April 9, 2019
| By: Andrew Escamilla
Associate Business Manager, Industrial Communications, Maxim Integrated
Voltage references serve an important role in ensuring that measured values from an electronic system that measures voltage are correct. In a measurement system, an analog-to-digital converter (ADC) compares input voltages against a reference voltage. The ADC then generates a code representing the relationship between an input signal and that reference voltage. If there’s an error in the reference voltage, this results in an error in the measured data.
Selecting the right voltage reference for a particular function calls for consideration of some top-line accuracy specifications:
With these parameters in mind, how do you choose? Say you require the voltage reference to have a total accuracy over temperature of ±0.2%. In this scenario, you might go with a reference with an initial accuracy of ±0.1% and a temperature coefficient of ±10ppm/°C. Between 25°C and 125°C, the temperature coefficient can vary 10ppm/°C x 100°C, or 1000ppm (0.1%). As a result, you can expect the total error (initial + drift) to be less than ±0.2%.
You might prefer to improve the total error. In this case, you can select higher precision voltage references with much smaller values of initial error and/or temperature coefficient. You can generally thank more sophisticated design and calibration techniques for the improved specifications.
As accuracy improves, other error sources become more apparent. Long-term drift (LTD) is one such error source; it is particularly critical in higher performance systems. LTD is marked by the output voltage shift from a given voltage reference at power up, measured at selected intervals over time.1 Data sheets generally show the typical drift after 1,000 hours of operation. LTD is caused by many factors, a major one being package stresses that happen during circuit board assembly. Exposure to high temperatures causes a slight change in the shape of plastic IC packages, and this applies pressure to the voltage reference die. Over the course of many hours, the assembly stresses settle and the voltage reference’s output changes. How much change occurs will be based on factors including circuit design, layout, and package; it is often on the order of 10s of ppm.
Figure 1. In industrial equipment, such as this automated machine, accurate voltage references are essential for high performance.
In Figure 2, which displays the LTD of a typical voltage reference, it’s apparent that, in a very high-precision measurement system, the LTD may be large enough to affect accuracy over time. To improve the system’s initial accuracy, you can perform a system calibration right after assembly. But changes will still occur over the course of weeks and even months.
Figure 2. LTD plot for plastic reference.
There are also various techniques you can apply to improve post-calibration LTD. Before calibration, you can burn in your board for a few months; however, this method isn’t super practical. You can also run the board through a temperature cycle or two over a period of a few hours. This approach will generally help stresses settle more quickly.
There are IC manufacturing considerations as well. For example, there are package types for voltage reference chips that offer more stable options than the conventional plastic package. Ceramic packages are a good option, as they exhibit far lower levels of post-assembly flexing than plastic packages. As a result, they can provide significant improvement in LTD. While earlier ceramic packages tended to be on the large side, the 3mm x 3mm dimensions of today’s more compact ceramic packages meet the requirements of dense circuit boards that require small components.
Using a better package type can produce significant benefits. In Figure 3, you can see the same voltage reference IC as in Figure 2 (with the operating circuit on the left), but it is mounted in the new ceramic package. Clearly, LTD (shown on the right of the figure) is much better with the ceramic package.
Figure 3. LTD plot for plastic reference.
For applications such as high-accuracy industrial and process control, precision instrumentation, and high-resolution ADCs and digital-to-analog converters (DACs), a low-noise, low-drift voltage reference like the MAX6079 provides a good option. Available in a small, 8-pin hermetically sealed ceramic package, the MAX6079 offers low drift over time, temperature, and humidity. A worthwhile option for applications such as cell phones, industrial process control systems, and portable, battery-powered equipment is the MAX6279 ceramic shunt voltage reference. A precision, two-terminal shunt mode, bandgap voltage reference, the MAX6279 is available in fixed reverse breakdown voltage of 1.225V and provides stable results versus time, humidity, and temperature.
In summary, for high-performance measurement systems, accurate and stable voltage references are their bread and butter. Improving system performance in a small footprint calls for enhanced LTD performance, and this is possible by mounting voltage references in compact ceramic packages.
A similar version of this blog post appeared on February 11, 2019, in Embedded Computing Design.