Digital multimeter vs. oscilloscope

What is the difference between a DMM and an oscilloscope?

Comparing the high-level block diagrams shows that both instruments have an analog-to-digital converter (ADC) at their core. In front of the ADC are some analog conditioning circuits, primarily an attenuator stage. While there are several fundamental differences between the instruments, each has a key block difference that separates their applications.

The multimeter block diagram has extra blocks, including a current source. Some of the DMM’s measurement modes use this current source. For example, the resistance mode uses this source to generate a known current through a shunt resistor and a resistor-under-test. Since the DMM knows the amount of current and the shunt resistor’s value, it can use Ohm’s law to determine the resistance value of the resistor-under test.

The oscilloscope has no such block, so an oscilloscope cannot measure resistance by itself. However, the oscilloscope does have a memory controller as well as memory for storing samples from the ADC. This buffer is how scopes can capture enough information to display waveforms.

In addition, oscilloscopes have sophisticated trigger systems that can help capture a wide variety of signal conditions. For example, an oscilloscope can update only when a pulse width violates a certain width (or time). Bench DMMs, on the other hand, may have a trigger input, but it is a simple start/stop signal.

DMMs and oscilloscopes also have different probing mechanisms, as you can see from their front panels.

DMMs generally have 4 mm banana-style jacks that accept a variety of low-frequency test leads or probes. The most common test lead has a silicon jacket with the 4 mm connector at one end and a metal pointed tip on the other. These test leads are helpful for handheld or quick measurements.

An oscilloscope front panel typically has a BNC-style connector. These connectors accept many different types of probes, most of which measure voltage. Most oscilloscopes come with passive voltage probes suitable for a wide range of general-purpose measurements. There are also probes that use sensors to measure other electric quantities. For example, a probe with a hall-effect sensor converts a wire’s electromagnetic field into a voltage, enabling the oscilloscope to measure current.

Direct cabling is an option for both DMMs and oscilloscopes. For example, if you’re using a text fixture, it may have 4 mm jacks or BNC (or other coaxial connectors) built in. You can connect the fixture to the DMM or oscilloscope using a cable with compatible connectors.

When to use an oscilloscope over a DMM and vice versa?

Perhaps your budget only allows for one of these instruments, or perhaps you have both but are unsure of which to use! The good news is that there are several measurement scenarios where it is obvious which one to use. The even better news is that both instruments work well in many cases.

A DMM is the better choice when you need to measure the characteristics of discrete devices. A DMM, such as the R&S®UDS, can measure specific aspects of a resistor, capacitor, diode or even a transistor. The R&S®UDS could measure, for example, the capacitance of a capacitor or forward drop of a diode. These measurements are helpful when designing a circuit or determining whether a component is damaged. For a complete characterization, however, you may want to consider an LCR meter, which measures the parasitics of a passive component.

DMMs are also good for measuring voltage or current with signals near DC. However, when measuring AC (if there is a frequency content above 100 kHz), a DMM might not correctly measure the signal since its bandwidth is limited.

Oscilloscopes, on the other hand, have very wide bandwidths and are excellent for measuring signals with high-frequency content. They can measure DC, but they present much lower loading to a DUT than a DMM. Oscilloscopes also have multiple time-correlated channels, so they are ideal for comparing a data signal with a clock or an enable signal.

In addition, an oscilloscope is often better for measuring the clock of a microcontroller. A DMM’s test leads have high capacitance, and its frequency measure might be limited to a few megahertz. An oscilloscope, however, puts less loading on the signal and typically has a much higher bandwidth.

An oscilloscope and DMM in one instrument

Nowadays, oscilloscopes often incorporate additional features. For example, it’s common for an oscilloscope to have function generation, either built in or as an option.

Similarly, there are also oscilloscopes that have built-in DMMs. An example of this is the two channel model of the R&S®Scope Rider handheld oscilloscope, which has a built-in DMM that measures AC/DC voltage, AC/DC current (with external shunt), resistance and capacitance.