Which Multi-meter to add to my tool kit?
From the simple interconnection of a few resistors to a large complicated circuit, every design needs to be tested for its electrical properties. The two channel scope and function generator software instruments in the ADALM2000 (M2k) and ADALM1000 (M1k) are great for making time domain and frequency domain measurements. The Scopy and ALICE software tools even include a simple two channel voltmeter instrument. Useful, as long as the voltage you want to measure is with respect to the same ground as the M2k / M1k. When that is not the case a good hand-held, battery powered meter is a must have.
In addition, it is difficult to measure current and resistance with USB instruments like M2k. The M1k SMU (Source Measurement Unit) analog frontend makes current and resistance measurements much easier, however, a good multi-meter can be an important debugging tool in an electronics designer's arsenal. Yet making sure you have the right meter at the right price might take some research.
Multi-meters are electronic test instruments that measure voltage, current, resistance and sometimes even parameters like capacitance, inductance and transistor gain. Although there are many different types of multi-meters with different functions and benefits, your first consideration should be analog vs. digital.
Analog Versus Digital
Analog multi-meters, as you might guess, use analog meter movements for displaying voltage, current and resistance. The resolution of an analog meter is a function of the size of the meter dial and the number of lines or tick marks around the scale. Digital multi-meters by contrast have LCD numeric displays. In Figure 1 are examples of what generic analog and digital meters might look like. Multi-meters with analog displays are not as commonly used and are actually becoming harder and harder to find. Simple analog meters passively measure voltage and current by extracting a small amount of energy from the circuit being probed to deflect the meter needle. Often this is only a minor effect but can adversely affect sensitive nodes in a circuit. This can lead to a lower input resistance than meters with active amplifiers as in most digital meters and some high end analog meters (some of you old timers may remember the old VTVM, vacuum tube voltmeters).
Figure 1 Analog vs. Digital multi-meters
Both kinds of meters generally have a switch to select the various functions and ranges, internal circuitry for signal conditioning and in the digital meter an analog-to-digital converter and display driver.
Generally speaking, DMMs are recommended, but there are still hundreds of models to choose from, so you need to narrow down the options and consider various factors when selecting the correct DMM for your needs. Here are some things to keep in mind:
Price: DMMs are available in a wide range of prices staring from less than $10 and going up to $1500 or more. This depends on the manufacturer and the included features. Higher priced meters tend to have more resolution and be more accurate than the lower cost models. Bench top multi-meters tend to be more expensive when compared to a portable or handheld DMM.
In general, the best recommendation is to look at where you will use the meter and the features that are needed. If it is going to be used along with the portable Active Learning Modules like M1k and M2k in an “anywhere, anytime” scenario, a lower priced battery operated hand held meter is probably a good option over an AC powered bench top model.
Display Count: The resolution of a meter, or number of display digits. It refers to how wide a range of readings the meter can display or the total number of digits that are displayed. The higher the display count, the better. Accuracy is generally specified as a percentage of the range (gain error) and +/- some number of counts (offset). The analog inputs on the M2k and M1k have a DC accuracy of a few mV so a DMM with similar accuracy is probably about right.
Most DMMs you are likely to come across are classified as 3 and 1/2 digits. This means the meter can display values from -1999 to +1999. This is a total range (resolution) of 4000. By comparison the 12 bit ADCs in the M2k fundamentally provide a total resolution of 0 to 4095, although the Scopy software will "process" the data and display floating point values with possibly more "digits". The 16 bit ADCs in the M1k provide a total resolution of 0 to 65535. Likewise, the ALICE software will display floating point values with possibly more "digits".
Versatility and Functionality: Multi-meters can have standard features such as measuring AC and DC voltages, current, resistance and even more advanced capabilities to measure frequency, capacitance and inductance. Many meters offer various other tests such as the forward voltage of diodes, battery test, continuity, transistor gain. Special functions like auto range, analog bar graph, USB interface to a computer, true RMS, can make them more useful than a standard multi-meter. Check out the meter's user manual to review the functions offered and select a meter based on the kinds of measurements you will be making.
Both the M2k and M1k can be configured along with the "software" instruments provided in the Scopy and ALICE suites to perform a similar wide range of test functionality but are not as simple, quick and easy to setup as using the features of one of the more advanced DMMs.
Typical specifications for 3½ digit hand held DMMs in the $10 (or less) price category:
We don’t necessarily endorse any of the following suppliers or products and can’t guarantee their price or availability but here are a few possibilities:
The Harbor Freight tool company offers a couple of low cost 3½ digit hand held DMMs such as this:
It is often on sale for $5.99.
Big box stores like Walmart etc., have a number of multi-meters on-line and in their stores.
Home improvement stores like Lowe’s and Home Depot have a number of low cost meters such as this analog display model for $12.98:
or from Home Depot
Newark and other distributors offer a wide range of digital multi-meters ranging from 1,999 count 3½ digit hand held meters like this one priced at $9.99:
To this 40,000 count 4½ digit, true RMS, 10Mohm input impedance, bench top multi-meter for $259:
I have owned and used a wide range of DMMs including the $5.99 ones from Harbor Freight and the $259 one from Newark. The portable hand held models are very useful and handy to have around for general measurements when doing experiments with the M1k and M2k but when you need more accuracy (and more features) the bench-top models are indispensable.
As always I welcome comments and suggestions from the user community out there.
What about a (portable) DMM in the mid-price range?