Today's digital multimeters have a wide array of features. Which ones will best help you overcome the problems you face, and which ones can you forego?
Why all the features? The first digital multimeters (DMMs) had full scale resolution of about 1% of scale. This didn't compare favorably to analog meters with their resolution of about 1000 parts per million (ppm) or 0.1% of full scale. Shortly after DMMs gained widespread acceptance, their basic accuracy reached the 0.5% to 0.25% range. Resolution was 31/2 digits or 500 ppm. DMM technology has improved considerably over the past 25 years, and continues to do so today.
Initially, features allowing you to compare DMMs and their performance in relation to other DMMs were accuracy, sensitivity, and resolution. Almost every manufacturer tried to capture market share by improving these with each generation. Eventually, they ran out of room for improvement. Consequently, designers looked for other ways to gain a competitive advantage. This meant adding new functions. Thus began a new round of design improvements incorporating a wide range of bells and whistles. The early "high-end" convenience features and productivity enhancements are staples in modern DMM designs.
Basic DMM functions. If you're using a modern DMM, understand DMMs typically include these basic measurement functions: AC and DC volts, AC and DC current, and DC resistance. Analog-to-digital converters process signals applied to the DMM. Signal-conditioning circuits then create certain functions. Let's take a closer look at these.
DC volts. Measures DC potential in volts. This is the de facto standard function for every multimeter. Service, design, and production applications all measure DCV. Different applications have different requirements in terms of accuracy, sensitivity, and resolution. Most designs strive for high input impedance to minimize input bias current (such current is a detriment to accuracy).
AC volts. Measures AC potential in volts. This function actually derives the amplitude of the AC waveform, usually with a true root mean square (TRMS) or averaging-type AC converter. Most DMMs calculate the AC voltage amplitude in terms of this rms value. A few measure the actual rms value itself. Common applications include measuring AC line voltage or a DC power supply's AC ripple.
DC current. Measures DC electron flow in amperes. Unlike voltage measurements, DC current measurements require breaking the circuit under test and placing the ammeter in series with circuit elements. When you apply full-scale current across the DMM, the maximum voltage it develops is the voltage burden. Voltage burden occurs because DMMs measure current by inserting a precision resistor in series with the incoming current and measuring the voltage drop across the resistor. Voltage burden is the key to the ammeter function's accuracy. You'll want a low-voltage burden so the meter is as non-intrusive as possible.
AC current. Measures AC electron flow in amperes. This function operates much like DC current, but uses signal-conditioning circuitry that includes a current shunt, AC converter, and input amplifier. An analog-to-digital (A/D) converter completes the signal processing.
DC resistance. Measures electrical resistance in ohms. To measure resistance, DMMs force a current through the test leads and measure the resulting voltage drop at the DMM terminals. Using Ohm's Law (R5V/I), the meter calculates the resistance. Some meters have a four-terminal resistance function that uses two pairs of measurement leads. You connect one pair of leads to force current through the resistance. Connect the other pair to measure the voltage across the resistance. This technique minimizes error due to voltage drop across the two current-carrying leads, which can be significant in low-resistance measurements.
Enter the bells and whistles. With sophisticated signal processing already in place, DMM designers built in new functions with modest cost increases. By using existing circuits and adding only those needed for new measurements, they created new functions including:
Temperature. You can measure temperature by using a thermocouple (T/C) or Resistance Temperature Detector (RTD). Most DMMs measure temperature by using T/Cs because they're rugged, reliable, relatively inexpensive, and able to measure across wide ranges of temperature. However, thermocouples have a relatively low accuracy, typically in the range of 60.5C. T/C output voltage is proportional to temperature, but the relationship isn't linear. Therefore, your DMM needs a reference signal to linearize the output voltage. This signal comes from another T/C maintained at a known lower temperature. We call that T/C a cold junction reference. DMMs also measure temperature by using RTDs. These devices are more accurate than thermocouples, but are also more expensive and have less range. RTDs measure temperature as a function of resistance. The more accurate the resistance measurement, the more accurate the temperature measurement.
dB or dBm. The dB function on a DMM makes it possible to compress a large range of readings into a much smaller scope. In audio and telecommunications applications, the dB function measures gain or loss from filters, amplifiers, and attenuators. DMMs normally use the equation shown in the sidebar below to calculate dB.
Frequency. Frequency is a relatively new function for multimeters. The frequency measurement range varies with DMM make and model. Typically, they range from 1 MHz to 10 MHz. This is a wide range for a general-purpose instrument, satisfying most applications. Still, the frequency function of DMMs does not eliminate the need for dedicated frequency counters. This function allows DMM users to perform a quick measurement of frequency without connecting another (more sophisticated) instrument.
