Voltage references 101
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:
- Provide a stable excitation voltage or current to a sensor or array of sensors
- A reference level for an analog to digital, (A/D) converter
- A reference level for a digital to analog, (D/A) converter
- 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.
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:
- 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."
Click on image to enlarge.