Data acquisition


ADAQ23875 AnD ADn4654 SAmpling CloCk Jitter iDeAl Snr CAlCulAtion The ADAQ23875 has an aperture jitter of 250 fs rms (typical), while the ADN4654 has an additive phase jitter of 387 fs rms


(fOUT = 1 MHz). In this case, we will not yet consider the jitter contribution of the reference clock and of the FPGA. Now, from the jitter specification of our ADC and isolator


we can calculate the total rms jitter by: Figures 14 and 15 illustrate the calculated maximum SNR


and ENOB performance of the isolated precision, high speed DAQ system. The SNR and ENOB degrade along with input frequency, which aligns with the profile in the theoretical SNR plot in Figure 13.


DLM custoM MuLti-purpose Data Logger for subsea equipMent testing


D


ynamic Load Monitoring (DLM), of Southampton, UK, has


provided a custom data logger and subsea measuring technology for use in a new tank where cable protection systems are tested. Darlington-based Subsea


Innovation, a designer of innovative subsea and offshore equipment, will primarily use the equipment for testing and verifying their in-house designed subsea equipment such as pipeline repair systems (clamps and connectors), hang- off systems, and other bespoke subsea and offshore equipment. It will also be used for internal


Figure 15. ADAQ23875 and ADN4654 maximum calculated SNR.


testing for Subsea Innovation’s purpose-designed and built submerged wear test rig. The company also maintains and manufactures deck equipment for cable lay vessels. This is the second major order that


DLM has delivered to the company recently, having provided two bespoke monitoring systems that were installed on subsea bend stiffeners at an offshore wind farm. However, DLM is increasingly responding to demand for bespoke and standard solutions from similar maritime technology and marine electronics companies. Martin Halford, managing director at


Figure 16. ADAQ23875 and ADN4654 maximum calculated ENOB.


ConCluSion Jitter in the signal (or clock) controlling the S&H switch in an ADC impacts the SNR performance of precision, high speed DAQ signal chains. Understanding the error sources that contribute to the overall jitter is impor tant when selecting the various components that are par t of the clock signal chain. When the application requires the DAQ signal chain to


be isolated from the backplane, selecting a digital isolator that has a low additive jitter is crucial to maintaining optimum SNR performance. ADI has lower jitter LVDS isolators that enable system-level designers to achieve high SNR performance in an isolated signal chain architecture. The reference clock is the first source of sampling clock


jitter, and it is important to use a low jitter reference clock to achieve the best performance of an isolated, high speed DAQ. It is also important to ensure the signal integrity of the path between the FPGA and reference clock to avoid additional error from the path itself.


Analog Devices 22 www.analog.com


DLM, comments: “We’re putting together systems like this with increasing regularity - incorporating load cells with other instrumentation and sensors. However, no two such jobs are the same. We start with the client’s problem, review what off-the- shelf instrumentation and sensors are available, and then customise the final package accordingly based on the


environment - subsea or otherwise - and demands of the application.” In this instance, a custom data logger


takes inputs from several sensors, initially feeding back information on load, rotation, and temperature. More sensors can be added to an expandable system should the need arise in the future. A stainless steel enclosure is mounted in Subsea Innovation’s workshop next to the test tank, which remains in construction. Within the enclosure, the logged data is collated and logged again as a package in a Programmable Logic Controller (PLC) on a memory card. An Ethernet port, meanwhile,


enables the user to plug in a PC to download the data periodically. Halford explains: “You can get a remote view on a web browser of the HMI [human- machine interface] screen to see both live values from the sensors and also download the data logs.” Integral to the system will be an S-


cell load cell from DLM’s standard range, which will be submerged in the tank at a depth of approx. 1m. The load cell is 250kg capacity and will measure in tension and compression. This is the lowest available capacity of a standard range than reaches up to 20t capacity and boasts welded covers, stainless steel construction, sealing to IP68, and high accuracy. It is suitable for use on test stands, to take measurements during certification, and in vessel and tank weighing applications, in addition to retrofit in original equipment manufacturer (OEM) technologies and machinery. Other standard equipment in this


order includes proximity and other sensors, and thermocouples, used for measuring temperature.


Dynamic Load Monitoring www.dlm-uk.com


March 2022 Instrumentation Monthly


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