TABLE OF CONTENTS As we saw in Chapter 14, the buried-zener reference is one of the most mature voltage reference topologies, and it was designed to meet an extremely high level of precision. Only in recent years have any of the other topologies come close to matching some of its outstanding characteristics. The buried-zener product was expensive to build, characterize, test, and burn in (many products received many hours of burn-in at 150 C as part of the preconditioning process), and so it resulted in being an expensive product. The earliest products used an on-board substrate heater, which required a significant amount of power to operate. Subsequent designs did not include a heater, and instead used various compensation techniques and laser trimming to attain similar (but slightly lower) specifications. Although many devices provided 6.9-volt and 10-volt outputs, newer devices brought the reference voltage down to the more popular 5 volts. Over time various products became available from National Semiconductor (which first introduced the topology), as well as Analog Devices, Burr-Brown, Linear Technology, Maxim, Precision Monolithics, and others. At its peak came various expanded buried-zener products in larger DIL packages (rather than the more usual eight-pin TO-99 metal can), which included additional features such as laser-trimmed resistors, precision op amp/buffers, Kelvin sensing, noise filtering, output voltage adjustment, and so on. Both Analog Devices and Maxim introduced such products at the time. (Maxim's MAX671MDD was a +10-volt precision buried-zener reference with 1 mV initial accuracy, a 1 ppm/ C tempco, and a 12
