April 23, 1998

Investment in voltage references pays big system dividends

Bill Schweber, Technical Editor

Unglamorous yet critical components, voltage references are increasingly showing up in high-volume applications. By choosing the right reference IC and applying it with care, you achieve superior system performance.

A voltage reference has a higher ratio of design-in subtlety to the number of active devices than any other linear component. For an IC with just two or three terminals, the voltage reference packs a lot of mystery into its design, packaging, and application. Yet, when you choose the right reference and apply it appropriately, you can achieve consistent performance and excellent accuracy and stability.

Although the designation "reference" suggests that the application of these devices is in just a few important niches, such as test instrumentation and transducer interfacing, engineers commonly use references, and this use is spreading. Communications and data-recovery systems, for example, use them to establish threshold levels in decoders. Even the commonplace battery charger now often needs a fairly precise reference for setting cutoff points when charging Lithium-ion cells.

To get the most benefit from a reference, you must not only choose a device with the appropriate specifications and trade-offs among these specs, but also use the device in a suitable circuit topology and physical installation. Otherwise, you risk seriously compromising the potential performance of your reference and thus your system.

Applications, convenience drive performance attributes

Since the beginning days of instrumentation in the 19th century, electrical systems have needed a compact source of well-defined potential difference--a voltage--that remains accurate, stable, repeatable, and inexpensive. Although laboratories have used carefully constructed, wet electrochemical cells, known as Weston cells, for primary references, these are hopelessly impractical for most systems. When solid-state technology developed through the 1950s and 1960s and as the physics of semiconductors became clearer, designers saw the opportunity for a more practical reference device based on diode characteristics.

Unfortunately, the temperature coefficients of diode junctions made them too inaccurate and variable for most system designs. As Dave Fullagar, vice president at Maxim Integrated Products, recalls, IC designers understood that a pn-diode's V_be has a negative temperature coefficient, and Greek delta,ucV_be has a positive coefficient. At the 1970 International Solid-State Circuit Conference, though, the legendary, late Bob Widlar described a reference that simply and cleverly employed the two opposing temperature coefficients together in a mutually canceling way (Reference 1).

The resulting device was a bandgap voltage reference, which Fullagar notes was "one of the most elegant pieces of design work in our industry."

Although early stand-alone discrete references were commensurate with 8-bit system performance, they soon reached 10- and even 12-bit levels. Simultaneously, manufacturers of A/D and D/A converters started embedding references in the converters, because nearly every converter needs a reference to establish calibration for its transfer function. Many 10- and 12-bit converters today are called "complete" when they include an internal reference, eliminating the need for a separate, external reference device in some applications.

However, as converters have become more precise, offering 14- and 16-bit resolution and linearity, the discrete reference is increasingly vital (see box "When disintegration is a better choice"). By using a 16-bit system in the quest for greater dynamic range than 12-bit converters offer with a single range, designers avoid the need to use programmable-gain amplifiers, gain ranging, and switching, along with these devices' settling times and range-matching difficulties.

Even with a function as basic as a reference, you have four basic architectural choices. References are available based on either the buried-zener or the bandgap principle, and you can tailor each of these principles into a shunt or series reference configuration. Relative to the bandgap device, buried-zener references generally have lower noise, better long-term stability, and lower temperature drift. However, their lowest output voltage is approximately 6 to 7V, and they need a voltage about 1V higher than this output voltage to operate. So, they are unsuitable for low-voltage systems; furthermore, their dissipation is greater than that of bandgap devices. Bandgap devices can provide reference voltages as low as approximately 1V and are available with final device values such as 1.235, 1.25, 2.048, 2.5, 4.096, and 5V to match both the application need and the converter resolution.

You must bias the two-terminal shunt-mode reference with a current value greater than the sum of the reference quiescent current and the maximum expected system load current (Figure 1a). You can operate the shunt reference from a wide span of supply voltages, because it is biased through a current-limiting resistor. However, the shunt reference wastes power through that resistor and can only source current. Note that some newer shunt references require very little quiescent current.

The three-terminal series-mode reference draws supply current equal to the quiescent current and the instantaneous load current, so this reference dissipates less power on average (Figure 1b). Series references can sink and source current without external components, as well. Current that this type of reference draws is independent of the current that the load draws.

Realizing potential

Unlike with most digital and some analog components, making sure you actually get the maximum per-formance and minimum difficulty from your reference requires consideration of numerous electrical and mechanical details. Start with the power supply for the reference. Although an independent supply isolated from the rest of your system supplies is often unfeasible, you should make sure that the reference is tied to a supply rail with local bypassing, because the power-supply rejection ratio of references is often too small to maintain their stated performance with a less-than-perfect supply. You may want to use a separate regulator for the reference in extreme cases.

