Section 7-1: Passive Components

This chapter, one of the longer of those within the book, deals with topics just as important as all of those basic circuits immediately surrounding the op amp, discussed earlier. The chapter deals with various and sundry circuit/system issues which fall under the guise of system hardware and housekeeping techniques. In this context, the hardware and housekeeping may be all those support items surrounding an op amp, excluding the op amp itself. This includes issues of passive components, printed circuit design, power supply systems, protection of op amp devices against overvoltage and thermal effects, EMI/RFI issues, and finally, simulation, breadboarding and prototyping. Some of these topics aren't directly involved in the actual signal path of a design, but they are every bit as important as choosing the correct device and surrounding circuit values. Introduction When designing with op amps and other precision analog devices, it is critical that users avoid the pitfall of poor passive component choice. In fact, the wrong passive component can derail even the best op amp or data converter application. This section includes discussion of some basic traps of choosing passive components for op amp applications. So, you've spent good money for a precision op amp or data converter, only to find that, when plugged into your board, the device doesn't meet spec. Perhaps the circuit suffers from drift, poor frequency response, and oscillations— or simply doesn't achieve expected accuracy. Well, before you blame the device, you should closely examine your passive components— including capacitors, resistors, potentiometers, and yes, even the printed circuit boards. In these areas, subtle effects of tolerance, temperature, parasitics, aging, and user assembly procedures can unwittingly sink your circuit. And all too often these effects go unspecified (or underspecified) by passive component manufacturers. In general, if you use data converters having 12 bits or more of resolution, or op amps that cost more than a few dollars, pay very close attention to passive components. Consider the case of a 12-bit DAC, where ½ LSB corresponds to 0.012% of full scale, or only 122 ppm. A host of passive component phenomena can accumulate errors far exceeding this! But, buying the most expensive passive components won't necessarily solve your problems either. Often, a correct 25-cent capacitor yields a better-performing, more cost-effective design than a premium-grade part. With a few basics, understanding and analyzing passive components may prove rewarding, albeit not easy. Capacitors Most designers …