EC-8.5 Sampling, Data Acquisition and the Digital Record
You can lay out the path from a sensor to a stored number, place the anti-alias filter in the right position, choose a sample rate and record length for the question you are asking, and store the result so someone else can use it.
Before:S4-5. Probability Theory & Stochastic ProcessesPE2-4. Sensing Techniques and Sensor SystemsEC-1. Circuit AnalysisUnlocks:EC-13. Laboratory Practice, in Sequence
Sampling was proved as a result about signals in the signals and systems module. Here it is a chain of physical parts, each of which can spoil the record, and the ordering of those parts is the thing most often got wrong. Once a record has been taken with the filter in the wrong place, no amount of later processing recovers it.
Work through these
The data acquisition chain, end to end
Sensor, signal conditioning, anti-alias filter, sample and hold, converter, and finally storage. Each stage limits what the next can do, so the weakest one sets the quality of the whole record.
NPTEL: Measurement and Instrumentation (IIT Guwahati) · CourseAnti-alias filtering, before the converter and not after
Anything above half the sample rate comes back disguised as a lower frequency and is then indistinguishable from real signal. The filter has to be in front of the converter, because after it the damage is already recorded.
NPTEL: Measurement and Instrumentation (IIT Guwahati) · CourseSample rate, record length and what you can resolve
The rate sets the highest frequency you can see and the total length sets the finest frequency difference you can separate. Wanting both at once is what makes acquisition files large.
NPTEL: Electrical Measurement and Electronic Instruments (IIT Kharagpur) · CourseResolution, range and the least significant bit
A converter divides its input range into steps, so a twelve-bit converter over ten volts has steps of about two and a half millivolts. Choosing a range far larger than your signal throws away most of those steps.
NPTEL: Electrical Measurement and Electronic Instruments (IIT Kharagpur) · CourseTriggering, and capturing something that happens once
A single event needs the instrument armed and waiting before it occurs. Pre-trigger storage keeps the moments before the trigger as well, which is usually where the cause of the event is visible.
NPTEL: Measurement and Instrumentation (IIT Guwahati) · CourseStoring a record so that someone else can read it
A column of numbers with no sample rate, units, range or date attached is unusable a month later. The metadata is part of the measurement, and writing it beside the data is cheaper than repeating the experiment.
NPTEL: Measurement and Instrumentation (IIT Guwahati) · Course
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