When you choose a reference, look at its specifications for both line and load regulation, just as you would with a power supply. Load stability is especially important when your load has transients. For example, many A/D architectures inherently have transient glitches at their reference inputs during various stages in their conversion cycle. You may need to buffer the reference output using a low-drift op amp if your analysis reveals potential problems here. Alternatively, you can add a filter capacitor on the reference output to provide some reserve capacity. However, many references are unstable with capacitive loads, so this tactic can backfire on your system performance.

Although all vendors specify load and line regulation, they may not do so at extremes. In other words, vendors may specify line regulation with a minimum, resistive-only load or load regulation at nominal line value. Depending on your application, you may need to see the load-regulation specifications at the minimum operating voltage as well, for example.

If you use the reference to supply operating current to other parts of the circuit, it's unlikely you'll find a reference that has the necessary output capability, because most references can provide only as high as about 10 mA. If you need more current, you need to buffer the reference output with a higher current output op amp, but these devices tend to have large drift, and it may negate the virtues of the reference performance.

Consider whether a trimmed, low-dropout regulator may be a better choice when you need to supply current. After all, a regulator is conceptually like a reference, providing a fixed, stable voltage. Compared with a reference, a regulator can generally source currents greater than 10 mA, has stability worse than 10 ppm, and has accuracy inferior to 0.5 to 1%; a reference supplies less current but with better stability and accuracy.

When your reference drives physically long lines and remote loads, regardless of load current, be prepared for IR drops. The voltage drop from even a few milliamps of load current to a few milliohms of track resistance can severely compromise your reference accuracy and perceived stability, especially if the load varies over a wide range. You have to resort to a buffer with a force/sense (Kelvin) four-terminal configuration to compensate, and you have to decide which of your various load points controls the output value. Some reference ICs have built-in force/sense terminals, allowing you to access their buffer inputs and so eliminating the need for an external buffer.

Be especially concerned about where you mount the reference, for both thermal and stress reasons (see box "Can cans really be better?"). References perform best when their ambient temperature is stable. It's almost always better to put the reference in a warmer location but at a nearly constant temperature than it is to put it where there are wide temperature fluctuations yet with a lower average temperature value. You can even get references with built-in heaters that maintain a more stable temperature. There are two disadvantages to this option, however. First, the reference consumes more power, and, second, you limit the number of vendors and models from which to select.

Some vendors specify the hysteresis for their parts, which indicates how closely the device retraces its output-value variation path as its temperature moves away from an initial value and then returns. Low hysteresis is critical in applications with temperature cycling. High hysteresis values can negate any initial precision or tight temperature-coefficient specs, so check this factor if your reference is in a fluctuating environment.

Resist the urge to save power by shutting down a reference when you don't need it; the thermal cycling from power-up/power-down cycles necessitates that you wait a relatively long time before getting an accurate reading. Instead, look for a low-power device that meets your other requirements. If you must conserve every microamp of power, look for a reference that settles and stabilizes relatively quickly to specified output.

Look at vendors' suggested guidelines for pc-board placement and routing around the reference. According to Reference 2, in a test using a surface-mount reference on a 7×9-in. pc board with deflection of 18 mils/in., the reference output had a 60-ppm p-p shift, compared with 4 ppm for an unflexed reference mounting. You can minimize the effect of board flex by routing the board area around the reference to decouple it from the rest of the board, using a thicker pc board, using flexible standoffs for mounting the pc board, and looking at the location and orientation of the reference package vs the location of the pc-board mounts and restraints.

In critical applications, you may need to consider burn-in of the assembled board. Powered burn-in helps age the reference and accelerate its trajectory to more stable performance. Even if your reference is sufficiently stable when you receive it from the vendor, you can use unpowered burn-in for 168 hours (one week) at 100°C to relieve latent stresses that can adversely affect reference performance in the final pc-board assembly.

When you look for a reference, the primary technical attributes to consider are initial absolute accuracy, temperature coefficient, long-term stability and drift, and noise (Table 1). In addition, you may need to consider operating-voltage span and power consumption. This factor is now often less critical because the power that a single reasonably good, low-power reference needs is a small fraction of most overall system-power budgets. Note that vendor specs are often package-dependent: Vendor A's SOT-23 reference may be better than vendor B's in the same package but inferior to vendor B's reference when housed in an SO-8. Make sure the vendor provides all the specs you need or can run special tests if you require them, because the precise and temperature-related nature of many reference specs makes it difficult to ascertain results yourself.

