Data acquisition
Newest sigma-delta ADC architecture averts disrupted data flow when synchronising critical distributed systems
Analog Devices’ Lluis Beltran Gil introduces the way distributed data acquisition systems have been traditionally synchronised for both SAR ADC-
based systems and sigma-delta (∑-Δ) ADC-based systems, and explores the differences among the two architectures. Additionally, he will discuss the typical inconveniences when trying to synchronise multiple sigma-delta ADCs. Finally, a novel approach based on AD7770’s sample rate converter (SRC) is presented, which shows how synchronisation is achieved on sigma- delta ADC-based systems without interrupting the data flow.
H
ave you ever imagined yourself in a supersonic aircraft breaking the sound barrier? Since Concorde was retired,
this seems like an impossible dream, unless you are a military pilot or an astronaut. As an electronic engineer, I am fascinated
with how everything works such as, for example, in a cuckoo clock, and how every single system works harmoniously in perfect synchronisation with the others. This applies to every single aspect of our
lives. As we live in an interconnected world, everything is synchronised—from bank servers to the alarms of our smartphones. The only difference is the magnitude or complexity of the problem to solve in each particular case, the synchronisation of different systems vs. the accuracy required—or tolerable error—or the size of the system to synchronise.
DISTRIBUTED SYSTEMS In a standalone design, the synchronisation is inherent to the local clock or oscillator used. But when the standalone design should be integrated into a wider system—let us call it a distributed system—the perspective of the problem changes and the standalone design should be designed accordingly for the use case. In a system, to calculate the instantaneous
power consumption of an electric appliance, both current and voltage must be measured simultaneously. By performing a quick analysis, you could solve the problem in three different ways:
Use two synchronised single-channel ADCs to measure current and voltage.
Use a multichannel simultaneous sampling ADC, either it has one ADC or one sample- and-hold circuit per channel.
Use one multiplexed ADC and interpolate the measurements, in order to compensate for the time shift between the voltage and the current measurement.
At this point, you may have a solid solution to
solve the problem, but let us expand the system requirements from the original single electric
24 Figure 1. Power grid synchronisation.
appliance to an application where the power must be measured in every single ac power socket in a factory. Now, your original instantaneous power consumption design should be distributed around a factory and somehow designed in such a way that the power in each ac power socket is measured and calculated at the same time. You are now dealing with a distributed system that consists of a set of subsystems located apart from each other, but closely interrelated. Each subsystem needs to provide data sampled at the exact same point in time, such that the total instantaneous power consumption in the factory can be calculated. Finally, if we keep expanding the
hypothetical application example, imagine that your original design is going to be integrated into your country’s power grid. Now, you are sensing millions of power watts, and any failure in a link could have terrible results, like damage to power lines due to stressful conditions that, in turn, may lead to a power outage, with dramatic consequences such as a wildfire or a hospital running without an energy supply. So, everything needs to be
precisely synchronised—that is, the data captured in the power grid shall be captured at the exact
same point in time, independent of their geographical situation, as shown in Figure 1. Under these circumstances, you may consider this as a critical distributed system, and you must get a continuous, fully synchronised stream of data from every single sense node. Similar to the power grid example, these requirements apply to many other examples of critical distributed systems that may be found within the aerospace or the industry market, among others.
Figure 2.
Synchronising a SAR ADC-based distributed system.
September 2020 Instrumentation Monthly
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