Data acquisition


µModule data acquisition solution eases engineering challenges for a diverse set of precision applications


Maithil Pachchigar, system applications engineer at Analog Devices, presents a few key aspects and technical challenges associated with designing precision data acquisition systems and how Analog Devices is leveraging its domain expertise in linear and converters to develop the highly differentiated ADAQ4003 signal chain µModule solution to solve some of the toughest engineering problems.


resources to develop high performance, discrete linear, and precision signal chain blocks for their end application (such as test and measurement, industrial automation, healthcare, or aerospace and defense) to measure and protect, condition and acquire, or synthesise and drive. This article will focus on a precision data acquisition subsystem, as shown in Figure 1. The electronic industry’s dynamics are rapidly


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evolving and there is less time to build and prototype analog circuits to verify their functionality as the control of R&D budgets and time to market (TTM) become challenging. Hardware designers are demanding advanced precision data conversion performance and increased robustness for complex designs in an ever-shrinking form factor amid thermal and printed circuit board (PCB) density limitations. Heterogeneous integration via system-in- package (SiP) technology continues to advance key trends within the electronics industry, including the move to higher density, increased functionality, enhanced performance, and longer mean-time-to-failure. This article will illustrate how Analog Devices is leveraging heterogeneous integration to change the precision conversion playing field and provide solutions that make a significant application impact. System designers face logistical challenges such


as component selection and design optimisation for final prototypes and technical challenges such as driving the ADC inputs, protecting ADC inputs from overvoltage events, minimising system power, and achieving higher system throughput with low power microcontrollers and/or digital isolators. With increased focus on system software and applications to differentiate their system solution, OEMs are assigning more resources to software development instead of hardware development. This is resulting in increased pressure on hardware development to reduce design iterations. System designers developing data acquisition signal chains typically require high input impedance to allow direct interface with a variety of sensors, which could have varying common-mode voltages and unipolar or bipolar single-ended or differential input signals present. Let us take a holistic view of the typical signal chain implemented using discrete components


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ystem architects and circuit-level hardware designers spend significant research and development (R&D)


Figure 1. High level data acquisition system block diagram.


and understand some of the system designer’s major technical pain points with the illustration in Figure 2. The key portion of the precision data acquisition subsystem is shown, where the 20 V p-p output of the instrumentation amplifier is applied to the noninverting input of a fully differential amplifier (FDA). This FDA provides necessary signal conditioning, including level shifting, attenuating the signal, and setting the output swing between 0 V and 5 V with a 2.5 V common mode, opposite in phase, resulting in a 10 V p-p differential signal to the ADC inputs to maximise its dynamic range. The in-amp is powered with dual supplies of ±15 V, whereas the FDA is powered from +5 V/–1 V and the ADC is powered from a 5 V supply. The ratio of feedback resistors (RF1 = RF2) to gain resistors


(RG1 = RG2) sets the FDA gain of 0.5. The noise gain (NG) of the FDA is defined as:


Where β1 and β2 are feedback factors:


This section will present how the circuit imbalance (that is, β1 ≠β2) or mismatch in


feedback and gain resistors (RG1, RG2, RF1, RF2) around the FDA influences key specifications such as SNR, distortion, linearity, gain error, drift, and input common-mode rejection ratio. The differential output voltage of the FDA depends on VOCM, so when feedback factors β1 and β2 are


Figure 2. Simplified schematic of a typical data acquisition signal chain.


May 2021 Instrumentation Monthly


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