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Weekly UpdatesBy definition, a DMM measures AC and DC voltage and current. Most meters also handle resistance and some form of continuity checking or diode test. This level of basic capabilities is readily available in a 3 1/2-digit handheld instrument for as little as $8.95. OK, to be fair, that was an online sale price reduced from the normal $12.
Most professionals wouldn't consider using such a DMM because they require better accuracy, resolution, safety, and durability. Also, many higher performance meters include an extensive list of additional measurements and features that enhance the user experience, as the marketing people say. Nevertheless, even with a large number of extras, an excellent DMM can be bought for only a few hundred dollars.
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Hand-held DMMs are tailored to suit the needs of a technician in a field test or troubleshooting application. Fully guaranteed safety characteristics to at least CAT III or IV are needed for any AC-power work although much more than 0.1 % accuracy generally isn't. Some meters are designed to be especially rugged, but protective rubber bumpers also are found on less robust equipment.
By far the largest category of features specific to hand-held DMMs could be classified as convenience items even though that description tends to minimize their importance. If you're working outdoors, a sunlight-viewable display is a big help. Similarly, auto-ranging and auto-polarity together with peak detect and measurement hold allow you to make a measurement while using both hands to position probes. A strong magnet attached to a hanging strap easily supports a meter when working among steel equipment racks.
In contrast, a benchtop DMM emphasizes extreme accuracy and may have as many as 8 1/2 digits of resolution. These instruments are used as secondary standards in a design lab. They provide 4- and possibly 6-wire ohms measurements, often frequency, and sometimes capacitance. Benchtop DMMs are the heart of many ATE systems, and models are available with extensive switching capabilities. Prices range up to several thousand dollars.
VTI Instruments' Business Development Manager Tom Sarfi explained that the 6 1/2-digit DMM development for the Model EX 1266 LXI Class A switching/DMM was complete. "The DMM is the core of a modular measurement system for a range of data acquisition applications. Much of our work is focused on adding capability that can work in conjunction with the DMM as part of a scanning measurement system," he said.
Plug-in PCI/PXI/VXI DMMs also are used in test systems, and generally a PC-based modular system has an economic advantage where an integral display and front-panel controls add little value. This kind of approach is more flexible than a proprietary LXI or benchtop system because you're not limited to the range of modules only available from the DMM manufacturer. Benchtop and LXI system manufacturers counter that their closely coupled instruments may offer better performance.
The trend toward greater functionality isn't limited to any one DMM format. In all cases, the intention has been to increase the types of applications that the instrument can address.
Tee Sheffer, president of Signametrics, commented in a 2006 EE-Evaluation Engineering article that his company saw "few applications that use more than 30% of the functionality of our DMMs. However, each application uses a different 30% than the previous one. You have to have a broad range of capabilities to address a major fraction of the applications even if no one application uses all of the capabilities."
Additional Measurements
True rms
Very low-cost meters often measure a signal's average value but display rms after changing the scale factor. While there's a fixed relationship between a sine wave's average and rms values, the ratio is different for every other wave shape. Only a meter that actually measures rms regardless of the waveform will display the correct result for common signals such as a distorted sine wave.
True rms has been offered on DMMs for many years, but it's easy to overlook the importance of this feature. Even though these measurements are insensitive to the shape of most signals, all DMMs have a crest-factor limitation. This means that errors begin to creep in when a signal shape starts approaching a pulse. Crest factor is defined as the ratio of peak to rms and typically must be below about 5 to ensure accurate measurements. A sine wave has a crest factor of 1.414.
Inductance
Inductance relates the voltage across a coil's terminals to the rate of change in current flow: V = L di/dt. Coils wound on nonmagnetic cores are much easier to measure because there are no core nonlinearities to deal with. However, although they commonly are used at RF frequencies, air-core coils have low inductance, so for practical reasons many larger inductors, such as those used in power supplies, are wound on a magnetic core.