Menus. Menus are a large part of DMM front panel operation and programmability. Some DMM menus are several layers deep. To navigate the menus, you need to know the measurement function you'll perform (where you are going) and understand the menu structure (the road map). The sidebar below gives an example of a menu's steps.
Buffer. Designers normally use memory registers for storing readings in the meter. This permits unattended testing. It also offloads some data storage functions from the computer so it can perform other tasks or run other applications. Normally, using the DMM buffer is the fastest method to acquire measurement data. The size of the buffer varies by manufacturer, and some meters come with an option to expand buffer memory. This allows you to tailor a DMM configuration as required and avoid unnecessary cost.
Triggering. To begin taking data, the meter normally requires a trigger. Triggering is very important for synchronizing events in a multi-instrument system. You can use various trigger sources for most DMMs. For example:
Timing trigger: Time-related applications require timing triggers. These will trigger the meter at exact time intervals.
Hardware trigger: Also known as an external trigger, the hardware trigger usually originates in a device other than the DMM. This external hardware device may be the object of the test. It could be another piece of test equipment, such as a switching matrix used for multiple device testing. Tightly coupled hardware triggers assure precise and synchronized data.
Software trigger: Some DMMs allow you to generate a trigger through computer software, either automatically or manually, to initiate measurements. The standard GPIB software trigger is a Group Execute Trigger (GET). You can synchronize this with the software operating the test system. The SCPI (Standard Commands for Programmable Instruments) defines several levels of triggering. Using this, you have flexible and intricate triggering capabilities.
Displays. Older designs had eight to 14 segment Light Emitting Diode (LED) displays. LEDs were bright, yet power-hungry. They had crude alphanumerics. Liquid Crystal Displays (LCDs) replaced some LED panels.
LCDs consume much less power, but the first ones were not very bright and had a limited viewing angle. Although LCDs have improved, Vacuum Fluorescent Display (VFDs) may be the display of choice in most DMMs.
VFDs are bright and have a level of power consumption between that of LEDs and LCDs. VFDs also have better alphanumerics. Today's DMM displays are information-intensive. Some include multiple functions. For example, displaying DCV, ACV, and frequency readings on the same screen is a feature on several models.
Help functions. Most high-performance DMMs include a help feature to help the user navigate menus and better understand the current operating mode of the instrument.
Emulation. These modes allow a newer meter to respond to an older meter's commands, minimizing test configuration software changes when replacing old instruments.
Continuity beeper. This function is almost literally a bell or whistle. It provides an audible tone output when you have a certain level of continuity. This is useful in production applications for a quick check of resistance levels, primarily to verify circuit continuity.
Analog bar graph. Some applications require you to monitor slowly varying signals. An analog bar graph makes visual tracking of signal magnitude easier than does a digital display. DMMs offering this feature usually have scale programming. For example, the bar graph could be set up for 100% and 10% of the measurement range. This would allow a magnified view on the lower range, which is useful when monitoring a low-level signal on a higher range.
Math. The microprocessors controlling most internal functions of today's DMMs make it easy for designers to add mathematical capabilities. Simple math routines include percent, slope of a line, and standard deviation.
Timestamp. This feature allows you to monitor readings relative to time. It verifies the multimeter's reading rate and is generally accurate within several hundred nanoseconds.
Relative reference. You can cancel out zero offsets with this feature. The relative operation subtracts a reference value from actual readings. Subsequent readings are the difference between the aggregate input signal and relative value.
Graphic displays. Graphical handheld multimeters combine multimeter capabilities with the power of visual waveform displays. Some can display signals with bandwidths up to 1 MHz and reveal details such as waveform distortion, noise, and transient failures/glitches. With a waveform display, component test results can take the form of a family of curves for diodes, transistors, varistors, etc.
Sleep mode. Sleep mode prolongs battery life, particularly in portable instruments. With this feature, a DMM automatically enters a power-conserving sleep mode after a period of no activity on the function keys or at the input terminals. This mode keeps only essential circuitry operating. The DMM remains in this mode until an input changes.
So what does the future hold? DMM users can probably count on a continuation in the trend of increasing features and functions, at least for the next few years. As each new generation of multimeters becomes richer in features, you can also expect performance improvements in measurement speed, accuracy, sensitivity, triggering flexibility, memory, and display capabilities.
While manual configuration will remain, more users will be linking DMMs to computers via data communication networks. As the Universal Serial Bus becomes more common in PCs, it will be even easier to create your own personal high-speed measurement network in a lab or production setting. These networks can include system meters, bench instruments and handheld multimeters for virtually any application. The variety of instruments available will allow you to select measurement hardware with specifications that closely match each application and budget.