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Voltage reference fundamentals

Posted: 06 Feb 2014 ?? ?Print Version ?Bookmark and Share

Keywords:embedded systems? DC? voltage reference? Zener diode? temperature coefficients?

We have seen, in the previous section, that we can get a pretty decent voltage reference using a biased Zener diode circuit for a relatively reasonable cost. However, in this modern age of single supply, low voltage and low power operation, the traditional shunt-biased Zener diode circuit may not be appropriate. In essence, there are two restrictions to using a Zener that may prove untenable in modern circuit designs. These restrictions are as follows:

1. We need at least 10 mA of current to properly bias the Zener diode. Even this small current may be too large for some low power applications.

2. We have found the optimum Zener breakdown voltage is nominally 5.1 to 5.6V. This voltage is too high in the era of 1.8V or 3.3V system operation.

To solve this problem, the silicon IC manufacturers introduced the band-gap reference many years ago. One of the most notable of these devices is the National Semiconductor LM336 device. This device operates in very nearly the same mode as a Zener diode. However, the reverse impedance of these devices is in the tenths of an ohm or less. Plus, these devices have the advantage of operating at very low power levels. These devices have a shunt voltage that can be 'configured' by the manufacturer to many voltage less than 3.3V.

Figure 6 shows a circuit realisation of the LM336-2.5 shunt regulated reference. The advent of new, lower power precision OP-AMPS has made using the newer shunt regulated references very attractive.

Series voltage references
We can save considerable printed circuit board real estate by using a series voltage reference. The series voltage reference is wired up in the same way as a 3-terminal regulator. Many of these references are very accurate, precise, show good PSRR and reasonable in cost.

Table 1 shows a representative sample of the costs for a 2.5V series voltage references by different manufacturers. A quick look at this table shows that this option will cost you about $1.50 and change.

Table 1: A small sample of various 2.5v series voltage references.

Table 2: A summary of costs and comments for the various reference voltage generation methods.

Manufacturers
In general, there are many effective ways to create a low-cost reference voltage with a reasonable level of performance. We have shown that even adding a simple resistor bias to a Zener diode can increase the PSRR by at least 20dB.

However, there are some problems with the biased Zener diode approach given the modern age of low-power, low-voltage operations. The most effective way to solve this problem is to go to any one of the linear manufacturers to purchase a dedicated voltage reference. Table 2 shows a summary of costs and comments for the various reference voltage generation methods.

About the author
Ken Wada is president and owner of Aurium Technologies, an independent product design and consulting firm in California's Silicon Valley. Ken has over 25 years of experience architecting and designing high-tech products and systems, including the FASTRAK vehicle-sensing system for toll roads and bridges. His expertise includes industrial automation, biotechnology, and high-speed optical networks. Ken holds four patents.

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