Voltage references hold steady

These ubiquitous parts keep getting better, and selecting them involves carefully weighing many specs and trade-offs.

Paul Rako, Technical Editor -- EDN, October 21, 2010

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Decades ago, these references provided initial accuracies of only ±10%, whereas modern reference ICs can provide initial accuracies of 100 ppm, or 0.01%. “We try to make the parts insensitive to line, load, and temperature variations for demanding tasks in the industrial, scientific, and medical markets,” notes Reza Moghimi, application-engineering manager at Analog Devices. Companies with expertise in those markets can also easily address the military and automotive markets, in which accuracy is critical.



As ICs became popular in the 1970s, it became essential that they integrate a shunt voltage reference. Companies such as Burr Brown, Analog Devices, and National Semiconductor then used the approach of burying zener diodes in their ICs (Figure 5). The IC-process steps create the device under the surface layer of the die. Like buried JFETs, buried zener diodes do not touch surface defects in the die, meaning that the diodes operate at low noise levels.

In 1974, Paul Brokaw, now senior technologist at Integrated Device Technology, designed a bandgap voltage that used feedback to improve accuracy and reduce errors (Figure 7). “I dreamed it up when trying to make a discrete power supply, and I wanted to use a lower reference voltage than a 6.8V zener diode,” says Brokaw.

In addition to the buried-zener and bandgap-type voltage references, JFET-based devices, such as the Analog Devices’ ADR440, are also available (Figure 8). The buried JFETs help these parts achieve noise specs of 1 μV p-p over 0.1 to 10 Hz. Analog Devices’ Moghimi also alludes to a new class of references that the company will introduce this year that employs a different architecture from any of the techniques this article describes.


Voltage-reference specs


You must understand several specifications to properly select and apply voltage references. You need not worry about the internal architecture. It is more important to know the specifications of the part, not how the IC company designs it internally. Besides deciding between shunt and series regulators, you must determine whether a zener diode would work in your system. In most cases, you are better off using a specialized voltage-reference IC from an analog-chip company. If you need ultralow power, you should use a series voltage reference, such as a floating-gate device from Intersil. Linear Technology offers the bipolar LT6656, which operates from less than 1 μA of supply current.

After you consider your power budget and select a series or a shunt reference and an output voltage, you must consider the device’s initial accuracy—that is, the accuracy at room temperature when you first apply power to the part. Some adjustable references let you set the output voltage or shunt voltage with one or two resistors. The accuracy of those resistors combines with the stated initial accuracy of the chip to give you a total initial accuracy of your output voltage. More commonly, you select a part with a fixed voltage output of 1.2 to 12V. The initial accuracy of the device determines how closely all the parts you buy approach that ideal voltage output. Using discrete zener diodes or older legacy references, you can expect to achieve 10% accuracy, meaning that you must calibrate or adjust the circuits in production. Modern parts, such as Analog Devices’ AD588, have initial accuracies approaching 0.01%. This spec is critical in data-acquisition systems that require accuracies of 16, 18, or even 20 bits. Another factor driving the adoption of parts with high initial accuracy is the requirement for charging lithium-ion batteries. The design of charger ICs or the process of measuring the charging voltage of a lithium-ion battery requires a total accuracy better than 0.5%. Thus, the voltage reference should have initial accuracies close to 0.2% to keep total system inaccuracy to less than the 0.5% figure that battery-cell manufacturers specify.


Once you have specified the initial accuracy and temperature drift of the part, you then have to look at the stability, or output-voltage drift, over time. Most parts change over the first six months of operation and then settle down to a smaller change over time. Again, the output drift adds to the initial inaccuracy and temperature drift. If you want your system to have a tight accuracy over its operating life, you must use parts that have a long-term drift speciation that keeps your system’s reference voltage within the desired limits. You can also average the output of multiple parts to reduce the effect of output drift over time (Reference 9). Some manufacturers take the extra steps of determining, specifying, and measuring temperature drift and long-term stability of a part, and these steps take time and come at a price. For example, Analog Devices tests the ADR425 voltage reference for a long-term stability of 50 ppm/1000 hours.

A related but less appreciated specification is the turn-on settling time of a reference IC. The output of an IC does not instantly stay within specified limits, so firmware engineers should not make readings or do calibration in the first few microseconds of a circuit’s operation. Many parts specify a 10-μsec delay after you apply power to the part.

Another important specification is noise. Because series references are simply op-amp-buffered shunt references, you can expect the output to have noise characteristics similar to those of an op amp. The noise spectrum is flatband at higher frequencies. Because you use voltage references for their dc output, however, most manufacturers specify their products with a peak-to-peak output-noise voltage over a frequency range of 0.1 to 10 Hz, for example. You might be able to reduce this noise by increasing the output capacitance, but you must be careful to not make the reference unstable. As with all op-amp circuits, driving a large capacitive load makes the amplifier oscillate. Analog Devices’ Moghimi wishes that analog designers would more carefully read modern reference data sheets. “Some customers still think it is good to put a large output capacitor on the part,” he says. “Even if it does not cause stability problems, it can cause the temperature coefficient to get much worse.”


