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True8DIGIT project aims to provide the groundwork for the next generation of precision digitisers
By Oliver Power, Metrologist, NSAI National Metrology Laboratory D
igitisers play a vital role in practically all areas of measurement. The conversion of real-world analogue signals into numbers that can be displayed, stored
and processed is central to nearly every measurement process. High-resolution digitisers, including sampling digital multi-meters that were first developed for metrology applications, are now used for signal analysis in a wide range of applications, including those in the automotive, aerospace, communications and medical fields. While Josephson voltage measuring systems now provide voltage measuring accuracies approaching 1 part in 109, the accuracy of currently available digitising instruments that use room temperature electronics has remained at the 1 part in 106 level for DC measurements for several decades. For ACV measurements, the accuracy decreases significantly with increasing frequency. Extending the performance of digitisers beyond the current state-of- the-art to deliver accuracy levels below the part per million level would widen the range of signals accessible to traceable, reliable and accurate measurement.
What prevents digitisers from achieving accuracies of 1 part in 107 or better? To answer this we need to look at the main components of a
precision digitiser which are the input signal conditioner, the analogue-to-digital converter (ADC), the reference voltage, the power supply and the timing circuitry. All of these parts exhibit non-ideal behaviour which adversely affect the accuracy of the digitiser, introducing noise, offsets, non- linearity, drift and sensitivity to external conditions. Realising a digitiser with a performance beyond the state-of-the-art requires a concerted attack on each of these elements. A co-operative research project with the short name
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True8DIGIT involving national metrology institutes, industrial partners and academic institutes is underway with the objective of breaking the ppm barrier for DC and low frequency digitisers (
www.true8digit.eu) . The project is planned in two stages. The first stage (True8Digit) is establishing the foundations for the development of such a digitiser while a follow-on project will build on these foundations to produce a digitiser with target noise levels and non-linearity of a few parts in 108. One important aspect of the current research focuses on
the digitiser’s ADC. There are, of course, a plethora of ADC designs to meet a wide range of applications, from very high- bandwidth, low resolution ADCs to ultra-high resolution ADCs used in precision measuring instruments. At DC and low frequencies (< 200Hz), a multi-slope integrating ADC (IADC) is still considered the most accurate option, despite the fact that this technique has not seen much development for the last three decades. As its name implies, an integrating ADC converts an
analogue signal into a digital signal by measuring the area under the curve of the input signal over a fixed period of time. Its operation is shown on Figure 1. This process inherently averages the input signal with a capacitor in the operational amplifier feedback loop, providing high noise immunity and outstanding accuracy for slowly varying or low-frequency signals. The key factors affecting the linearity of an IADC are: • operational amplifier non-idealities, like offset voltage drift, finite gain and bandwidth limitations
• capacitor non-idealities like dielectric absorption, leakage current and non-linear capacitance • reference voltage drift and noise
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