1.7 System components  (Page 2/3)

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.

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).