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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?

Many embedded systems call for a stable, low-noise DC reference. This reference can be a stable current or voltage source. These references are used in many ways. Some typical uses are listed as follows:

???Provide a stable excitation voltage or current to a sensor or array of sensors
???A reference level for an analogue to digital, (A/D) converter
???A reference level for a digital to analogue, (D/A) converter
???Provide a stable reference for a timing circuit

In this article, I will go over the basic DC voltage source or a voltage reference. Also, some discrete designs will be reviewed. A very brief compendium of some of the off-the-shelf IC's will be discussed.

Desirable characteristics
In general, a voltage reference must provide a stable low noise DC level to some part of a system. In some cases, a high accuracy is desired. However, a high accuracy usually entails some increase in costs. These costs can be manifested as an increase in unit costs, due to a more expensive part, or an increase in direct labour during manufacturing, due to manual or automated trimming during final assembly. Some of the desired characteristics of a voltage reference are as follows:

???High precision
???Low temperature coefficient
???Good power supply rejection
???Very good load regulation
???Low power consumption
???Reasonable cost

What not to do
Given the desired characteristics of a voltage reference, we can see that not achieving or straying from these characteristics may lead to undesirable properties. I have seen a lot of 'strange', or actually, I shall say, 'lazy' designs when it comes to the application of circuitry for a voltage reference. Figure 1 shows some of the 'simple' not-to-be-recommended designs for a voltage reference. In short, I call the examples shown in this figure the voltage reference "Hall of Shame."

Figure 1: Voltage Reference "Hall of Shame".

The Zener diode
Review / Overview: One of the oldest and well known methods to create a stable voltage reference is to reverse bias a Zener diode. Going back to EE101, I show the IV curve for a typical Zener diode in figure 2.

Figure 2: IV characteristic of a Zener Diode.

One of the reasons why I show this well-known curve is to point out that the Zener voltage is really dependent on the current flowing through the reverse biased junction. In the classical case, as we were taught in university, we modelled the behaviour as a constant Zener breakdown voltage for all currents flowing through the junction. This is simply not true. There is a small, yet significant slope in the Zener voltage vs. current. This slope is modelled as a reverse Zener impedance. This impedance is usually in the 30-100 ohm range.

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