Technical Insights > Benchtop

Digital Multimeters Are Different by Design

2020-01-02  |  6 min read 

When you, as a product designer, buy a digital multimeter (DMM), the last thing you want to worry about is the quality of your measurement. Your focus should be on the product you are designing, making the right design decisions, verifying your design working as expected, fixing all the bugs in your product, and getting them ready for production.

When you pick out a DMM from an instrument store, do you ask yourself if you can measure with unquestioned confidence with this DMM? Or do you choose the one that costs you the least? There are plenty of DMM brands and models to choose from when you go into an instrument store. However, the underlying truth is, DMMs are different by design.

In this blog today, I want to share with you three key aspects of Keysight’s Truevolt DMM designs that enable you to make measurements with total confidence.

1. Noise and injected current

In a rack or on a bench, real-world signals are never flat. They have some level of AC signal riding on top from power line noise, other environmental noise, or injected current from the DMM itself. Residual capacitances in the multimeter’s power transformer cause small currents to flow from the LO terminal to earth ground. The frequency of the injected current is the power line frequency or possibly harmonics of the power line frequency. The injected current is dependent upon the power line configuration and frequency. See Figure 1.

Noise-and-injected-current-schematic-representation
Figure 1. Injected current introduced by DMM's LO terminal to earth ground

How well the DMM deals with these extraneous factors and eliminates them from the true measurement makes a big difference to the overall accuracy. The Truevolt DMMs have the smallest noise level of anything in the test and measurement industry.  Compared to some of the lower-cost alternatives in the market, Truevolt DMMs offer almost 100% less noise. See Figure 2. Keysight Truevolt DMMs contribute less than 30% of the injected current than alternatives.

Noise-and-injected-current-graph
Figure 2. Keysight’s Truevolt noise and injected current comparison with other alternatives in the market

2. Input bias current

Ideally, no current flows into the measurement terminals of your DMM. In real measurement situations, there are always input currents creating additional measurement errors. A typical DMM has input capacitance that will “charge up” due to input bias currents when the terminals are open-circuited (if the input resistance is 10 GΩ). This input bias current generates small voltage offsets dependent upon the source resistance of the device under test, hence causing a loading error. This effect becomes evident for a source resistance of greater than 100 kΩ, or when the multimeter’s operating temperature is significantly greater than 30°C. See Figure 3.

Input-bias-current-schematic-representation
Figure 3. Diagram showing the effects of loading errors due to input bias current

Truevolt DMMs take care of input bias current. Some alternative DMMs offer 20% to infinitely poorer performance (some are too noisy to measure). See Figure 4.

Input-bias-current-graph
Figure 4. Keysight’s input current when compared to other alternatives in the market

3. Digital AC RMS measurement

For meters in this class, only Keysight uses digital direct sampling techniques to make AC RMS measurements. This technology results in a true RMS calculation technique that avoids the slower response of analog RMS converters used in all other vendor’s 6½ digit DMMs. This unique, patented technique – only used by Keysight, allows for crest factors up to 10 without additional error terms.

Crest factor is the ratio of the peak value to the root mean square (RMS) value of a waveform. See Figure 5 below.

Peak-versus-RMS-value
Figure 5. Peak value versus RMS value of a waveform

AC measurement accuracy degrades when signals have high energy contained in higher frequencies than a typical sine wave. Figure 6 shows an exponential shaped or “Hershey’s kiss” periodic signal with a high crest factor that poses a challenge for many multimeters in the market to measure AC measurements accurately.

Periodic-waveform-with-high-frequency-energy-content
Figure 6. An exponential shaped or “Hershey’s kiss” periodic waveform with high crest factor

Bottom line: Measure with unquestioned Truevolt confidence

Behind the scenes, Keysight’s Truevolt technology accounts for measurement errors created by these real-world factors, such as noise and injected current, input bias current, and imperfect analog-to-digital sampling architecture so that you can be confident in your measurements.

All Truevolt DMM specifications are tested and guaranteed for compliance with ISO/ IEC 17025 standards so you can prove the effectiveness of your lab or production line’s quality management system. Many lower-cost DMMs in this class do not carry a guarantee of their measurement specifications.

To learn how to make accurate measurements with Keysight Truevolt DMMs, go to Keysight Truevolt DMMs.

Truevolt-DMM (Keysight Digital Multimeter)
Figure 7. Keysight’s Truevolt DMMs