The vital role of the voltage reference in precision measurement systems

In this 21st century environment, we change gadgets every year and expect them to work correctly, even if they don’t last all that long. They usually only survive until the next revision comes out, but that seems to be acceptable to most people. In contrast, there are many applications, such as field sensors, that demand high accuracy over a long period of time. A quick replacement for these devices would usually be unreasonable, and many times, impossible.

Within the hostile environment of a field sensor, a key device in the electrical systems that monitor and measure oil, gas, pressure and/or temperature is the seemingly insignificant voltage reference. The role of the voltage reference in these systems is to be the analog gatekeeper by maintaining an absolute point of reference for all measurements and calculations. This can be quite a challenge given the extreme operating conditions experienced by these systems (Figure 1).

Figure 1 High Mountain Weather Station

In this article, we will address not only the impact of two critical environmental variables, relative humidity (RH) and long-term drift (LTD) over time, but on the voltage reference gatekeeper and a winning solution that makes these variables a “nonissue”.

The Voltage Reference Gatekeeper

At its root, system accuracy depends on voltage reference devices. For instance, the system’s analog and digital converters use reference voltages to produce their digital results where the voltage reference value is an integral part of the output digital word (Figure 2).

Figure 2 The SAR ADC Signal Path (A) and ΔΣ ADC Signal Path (B) Both Require Voltage References (REF) in the Circuit

In Figure 2, the left column shows a few of the more popular sensors. These signals can proceed through the (A) path, which finishes with the successive-approximation, analog-to-digital converter (SAR ADC) or the (B) path which finishes with a delta-sigma analog-to-digital converter (ΔΣ ADC). Both converters require a precision voltage reference (REF) to complete their conversion.

In signal path (A) with the SAR ADC and in signal path (B) with the ΔΣ ADC, the converter’s LSB size is equal to:

LSB = Vref /2N (Equation 1)

where LSB = Least Significant Bit

V ref = Voltage reference voltage

N = Number of ADC bits

From Equation 1, it is clear that the voltage reference has a direct influence on the accuracy of the converter’s system.

Humidity and Drift vs. Time

In field sensor applications, the electronics can experience large humidity excursions over time. As an example, the RH at the Mount Everest base camp can range from a maximum of 98% down to 20% throughout the year. This is equivalent to an RH delta of 78%. With systems in this environment, it is essential to guarantee equipment accuracy.

The pathway to this guarantee is to utilize the most stable voltage reference. By reviewing the voltage reference alone, one can predict the stability of the system over time.

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