Using Your DMM and Scope Wisely
Nov 1, 1999 12:00 PM, By John A. DeDad, Editorial Director
Here's a primer on symbology, terminology, connections, and operation for modern digital multimeters and oscilloscopes.
Your troubleshooting skills should be getting better and better. Why? Because you have so many more sophisticated tools at your disposal. However, it takes a knowledgeable individual to properly operate these powerful devices; and knowing basic symbols and terminology is astep in the right direction. More importantly, you should know how to properly connect and operate these test tools in a variety of situations to maximize your efficiency. Let's go through a quick primer to keep you up to speed.
What do all those symbols mean? You'll find numerous electrical symbols on your digital multimeter (DMM) or oscilloscope (scope). Many of them are internationally accepted. In the original article, Table 1 lists the symbols most commonly found on DMMs, along with an explanation of each. Table 2 (in the original article) does the same for scopes. Make sure you're completely knowledgeable of these symbols and functions before you set out to analyze a system or pinpoint that trouble spot.
Here's a more detailed explanation of the most commonly found (and misunderstood) terms.
• AC coupling: a mode of signal transmission that passes the dynamic AC signal component to both inputs of a scope but blocks the DC component. You'll find this useful when you want to view an AC signal that's normally riding on a DC signal.
• Attenuation: the decrease in amplitude of a signal.
• Bandwidth: the range of frequencies a test tool can display accurately with no more than a manufactured-specified amount of attenuation of the original signal.
• Average: a technique to obtain the average value of a repetitive signal.
• BNC: a coaxial-type connector used for the inputs of your test tool.
• DC coupling: a mode of signal transmission that passes both AC and DC signal components to both inputs of a test tool.
• Digital storage capability: Because of the design of digital oscilloscopes, these test tools do not display signals at the moment they're acquired. Instead, digital oscilloscopes store these signals in memory and then send them to the display.
• Dual trace: a feature that allows a test tool to display two separate live waveforms at the same time.
• Duty cycle: the ratio of a waveform with respect to the total waveform period. This is usually measured in percent.
• Frequency: the number of times a waveform repeats in 1 sec, measured in Hertz (Hz), where one Hz is one cycle per second.
• Maximum peak: the highest voltage value of a waveform.
• Minimum peak: the lowest voltage value of a waveform.
• Percentage of pulse width: the ratio of signal on-time to its total cycle time, as measured in percent.
• Root mean square (rms): the conversion of AC voltages to the effective DC value.
• Sampling rate: the number of samples taken from a signal every second.
• Time base: the time defined per horizontal division on a test tool display, expressed in seconds per division.
• Trace: the displayed waveform showing the voltage variations of the input signal as a function of time.
• Trigger level: the voltage level that a waveform must reach before a test tool will read it.
"Isolated" and "grounded," what's the difference? All manufacturers use the term "isolated" (and sometimes "electrically floating") to denote a measurement where you connect your test tool's common (COM) to a voltage different from earth ground.
They use the term "grounded" to denote a measurement where you do connect the COM to an earth ground potential. Knowing this difference is important for your safety and the life of your test instrument.
Do not use an isolated test connection, as shown in Fig. 1 (of the original article) while taking measurements on an AC or DC circuit of several hundred volts to ground. Instead, use the differential three-lead connection system, as shown in Fig. 2 (of the original article) for dual input measurements.
On almost all handheld scopes, you should connect the A-channel to the higher voltage (in relation to ground) and the B-channel to the other. If both test points are equal in voltage to ground, it makes no difference. Regardless, you should connect the A-channel to the signal (voltage) designated as the phase or zero-crossing reference (such as for the trigger sweep). Then set both of your test tool's channels to the same input attenuation, AC or DC setting, and V/cm levels.
Safety first. Every manufacturer includes specific cautions and warnings to encourage safe use of its test instrument. Usually, a caution points out a condition and/or action that may damage your test tool. A warning, on the other hand, actually calls out a condition and/or action that may pose a hazard to you.
You'll find cautions and warnings throughout a user manual. And where absolutely necessary, you'll find them marked on the specific test tool.
Here are some other helpful safety tips:
• Don't exceed the working voltage of the input channel probes to ground. (Check your instrument manual to find this value.)
• Use available accessories to safely make differential measurements if your instrument doesn't have differential input capability or sufficient voltage rating.
Sidebar: Know the difference between DMMs
All digital multimeters (DMMs) are not the same. Yes, they're calibrated to give an rms indication for the measured signal. However, the difference between them is the method of calculation. There are three commonly used methods:
Peak method: Here, the meter reads the peak of the signal and then divides the result by 1.414 (the square root of 2) to get the rms value.
Averaging method: With this approach, the meter determines the average value of the rectified signal. For a clean sinusoidal signal, this average value is related to the rms value by the constant k, which is equal to 1.1. The meter uses this value k to scale all the waveforms it measures.
True rms: The meter measures the heating that will result if the voltage is impressed across a resistive load. One method to detect the true rms value is using a thermal detector to measure a heating value. However, modern DMMs use a digital calculation of the rms value by squaring the signal on a sample-by-sample basis, averaging over a period, and then taking the square root of the result.
All these methods give the same result for a clean, undistorted, sinusoidal signal. But, they can give different answers for distorted signals, and significant distortion levels are not uncommon.
Sidebar: Oscilloscope — an effective PQ analysis tool
You can use an oscilloscope to do real-time testing. Without doing a detailed harmonic analysis, you can tell what's going on just by looking at the voltage and current waveforms. You can get voltage and current magnitudes, look for obvious waveform distortion, or detect major signal variations.
A scope with data storage capability is valuable in many applications because you can save the waveform for future analysis. Scopes in this category very often have waveform analysis capability, which includes energy calculation and spectrum analysis.
Also, you can usually get a DSO with communications capability so you can upload the waveform data to a PC for additional analysis with a software package.