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Sensors & transducers


The perilous path from the transducer to the ADC


Hooman Hashemi, product applications engineer at Analog Devices, explores a new family of instrumentation amplifiers that have been introduced to help bridge the gap between a transducer and data acquisition


family of ADI instrumentation amplifiers can reject the CM, gain-up the differential signal, translate the voltage to the ADC input voltage requirements, and protect the ADC against overvoltage in one fell swoop! One of the most ubiquitous challenges in


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countless industrial, automotive, instrumentation, and numerous other applications is how to properly connect a minuscule transducer signal to an ADC for digitisation and data acquisition. The transducer signal is usually weak, fragile, could be noisy, may look like a very high impedance source, and could ride atop a huge common-mode (CM) voltage. None of these are conducive to what an ADC input likes to see. This article will introduce recent integrated solutions that might once and for all answer the poor engineer’s plea for help beyond what is currently available. The article will also go through the detailed design steps to configure a complete transducer interfaced instrumentation amplifier (in-amp) driving an ADC input.


s there a building block that allows me to take a tiny transducer output signal directly to an ADC input voltage? Yes, the latest


no unwanted CM. In-amps have a huge input impedance that will not load the transducer, ensuring that the fragile signal is not affected by signal processing. Furthermore, in-amps allow large gains and a large selectable gain range, usually with a single external resistor, for maximum flexibility to adapt the small signal of interest to voltages far above the signal path noise level and fit for ADC analogue inputs. Because in-amps are designed for precision, they are internally trimmed and maintain their performance over a wide operating temperature and are immune to variations in supply voltages as well. They also maintain their accuracy by having very low gain error, which limits the measurement or signal error as the swing varies.


Figure 1. The challenge of getting from the transducer to the ADC.


WHAt SuitS tHe trAnSDucer AnD WHy iS tHere An iSSue? The short answer to this question is an instrumentation amplifier. That is what a transducer prefers to look into: an in-amp. In-amps potentially have the precision (low


offset) and the low noise to not corrupt the small input signal. They have differential inputs suitable for many transducer signals such as strain gauges, pressure sensors, etc. and will be able to reject any CM present, leaving only the pristine small voltage we are interested in and


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WHAt WoulD tHe ADc input like to See? An ADC input is not the easiest load to drive. There is charge injection from the internal capacitor, CDAC in Figure 2, switching action at the front end that makes it a challenging task to deliver a highly linear and settled signal for quantisation by the ADC. What drives the ADC input must be able to handle these large charge injections and settle quickly before the


next conversion cycle. Furthermore, the driver noise and distortion should not be a limiting factor depending on the ADC resolution (number of bits). Taken together, these requirements are not


trivial tasks, especially with a low power consumption driver. Furthermore, ADC operating supply voltages are shrinking day-by- day as part of the modernisation of semiconductor processes. As an unwanted side effect of such a trend, ADC inputs tend to be more susceptible to input overvoltage stress and possible harm or damage. This necessitates that there is external circuitry that protects against such overvoltage. Any such external circuitry should not limit the bandwidth or cause any sort of distortion in addition to not add any measurable noise to the signal. It is also highly desirable that the overall circuit reacts fast and recovers gracefully and quickly from an overvoltage event. There is also the challenge of shifting the input


signal to conform with the ADC analogue input voltage range. Any circuit elements added to perform this task is subject to all the constraints listed earlier (that is, low distortion, low noise, sufficient bandwidth, etc.).


Figure 2. ADC input driving is challenging.


tHe iSSue: if only tHe in-Amp coulD Directly Drive tHe ADc! With all that in-amps bring to the table, they have some shortcomings that necessitate more circuit elements to complete the path from the physical world (transducer) to the digital world (ADC). Traditionally, an in-amp would not be the


February 2021 Instrumentation Monthly


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