You wouldn't try to make a resistor or capacitor yourself, but prototype inductors are easy to build from readily available cores and copper wire. What appears to be a simple component actually can have very complex behavior.
DC current changes the operating point on the core-material B-H characteristic curve. As the DC level is increased, the slope of the B-H loop gradually becomes less steep. This means that the coil inductance will be smaller. In the extreme, when the core is saturated, the inductance falls to its air-core value. Consequently, a coil intended to be used at a certain DC current level must be biased at that level to measure the correct inductance. An Agilent Technologies' application note covers DC current-biased inductance measurement in detail. (1)
Even if a coil operates in a circuit with no DC current flowing, the level of the AC current still may affect the inductance measurement. Magnetic core material requires an amount of energy to activate it--the so-called magnetizing current in power transformers. So, depending on the core size and material, accurate measurements require AC current to be above a minimum level. A coil's inductance also varies because magnetic core material properties are not constant with frequency.
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The electrical model of an inductor includes components that represent the copper loss of the coil, the core loss resulting from energy dissipated to excite the magnetic material, and a shunt interwinding capacitance. In general, these elements are frequency-dependent. For example, the equivalent series resistance (ESR) at DC is approximately equal to the coil resistance, but at high frequencies, skin effect accounts for a much higher value. Also, at some high frequency the inductance will resonate with the interwinding capacitance.
Dedicated LCR meters, also called bridges, are capable of providing a DC bias and measure inductance as well as ESR over a wide range of frequencies. The ratio of the inductive reactance to ESR is the inductor's quality factor (Q) or Q = 2 [pi] f L/ESR. In high-frequency designs, it's common to specify the Q of a coil at a certain frequency. Measuring the coil inductance at a much lower frequency simply isn't equivalent.
National Instruments' (NI) Model PXI-4072 6 1/2-digit DMM includes inductance and capacitance measurement based on a technique documented in an NI technical paper and described by Travis White, product manager for precision DC and switches: "We use a very stable, harmonically limited square wave as the current source for the component under test. Then, the digitizer capability of the on-board FlexADC is used to acquire a wave-form of the resulting voltage. Finally, an FFT is performed and an algorithm applied to derive the inductance or capacitance.
"The test method offers the added benefit of compensating for losses in the front end, cabling, and the component under test," he explained, "by extracting the magnitude and phase of the impedance at the excitation source fundamental and third harmonic frequencies and then comparing the differences to calculate the losses in the system and derive a corrected result." (2)
Part of the process involves open/short compensation that minimizes the effect of the DMM probe wires without using a Kelvin 4-wire connection. On the other hand, the highest fundamental test frequency is 10 kHz used to measure inductances less than 1 mH. Above 1 mH, the test frequency drops to 1 kHz and again to 91 Hz for inductances greater than 100 mH.
Obviously, you may not get the same result from this approach that an LCR meter would measure at 1 MHz or 10 MHz. The comment is made in the technical paper that "because of the amount of magnetization current required, you may see an increase in sensitivity to frequency changes and other dependency factors in inductors with cores of larger dimensions, such as those used in transformers and power inductors."
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The point of comparing a DMM's inductance measurement to an LCR meter's capabilities isn't to disparage the DMM. Rather, it's important to understand the limitations of such a measurement. If an inductor is intended for use with a significant DC bias, you can't measure its performance without the bias. Similarly, if a coil's characteristics at 20 MHz are important to you, measuring the inductance at 10 kHz won't give an accurate picture.
Capacitance
Similar to an inductor's sensitivity to DC current level, many capacitors exhibit sensitivity to voltage levels. A Murata technical paper specifically addresses multilayer ceramic chip (MLCC) capacitor characteristics, which can include large sensitivities to voltage depending on the dielectric material. (3) For example, the capacitance of a type X7R 10-[micro]F 10-V size 1206 surface-mount MLCC was reduced by 60% when the DC bias was raised from zero to 10 V. The capacitance of a similar device made with type Y5V material dropped by 90% under the same conditions.
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