Electronic Measurements and Instrumentation
Electronic Measurements and Instrumentation tests instrument selection, reading interpretation, loading effects, accuracy and basic measurement practice. It covers the digital multimeter (DMM), cathode-ray oscilloscope (CRO) and digital storage oscilloscope (DSO), signal generators, frequency counters and bridges.
Core rules
- A voltmeter is connected in parallel and should have high resistance; an ammeter is in series and should have low resistance.
- Accuracy is closeness to true value; resolution is the smallest display change; sensitivity is response per unit input.
- A DMM on AC may measure average and display RMS for a sine; a true-RMS meter is needed for distorted waveforms.
- CRO controls: time/div sets horizontal scale, volts/div sets vertical scale; trigger stabilises the trace.
- Probe compensation is adjusted on the calibrator square wave; a miscompensated probe rounds edges.
- A Wheatstone bridge measures unknown resistance by balancing.
Worked example
A 10 V source has a 100 kΩ series resistor and a 100 kΩ load. Without a meter, load voltage = 10 × 100/(100+100) = 5 V. A voltmeter with 1 MΩ input is connected across the load: 100 kΩ || 1 MΩ = 90.9 kΩ. Total = 190.9 kΩ; measured voltage = 10 × 90.9/190.9 = 4.76 V. The meter reads 4.76 V instead of 5 V, a loading error of about −4.8%.
Traps
- Quoting a DMM reading as exact; include range, resolution and accuracy.
- Using a cheap meter on mains without a CAT rating.
- Forgetting that the oscilloscope ground clip is connected to earth.
- Reading peak-to-peak as RMS; for a sine, Vrms = Vpp/(2√2).