Still another specification, which relates to the temperature coefficient, is hysteresis—an effect in which the output drifts to another level when you heat the part and then cool it down to its original temperature. Manufacturers often specify it as a part-per-million value over a temperature traversal such as 0 to 50 to 0°C. Like all other analog circuits, voltage-references chips also have a PSRR (power-supply-rejection ratio)—how much attenuation the part applies to any noise or changes in the power supply that is feeding the part. This spec is important now that more systems use switching voltage regulators to supply the reference IC. Manufacturers often specify this characteristic as a voltage ratio in decibels at dc or over a frequency range. PSRR always drops off at higher frequencies, often falling to 20 dB or less at 1 MHz. If your voltage-reference chip is operating from a power supply comprising a switching regulator operating at these high frequencies, you must ensure that the ripple and noise on the power supply do not bleed into the reference-voltage output due to poor high-frequency PSRR. You can often fix these problems by putting a linear preregulator IC on the switching-power-supply output that feeds your reference chip. You can also put RC (resistance/capacitance) or RLC (resistance/ inductance/capacitance) filters before the power-supply pin of the reference IC. This approach prevents the occurrence of high-frequency noise in the reference voltage.

Some engineers use Spice models of voltage references; keep in mind, however, that these models have varying quality. Analog Devices, for example, puts the effects of most of the specs into the model. Other companies do not model references at all. Be sure that your model run takes into account all the specs that will affect your design. You may have to do a Monte Carlo Spice run to see the limits of accuracy, but at least you will know the limitations of the parts you are evaluating.

Trade-offs abound

The accuracy and noise of a voltage reference are important parts of the system-design trade-offs you make. For instance, LCD televisions are adopting Class D audio subsystems. Class D amplifiers are similar to switching regulators in that they are more efficient than conventional Class AB amplifiers. One trade-off with Class D amps, however, is that they have worse PSRR than do linear output stages. As a result, you must use either a higher-quality power supply or a more expensive Class D IC with feedback that corrects errors due to power-supply-voltage changes. This trade-off directly affects your choice of voltage reference. You might use a low-noise voltage reference in a power-supply circuit with low ripple and good regulation. You can then use a less expensive, open-loop Class D-amplifier circuit in your audio systems. On the other hand, it may be less costly to use a Class D-amplifier IC that has feedback and good PSRR so that you can use a less expensive power-supply circuit. This trade-off will change over the years and over the power and cost targets that you have for the TV. Using a more expensive voltage-reference circuit can save money in other subsystems or in factory calibration or testing when you manufacture the product in volume.

As with anything in analog design, applying a voltage reference is more complex than you might think. Even though it has only two or three pins, many specs affect its quality (Table 1). Be sure you understand all the specs and why they are important. If you have any doubt, consult the application- engineering departments of the reference- and data-converter-IC manufacturers. They will be glad to help you understand the intricacies of applying voltage-reference ICs. Remember that, in the analog world, an initialaccuracy spec is just the starting point. Actual accuracy depends on time, temperature, power-supply quality, and a host of other factors. Factor the specifications of the reference circuit into error budgets at the start of your design to ensure that no problems arise when the circuit enters volume manufacturing. Then, you can celebrate instead of rushing around doing ECOs (engineering change orders) to get your reference to behave properly.

You can reach Technical Editor Paul Rako at 1-408-745-1994 and paul.rako@cancom.com.

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References

“Series or Shunt Voltage Reference?” Application Note 4003, Maxim Integrated Products, March 19, 2007. Bauman, Edward, Applications of Neon Lamps and Gas Discharge Tubes, Carlton Press, September 1966. “Clarence Zener,” Wikipedia. Van Zeghbroeck, Bart, “Principles of semiconductor devices,” 2007. “ECE 3950,” Slides 25 to 29, Villanova University. Kruger, Anton, “Zener and Avalanche Diodes." Pease, Bob, “The Design of Band- Gap Reference Circuits: Trials and Tribulations,” 1990. Rako, Paul, “Analog floating-gate technology comes into its own,” EDN, Dec 15, 2009, pg 29. Pease, Bob, “What’s All This Long- Term Stability Stuff, Anyhow?” Electronic Design, July 20, 2010.

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For More Information
Analog Devices	Cirrus Logic	Fairchild Semiconductor	Integrated Device Technology
Intersil	Linear Technology	Maxim Integrated Products	National Semiconductor
Renesas	Texas Instruments

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