For micropower applications with precision performance, Linear Technology offers the LT1634 family, with 1.25, 2.5, 4.096, and 5V members. These shunt devices require just 10 µA and guarantee initial accuracy to 0.05% and maximum drift of less than 25 ppm/°C (Figure 2). The company's LT1460 series reference family strives for high performance in an SOT-23 package with 0.2% initial accuracy and a 20-ppm/°C temperature coefficient. The family operates with an input-to-output-voltage differential as low as 0.9V or as high as 20V, and the vendor also specifies this family as having a typical pc-board solder shift of 0.02%.

If you want low noise, the Maxim MAX6325 buried-zener series reference features 1.5/2.4 µVp-p (typical/maximum) noise over the 0.1- to 10-Hz span and suits 16-bit applications. This 0.02%-accurate, 1-ppm/°C temperature-coefficient device operates from 8 to 30V and includes an optional noise-reduction pin to which you can connect a 2.2-µF capacitor to reduce the noise by a factor of two. The long-term stability of this reference is 20 ppm/1000 hours, and its temperature hysteresis for a 25° shift is also 20 ppm. For applications that need the lower power of a bandgap device, the MAX61XX family operates from an 80-µA supply current and provides initial accuracy of ±1% and maximum drift of 50 ppm/°C; typical drift is half that figure (Figure 3).

Also for power-constrained applications, Analog Devices' AD158X family of series bandgap references has quiescent current of 65 µA maximum and operates from a supply that ranges from 200 mV to 12V above the nominal output. Worst-case initial inaccuracy is less than ±0.1%, and the reference can sink or source as much as 5 mA. Analog Devices further pushes the low-power envelope with minimal performance compromise with its ADR29x family. This family employs a new internal architecture with performance that minimizes the compromises of both zener and bandgap designs (Figure 4). The family specifications include 6-µVp-p noise from 0.1 to 10 Hz, 12-µA supply current, initial accuracy of ±2 mV, and 8-ppm/°C maximum temperature coefficient (5 ppm typical).

If you prefer a bandgap shunt device, National Semiconductor has the LM4041 1.2V device, which is stable with any capacitive load. Minimum operating current is 60 µA, and maximum current is 12 mA. Output-voltage error is ±0.1%, and output noise is less than 20 µV rms. This reference also has a maximum drift of 100 ppm/°C.

Some references are available from several sources, and thus vendors have carefully characterized them over years of experience. Burr-Brown's REF102, for example, provides 10V ±0.0025V output with drift of less than 2.5 ppm/°C. Long-term stability for this part is better than 5 ppm/1000 hours, and noise is typically 5 µVp-p. This reference operates from 11.4 to 36V dc, requiring quiescent current of 1.4 mA.

It seems almost contradictory, but sometimes you need a reference that is not only stable but also adjustable. For this reason, many vendors offer some of their references with an adjustment pin brought out to the IC package to connect to a trimming potentiometer. This pin gives you the adjustment you need, but it also shifts the burden of stability from the reference IC to you and your adjustment. If possible, avoid the need for an adjustable reference by using a different calibration algorithm or by looking at alternatives to hardware reference variation. Once you add an external trim device to your reference, you have to worry about the trim device's thermal stability and settability. You also have trouble isolating the cause of system-performance drift if it exceeds your limits.

If your application demands a current source rather than a voltage source, you can use many of the voltage references supplemented by additional components or seek a current-reference IC (see box "What about current affairs?").

References differ from most components in another practical consideration. Because they are basic building blocks, many devices are pin- and function-compatible. These alternative sources mean that in your quest for the best system performance at the lowest cost, you can substitute other devices from the same vendor or try devices from another vendor without board redesign or software changes.

Invest in a good reference, and treat it with care, consideration, and respect, and you'll get the system performance you need. Treat a reference IC just as you would any other IC, and you'll probably be disappointed and frustrated. Be sure to study vendor application notes and specifics--their reference reference designs--for general guidelines as well as device-specific advice.

Acknowledgments

Thanks to Jim Chase, Roya Nasraty, and Derek Bowers of Analog Devices Inc; Bill Gross of Linear Technology Corp; and Dave Fullagar and Ron Clark of Maxim Integrated Products for their insight and perspectives.

References

1. Widlar, RJ, "New Developments in IC Voltage Regulators," IEEE International Solid-State Circuits Conference, 1970, Session FAM 13.3.

2. Kester, Walt, "Linear Design Seminar," Analog Devices Inc, 1995, Chapter 8.

3. Lee, Mitchell, "Understanding and Applying Voltage References," Linear Technology Magazine, June 1997 (Part 1), August 1997 (Part 2).

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