Whenever you measure something, you need a reference to measure against. In an electronic system that measures voltage – whether it’s a general-purpose instrument like a digital multimeter (DMM), a circuit that measures the voltage drop on a resistor to determine current, or a sensor interface that measures the output voltage from a specific type of sensor – you need a voltage reference to ensure that your measured values are correct.
In a measurement system, an analog-to-digital converter (ADC) compares input voltages against a reference voltage and produces a code that represents the relationship between an input signal and that reference voltage. Any error in the reference voltage directly produces an error in the measured data.
When we choose a voltage reference for a particular function, we generally begin by focusing on a couple of top-line accuracy specifications: the initial accuracy (the accuracy at room temperature) and the temperature coefficient (the variation in output voltage as a function of temperature). As a simple example, if we need the voltage reference to have a total accuracy over temperature of ±0.2%, we might choose 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%), so we can expect the total error (initial + drift) to be less than ±0.2%.
To improve the total error, we can choose higher precision voltage references with much smaller values of initial error and/or temperature coefficient. The improved specifications are the result of more sophisticated design and calibration techniques. There are additional error sources, however, that become apparent as the accuracy improves. One that becomes critical in higher performance systems is long-term drift (LTD). You can find LTD specifications listed in most voltage reference data sheets. These generally show the typical drift after 1,000 hours of operation. There are several causes for LTD, but a major cause is the stresses on the package that occur during circuit board assembly. Plastic IC packages change shape slightly due to exposure to high temperatures, and this applies pressure to the voltage reference die. As the assembly stresses settle over periods of many hours, the voltage reference’s output changes. The degree of change depends on circuit design, layout, package, and other factors, and is often on the order of 10s of ppm.
Figure 1 shows the LTD of a typical voltage reference. As you can see, in a very high-precision measurement system, the LTD may be large enough to affect accuracy over time. System calibration immediately following assembly can improve the system’s initial accuracy, but there will be changes over weeks and months.
There are ways to improve post-calibration LTD. You can burn in your board for a few months before calibrating, but that approach has limited practicality. An alternative is to run the board through a temperature cycle or two over a period of a few hours, which will generally help stresses settle more quickly.
On the IC manufacturing side, the voltage reference chip can be built into a package that is more stable than a conventional plastic package. Ceramic packages exhibit far lower levels of post-assembly flexing than plastic packages and can, therefore, provide significant improvement in LTD. An example of such a ceramic package is shown in Figure 2. In contrast with earlier ceramic packages, which tended to be impractically large, the 3mm x 3mm dimensions are compatible with dense circuit boards that require small components.
The effect of using an improved package can be striking. Figure 3 shows the same voltage reference IC as in Figure 1 (with the operating circuit on the left), but mounted in the new ceramic package. LTD (shown on the right of the figure) is clearly much better.
Accurate and stable voltage references are critical to high-performance measurement systems. LTD can be improved by mounting voltage references in compact ceramic packages, thereby improving system performance in a small footprint.