Voltage references 101

January 23, 2014

Ken.Wada-January 23, 2014

Many embedded systems require 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:

  1. Provide a stable excitation voltage or current to a sensor or array of sensors
  2. A reference level for an analog to digital, (A/D) converter
  3. A reference level for a digital to analog, (D/A) converter
  4. Provide a stable reference for a timing circuit

In this Roundtable, 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 labor during manufacturing, due to manual or automated trimming during final assembly. Some of the desired characteristics of a voltage reference are as follows:

  1. High precision
  2. Low temperature coefficient
  3. Good power supply rejection
  4. Very good load regulation
  5. Low power consumption
  6. 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."

Click on image to enlarge.

Figure 1 Voltage Reference "Hall of Shame"

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