Choosing a voltage reference

Technology News | February 16, 2014

By eeNews Europe

In analog and mixed-signal circuits, the voltage reference acts as the standard against which other signals are measured. Inaccuracy and variation in the voltage reference directly impact the accuracy of the whole system. Let’s look at how accuracy specifications and other criteria apply when selecting a voltage reference.

Initial Accuracy indicates the variance of output voltage as measured at a given temperature, usually 25°C. While the initial output voltage may vary from unit to unit, if it is constant for a given unit, then it can be easily calibrated.

Temperature Drift is probably the most widely used spec to evaluate voltage reference performance, as it shows the change in output voltage over temperature. Temperature drift is caused by imperfections and nonlinearities in the circuit elements. For many parts, the temperature drift specified in ppm/°C, is the dominant error source. For parts with consistent drift, some calibration is possible.

A common misconception regarding temperature drift is that it is linear. But one should not assume that a reference will drift a lesser amount over a smaller temperature range. Temperature coefficient (TC) is generally specified with a “box method” to convey the likely error over the entire operating temperature range. It is calculated by dividing the min-max voltage difference over the entire temperature range, divided by the total temperature range (Figure 1).

Figure 1. Voltage Reference Temperature Characteristics

These min and max voltage values may not occur at the temperature extremes, leading to regions where the TC is much larger than the average calculated for the entire specified temperature range. This is especially true for the most carefully tuned references, often identified by their very low temperature drift, where the linear drift components have been compensated leaving a residual nonlinear TC.

The best use of the temperature drift specification is to calculate maximum total error over the specified temperature range. It is generally inadvisable to calculate errors over unspecified temperature ranges unless the temperature drift characteristics are well understood.

Long Term Stability is a measure of the tendency of a reference voltage to change over time, independent of other variables. Initial shifts are largely caused by changes in mechanical stress, from the difference in expansion rates of the lead frame, die and mold compound. This stress effect tends to have a large initial shift that reduces quickly with time.

Initial drift also includes changes in electrical characteristics of the circuit elements, including settling of device characteristics at the atomic level. Longer-term shifts are caused by electrical changes in the circuit elements, often referred to as “aging.” This drift tends to occur at a reduced rate compared to initial drift, and decreases over time. It is therefore often specified as drift/√khr. Voltage references tend to age more quickly at higher temperatures.

Thermal Hysteresis is an often-overlooked specification which can be a dominant source of error. It is mechanical in nature, and is the result of changing die stress due to thermal cycling. Hysteresis can be observed as a change in output voltage at a given temperature after a large temperature cycle. It is independent of temperature coefficient and time drift, and reduces the effectiveness of initial voltage calibration.

Most references tend to vary around a nominal output voltage during subsequent temperature cycles, so thermal hysteresis is usually limited to a predictable maximum value. Each manufacturer has their own method for specifying this parameter, so comparing typical values can be misleading. Distribution data, as provided in data sheets such as the LT6654 and LT6656, is far more useful when estimating output voltage error.

Recently a new class of voltage references has been introduced to the market. Housed in a hermetic surface mount package, these products exhibit significantly improved long term stability and thermal hysteresis performance when compared with the same products in traditional plastic SOT-23 and MSOP packages. Such products include the LTC6655BHLS8-5 #PBF, LT6656AILS8-1.25#PBF, and LT6654AHLS8-2.5#PBF.

Other Specifications

Additional specifications that may be important, depending on application requirements include:

Voltage Noise
Line Regulation/PSRR
Load Regulation
Dropout Voltage
Supply Range
Supply Current

Reference Types

Most voltage references are series (Figure 2) or shunt (Figure 3). Series references are three (or more) terminal devices similar to low dropout (LDO) regulators, sharing many of their advantages. They consume a relatively fixed amount of supply current over a wide range of supply voltages, and they only conduct load current when the load demands it. This makes them ideal for circuits with large changes in supply voltage or load current. Series products available from Linear Technology include the LT6654, LTC6655, LT6656, and many others.

Figure 2. Shunt Voltage Reference

Figure 3. Series Voltage Reference

A shunt reference is a 2-terminal type, usually designed to work over a specified range of currents. The most common circuit ties one terminal of the reference to ground and the other terminal to a resistor. The remaining resistor terminal is tied to a supply. This creates a three terminal circuit. The shared reference/resistor terminal is the output.

The resistor must be chosen such that the minimum and maximum currents through the reference are within the specified range over the entire supply range and load current range. If either the supply voltage or load current may change substantially, then the resistor must be chosen to accommodate this variance, often forcing the circuit to dissipate significantly more power than required for the nominal case.

Advantages of shunt references include simple design, small packages and good stability over wide current and load conditions. In addition, they are easily designed as negative voltage references and can be used with very high supply voltages, as the external resistor holds off most of the potential, or very low supplies, as the output can be as little as a few millivolts below the supply. Examples include the LT1004, LT1009, LM399 and LTZ1000.

Products such as the LT1021 and LT1019 may be operated either as a shunt or a series voltage reference.

Choosing a Reference

Let’s now consider how you might use this information to select your next voltage reference. If the power supply voltage is very high, a two-terminal shunt reference may be the best choice. On the other hand, if the supply voltage or load current is expected to vary widely, a series reference is probably a better choice.

In calculating the total accuracy of your reference, consider the precision required by the application. Multiply temperature drift by the voltage reference’s specified temperature range (Linear Technology provides guaranteed specifications over various temperature ranges including 0°C to 70°C, –40°C to 85°C and –40°C to 125°C). To get an idea of total accuracy, add initial accuracy error, thermal hysteresis, and long term drift over the intended product life.

For the most demanding applications, noise, line regulation and load regulation errors may also be added. As an example, a reference with 0.1% (1000ppm) initial accuracy error, 25ppm/°C temperature drift over –40°C to 85°C, 200ppm thermal hysteresis, 2ppm peak-to-peak noise and 50ppm/√kHr long term stability would have a total uncertainty of over 4300ppm at the time the circuit is built. This uncertainty increases by 50ppm in the first 1000 hours the circuit is powered. The initial accuracy may be calibrated, reducing the error to 3300ppm + 50ppm • √(t/1000 hours).

Also consider your maximum expected supply voltage, power consumption, and load current. Will the load draw substantial current or produce current that the reference must sink? Many references can provide only small currents to the load and few can absorb substantial current. The load regulation specification is a good guide.

References come in a wide variety of packages, including plastic packages (e.g. SOT-23), hermetic surface mount (e.g. LT6654AHLS8-2.5#PBF) and even metal cans. Careful attention to mounting methods and locations can minimize stress and its associated inaccuracies. See application note AN82, “Understanding and Applying Voltage References,” available from Linear Technology for more details.

Conclusion

Selecting a voltage reference requires a good understanding of the accuracy needed for your system and including the relevant error sources in your calculations. In addition to initial accurate and temperature drift, long term stability and hysteresis may be significant sources of variation. Other specifications such as load current may also be determining factors. Linear Technology offers a wide variety of voltage reference products to meet the needs of almost any application.