What is a Voltage Reference?
A semiconductor voltage reference is a circuit that is designed to provide a constant voltage with certain accuracy regardless of variations on parameters such as power supply, temperature, load current, or different external conditions. Voltage references are considered an essential building block in SoCs because they provide the reference currents and voltages for all the system blocks. Because voltage references are intended to produce an accurate, low-noise, and stable voltage, there are several specifications that provide us with an indication of how the reference voltage should react in regards to certain conditions. The following specifications should be considered when selecting a voltage reference.
7 Important Voltage Reference Specifications
- Initial Accuracy
The initial accuracy of a voltage reference represents the initial output reference voltage error and is an indication of how close the output voltage is to the nominal voltage at a given temperature (usually room temperature) within a defined load current and input voltage. The initial accuracy is a cornerstone for other specifications, as many specifications may require a maintained accuracy in order to measure other characteristics of the voltage reference.
- Temperature Coefficient (TC)
The temperature coefficient is the variation in the output voltage reference with respect to changes in the temperature due to imperfections in the circuit elements which cause nonlinearities, and it is expressed in parts-per-million per degree Celsius (ppm/°C). This output voltage reference deviation can be positive or negative depending on how that output voltage reference is trimmed during initial calibration. Curve compensation techniques with special circuitry are used to improve the TC of the devices.
- Supply Current
The supply current, also specified as “quiescent current”, is the required current for correct operation of the voltage reference. Voltage reference designs should maintain a low supply current to avoid the risk of degradation in the temperature coefficient due to self-heating of the devices.
- Long-Term Stability
Long-term stability is measured by the change in the reference voltage under normal conditions over a period of time, specified as ppm per 1000 hours. This is an important specification if the reference voltage must continue operating accurately for an extended period of time for a certain application. The drifts in the reference voltage are due to changes in the electrical characteristics of the circuit elements and devices, which is often referred to as “aging.” The effect of aging is more significant at higher temperatures.
The reference noise is a random signal generated by the circuit’s active and passive devices which affect its accuracy. This noise is an issue because it can cause dynamic errors that degrade the signal to noise ratio.
- Power Supply Rejection Ratio
The power supply rejection ratio (PSRR) is quantification of the ability of the voltage reference to reject the noise and uphold its output voltage at its fixed value when the input supply voltage varies. It is calculated as the ratio of the change in the power supply voltage with respect to the change in the output voltage.
- Supply Range
Supply range specifies the range of the input supply voltage needed to maintain proper operation of the devices. The maximum supply voltage puts limitations on using low voltage devices as they may be damaged due to overstress when the maximum supply voltage exceeds the absolute maximum rating of the devices.
Voltage references are used in many applications and provide different features that make them indispensable in most power management systems. The main challenge in voltage references for designers is to achieve an accurate, low noise and stable voltage reference with low current consumption and small area. Different applications require different specifications, each of which requires the use of a specific voltage reference topology. By analyzing the key voltage reference specifications, designers should be able to determine what parameters are important for a certain application.
Vidatronic offers a portfolio of voltage references in a variety of process nodes.
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