "Power" chapter in this book introduces the various components for power management separately, yet the application of these elements to a real-world application can be complex.

Consider the challenge of creating a lithium-ion battery charger for your project. The physics of the Li-ion battery charging under load are complex and require a detailed solution. While the creation of such a charger would have once been a suitable senior project, the bq24040 is a single-input, single-cell Li-ion battery charger that provides the solution in a single device. The bq24040 charges the battery while it is powering a system load. The battery is charged in three phases: conditioning, constant current and constant voltage. Clearly, this is a sophisticated solution, which when used in your project frees team resources to address other basic elements.

If you decide to choose the bq24040, you must make sure that the basic parametric and functional attributes of the device meet your system's needs. As described in previous chapters, a close inspection of the data sheet is a requirement for successful implementation. If you find yourself with a challenge you cannot address, then take advantage of the application notes associated with the device, or use the E2E Community, http://e2e.ti.com.

To explore the use of system components further, let's consider a more complex device. The ADS1294 family incorporates all of the features commonly required in medical electrocardiogram (ECG) and electroencephalogram (EEG) applications. The ADS1294 family of analog front-ends (AFEs) provides multichannel, simultaneous sampling; 24-bit, delta-sigma (ΔΣ) analog-to-digital converters (ADCs) with built-in programmable gain amplifiers (PGAs); internal reference; and an onboard oscillator. Figure 2 is a block diagram of this device.

Figure 2

Reviewing the data sheet for this family of devices shows a complex solution to a sophisticated application. With devices as complex as the ADS1298, an in-depth exploration of the data sheet exceeds the scope of this chapter. However, the concepts previously described are applicable to the specific functions on the device.

If you decide to use an application such as the ADS1298, you will be designing the rest of your system to match the input and output characteristics – functional and parametric – of the ADS1298. When using a device as complex as the ADS1294, you must pay special attention to the timing relationships of the various signals; failure to do so will result in a bad outcome. Again, if you find yourself with a challenge you cannot address, take advantage of the application notes associated with the device or use the E2E Community, http://e2e.ti.com.

As the bq24040 and ADS1298 devices illustrate, integrated system components incorporate a plethora of functional solutions. These solutions cover all aspects of devices that have been discussed in previous chapters of this book, such as processors developed to support applications like motor control, battery-management systems with embedded controllers, touch-screen controllers, and many more.

Line drivers

At some point, you will be faced with a project that requires transmitting data across a several-inch-long backplane on a printed circuit board, or perhaps uses a cable that's several feet long. Electrically, you should think of the backplane or cable as transmission lines. Pay careful attention, because lack of signal integrity can result due to mismatched impedances.

In the bus interface environment, signal integrity is simply maintaining the characteristics of the input signal at the receiving end of the bus, where the bus is represented as a distributed RLC network. The impedance of the line driver, the bus network and the load on the bus can interact with each other to create signal distortions caused by energy reflections. The mathematics of these interactions are complex but can be simplified by modeling the system, as shown in Figure 3, and its associated reflection coefficient, shown in Equations 1(a) and 1(b).

Figure 3

R(Source) = Z(S) - Z(t) Z(S) + Z(t) R(Source) = Z(S) - Z(t) Z(S) + Z(t) (1a)

R(Load) = Z(L) - Z(t) Z(L) + Z(t) R(Load) = Z(L) - Z(t) Z(L) + Z(t) (1b)

Where:

R(Source) = the reflection coefficient at the source

R(Load) = the reflection coefficient at the load

Z(S) = impedance of the source

Z(L)= impedance of the load

Z(t) = impedance of the transmission line

Inspection of the reflection coefficients shows that if the impedance of the source driving the transmission line is matched with the impedance of the transmission line itself, there is no reflection. The same holds true for the load. Similarly, if the impedances are not matched, you can expect reflections to occur, resulting in distortions of the signal being propagated on the transmission line.

Signal characteristics such as the rise and fall time (slew rate) of the signal can also impact the integrity of the signal being transmitted. Slew-rate limitation also limits the data transmission rate of a given bus configuration. Therefore, bus interface products are designed to optimize not only bus impedance matching but signal slew rate as well.

In practice, perfect matching is difficult to achieve because of the trade-offs between drive currents, output voltages and frequencies. You can optimize the performance of the bus and minimize the impact of these distortions by carefully selecting the line drivers or bus interface devices. For buses that follow industry standards such as LVDS, PECL and many others, devices exist that provide a robust switching solution. The "SLL Advanced Bus Interface Logic Products Selection Guide and Reference," http://www.ti.com/lit/sg/scyt126/scyt126.pdf, provides an in-depth guide to the various options available.

Finally, when dealing with buses and signal-integrity challenges, one of the best things that you can do is use an oscilloscope to observe the impact of the various options available on the actual signal. Experiment with different drivers and termination schemes to determine the best solution for your project.

Basic functions

Basic functions incorporate logic gates, multiplexers, analog switches and many other relatively simple functions. This category of devices is discussed separately from integrated solutions, primarily because basic functions tend to be relatively small circuits that perform a simple action.

Examples of these types of devices include simple logic gates (NAND, AND, OR), stand-alone registers and flip-flops, and clock drivers. Use keywords that describe the function you need in the www.ti.com search engine to view the available options.

As with all products, it's imperative that you read the data sheet to confirm that the recommended operating conditions meet your application. First, look at the supply voltage ratings and input voltage levels to ensure that the device will function in your system. Then review the output current properties to confirm that the device can drive the load you need to drive. For storage elements such as flip-flops and register files, frequency characteristics are also key design criteria.

Conclusion

System components encompass a large variety of products. We recommend that you explore the options available to you at www.ti.com. Indeed, one of the pleasures of the engineering design process is sorting out the available solutions to meet your project’s needs.

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Last edited by Ed Woodward on Jan 11, 2013 8:07 am -0600.