High-precision test equipment depends on accurate data converters to capture exactly what's happening inside the device being tested.

In battery factories, for instance, these parts are fundamental to quality control and safety since they help identify defective cells and ensure consistency when the cells are bundled into battery packs. In semiconductor fabs, high-precision testing is equally critical: Accurate measurements of voltage and current help verify chip specifications and identify potential faults.

Separating good dies from bad can help prevent costly failures later on in the process, particularly when integrating chiplets in a package. 

However, it's impossible to eliminate every source of error in high-precision test equipment or data-acquisition systems. Everything from temperature changes and electrical noise to other external factors can cause gain and offset errors that may impact the system's accuracy or reduce the effective number of bits (ENOB) in the analog-to-digital (ADC) and digital-to-analog converters (DACs) inside. As a result, it must be calibrated and recalibrated over time to make sure that its voltage and current measurements remain accurate.

But calibrating the system requires a stable, temperature-compensated voltage to compare measurements. This is often called a voltage reference, or informally, a “golden reference.”

The ADC measures these known voltage levels, compares the results, and then uses any differences to determine the gain and offset error. After quantifying these errors, the system can recalibrate itself by compensating for the difference.

Since the accuracy of the entire system depends on the golden reference, it's important to choose a high-precision voltage reference that minimizes errors caused by temperature, noise, and other factors.

Voltage Reference Based on Buried Zener Diode

One potential solution is a voltage reference such as the REF81 from Texas Instruments, which is based on a buried Zener diode. By burying the diode deep inside the device, it can supply a very stable reference voltage with minimal drift over time and temperature as well as very low noise.

The REF81 can also regulate its internal temperature to keep the voltage output constant. By adjusting the temperature to the situation, it can achieve extremely low temperature drift of 0.05 ppm/°C.

The device is designed to deliver a 7.6-V output to drive precision data converters used in test and measurement. But it also integrates precision resistor dividers and a buffer to output additional voltage references.

Furthermore, it can be configured to provide a gain of two for the output voltage. That means the 5-V version of REF81 could be arranged so that the buffer delivers an output voltage of 10 V. This integration eliminates the need for external precision components, reducing the complexity of the overall system, according to TI.

While a golden reference must remain as close to unchanging as possible, its output still inevitably drifts over time, potentially resulting in calibration errors. To limit the long-term drift, the REF81 is packaged in a 20-pin, hermetically sealed LCCC ceramic package, enabling a long-term stability specification of 1 ppm. The decrease in long-term drift decreases the number of calibrations needed over time, said TI.

A voltage reference may be a small component in a test and measurement system, but its role is far from insignificant. Its accuracy and stability can determine how confidently engineers can go about developing high-density battery cells, high-speed circuit boards, and new generations of AI chips.