Voltage references are critical components that ICs use to benchmark voltage against. They allow for the reconciliation of analog and digital signals, and contribute to the ability for electronics to interact with the “real” world. In this article, learn about the different types of voltage references and their specifications.
Key Reference Specifications
When designing a circuit with a voltage reference, there are some key specifications to consider:
Initial Accuracy
Most references have an output voltage variance that is measured at 25°C. If this is a constant for a given unit, it can then be easily calibrated.
Temperature Drift
An important measurement to calculate voltage reference performance is temperature drift, or how the output voltage changes with temperature variations. Due to imperfections in circuit elements, the curve of this temperature drift can be nonlinear. Therefore, temperature drift estimations are usually presented in a “box method”, which calculates max/min voltages expected in a system without consideration for temperature fluctuations.
Long Term Stability
This is a measure of the tendency of a reference voltage to change over time, independent of other variables. There can be many reasons for changes in long term stability, including mechanical stress, changes in circuit element composition, and temperature degradation.
Thermal Hysteresis
This is a measurement of voltage fluctuation as a result of changing die stress due to thermal cycling. It’s important to know your reference’s nominal voltage output over subsequent temperature cycles, as this will allow for accurate output error calculation.
Types of Voltage Reference
There are two main types of voltage references - shunt and series. Shunt references are of a 2-terminal type, designed to work over a specified range of currents. Series references are three (or more) terminal devices that are designed to work in circuits with large changes in supply voltage or load current.
Reference Circuit Design
There are many ways to design a voltage reference IC. Each has specific advantages and disadvantages.
Zener-Based References
The buried Zener type reference is a relatively simple design. A Zener (or avalanche) diode has a predictable reverse voltage that is fairly constant over temperature and very constant over time. These diodes are often very low noise and very stable over time if held within a small temperature range, making them useful in applications where changes in the reference voltage must be as small as possible.
Bandgap References
Bandgap references can produce a wide variety of output voltages with little supply headroom—often less than 100mV. They can be designed to provide very precise initial output voltages with low temperature drift, eliminating the need for time-consuming in-application calibration.
Fractional Bandgap References
Fractional bandgap references are references based on the temperature characteristics of bipolar transistors, but with output voltages that may be as low as a few millivolts. They are useful for very low voltage circuits, especially in comparator applications where the threshold must be less than a conventional bandgap voltage (approximately 1.2V).
Choosing a Reference
Consider the following when choosing a reference for your next design:
● Is the supply voltage very high? Choose a shunt.
● Does the supply voltage or load current vary widely? Choose a series.
● Require high power efficiency? Choose a series.
● Figure your real-world temperature range.
● Be realistic about the required accuracy.
● What is the real supply range? What is the maximum expected supply voltage? Will there be fault conditions such as battery load dump or hot-swap inductive supply spikes that the reference IC must withstand?
● How much power can the reference consume?
● How much load current? Will the load draw substantial current or produce current that the reference must sink?
● How much room do you have?