Understanding the Resolution vs. Speed Tradeoff for 6 ½ Digit Digital Multimeters Tutorial

Two of the most important factors in choosing a digital multimeter are resolution capabilities and measurement speed. These aspects play a large role in determining the total throughput of an automated test system and in these automated test systems, measurement speed = equipment utilization = cost. However, with increased speed and lower cost, resolution is often penalized thus intertwining both factors.

Engineers have traditionally dealt with the problem of high resolution, decreased speed measurements in a couple of ways:

• You can live with the slow measurements, but this can be unacceptable on the manufacturing floor where time is money.
• Reducing the accuracy requirements, but that can lead to units passing tests when they should be failing.
• Buying multiple high resolution digital multimeters to improve the overall throughput, but this dramatically increases the system cost.
  

This tutorial illustrates how the NI 4070 FlexDMM can achieve higher resolutions and measurement rates as compared to traditional 6 ½ digit digital multimeters. You can also view an interactive presentation that takes you through this tutorial's material at your own pace.

For additional information and more interactive tutorials on the NI 4070, visit the Digital Multimeter Measurement Tutorial.

Digital Multimeter Function and Operation

Every digital multimeter has a tradeoff between measurement speed and measurement resolution. This tutorial discusses traditional digital multimeter limitations and how the NI 4070 FlexDMM achieves superior resolution and accuracy at any measurement rate.

A digital multimeter is similar to a digitizer or data acquisition hardware but with some important differences. Figure 1 below illustrates basic digital multimeter functionality. Like a digitizer, a digital multimeter has a fast sample clock which sets the rate at which samples are acquired. However, unlike a digitizer, a digital multimeter does not return every analog to digital conversion. Rather, multiple samples are buffered and then averaged to achieve very high accuracy and resolution.

Two factors then affect how fast a digital multimeter can operate at a given resolution, the acquisition rate and the number of samples required. The acquisition rate is determined in part by the sample clock of the digital multimeter and the settling time of the analog-to-digital converter (ADC). The second factor, the number of samples that are needed to achieve the desired resolution through averaging, is strongly affected by the noise of the digital multimeter’s analog front end.

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Traditional Digital Multimeter Design

The basic limitation with digital multimeters in both speed and accuracy continues to be the analog-to-digital converters (ADCs) used in these products. Most traditional digital multimeters are still being built with 15 to 20 year old ADC technology. These conventional ADCs must operate at slow sample rates due to settling times and noise factors. In addition, these conventional ADCs must disarm and rearm the integrator between each measurement which results in significant dead time between each measurement. Recall that a set number of samples are buffered and then averaged to achieve the desired resolution, thus a traditional digital multimeter might take up to 166 ms to resolve 1 V to 6 ½ digits.

Another one of the main digital multimeter speed limitations is driven by the traditional hardware platform, the GPIB (IEEE 488) interface bus. This interface, in use since the 1970s, is often considered the standard, despite tradeoffs in speed, flexibility, and cost. Most traditional "box" digital multimeters employ this interface, although we are beginning to see alternative interface standards offered, such as USB and Ethernet. All of these interfaces communicate with the digital multimeter by sending messages to the instrument and waiting for a response, which is inherently slower than the register-based access used in PCI or PXI/CompactPCI buses.

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Traditional Digital Multimeter Speed vs. Resolution

A traditional digital multimeter has a huge tradeoff between speed and resolution. For example, if the digital multimeter is configured for 6 ½ digit resolution, the sample rate might be 5 samples per second (S/s) as shown by the left-most red dot in Figure 3 below. If a faster sample rate is needed, then the traditional digital multimeter architecture dictates that the resolution must drop down by an entire digit. At 5 ½ digits then, the reading rate becomes about 90 S/s (middle red dot). If an even higher speed is necessary, the resolution again drops down by an entire digit and the speed goes to about 600 S/s for 4 ½ digit resolution (right-most red dot). So a traditional digital multimeter is limited by the vendor-defined resolutions vs. speed choices.
A similar process could be completed to compare the range of speeds vs. resolutions for the market of traditional digital multimeters. The red stepped line shows very dramatic tradeoffs between speed and resolution offered in the marketplace of traditional 6 ½ digit digital multimeters. At 6 ½ digits, the sample rate is between 1 S/s and about 40 S/s. At 5 ½ digits, the rates available are between 40 and 250 S/s, and so on for 4 ½ digits. Typically, you only get one point at each resolution as represented by the larger dots. Therefore, you will usually lose a full digit of resolution to achieve a significantly faster reading rate on your digital multimeter, resulting in very striking and sometimes unacceptable tradeoffs between speed and resolution.

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NI FlexDMM Design

National Instruments evaluated a number of modern ADC architectures for the 6 ½ digit NI 4070 FlexDMM. However, no existing ADC provided the noise, linearity, speed, and flexibility that were needed to achieve the goal of high-speed, high-precision measurements. To meet these requirements, NI developed a converter using a combination of off-the-shelf high speed ADC technology and a custom-designed sigma-delta converter as shown here. This combination optimizes linearity and noise for 7-digit precision and stability, yet offering extremely fast sampling rates up to 1.8 MS/s.

The block diagram in Figure 4 shows a simplified model of how the FlexADC operates. At low speeds, the circuit exploits the advantages of the sigma-delta converter. The feedback DAC is designed for extremely low noise and exceptional linearity. The lowpass filter provides the noise shaping necessary for good performance across all resolutions. At high speeds, the 1.8 MS/s modulator combines with the fast-sampling ADC to provide continuous-sample digitizing. The DSP provides real-time sequencing, calibration, linearization, AC true-rms computing, decimation.
The FlexADC has several advantages over traditional digital multimeters including:

• A continuously variable reading rate from 5 S/s at 7 digits to 10 kS/s at 4 1/2 digits
• An isolated digitizer mode operation with a sampling rate of up to 1.8 MS/s
• Continuous A/D conversion for contiguous signal acquisition
• Advanced host-based functions through virtual instrumentation with LabVIEW such as FFTs, AC crest factor, peak, AC averages, and others.

A typical 1 V measurement with a 6 ½ digit resolution setting on the NI 4070 FlexDMM would look like Figure 5. There are a number of important distinctions from the measurement in Figure 2. The first difference is in the shorter individual sample time. The second distinction of the FlexDMM is the continuously sampling ADC. Thus, the FlexDMM has addressed two of the major limitations in traditional digital multimeters. The difference in the case of 6 ½ digit resolution of a 1 V signal is quite remarkable. It could take the traditional digital multimeter up to 10 times longer than the FlexDMM to resolve 6 ½ digits. Again, this disparity is a result of the ADC in the FlexDMM which never stops and has very low noise.

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NI FlexDMM Advantages

Figure 6 shows the some of the performance advantages of the NI 4070 FlexDMM. Recall that the red stepped curve shows the tradeoffs between speed and resolution offered in the marketplace of traditional 6 ½ digit digital multimeters. The blue curve shows the FlexDMM’s advantage in resolution at all reading rates over a traditional 6 ½ digit digital multimeter. By selecting a sample rate for comparison, it is possible to illustrate the performance improvements of the FlexDMM. For example, at 5 S/s, a traditional digital multimeter may have 6 ½ digit resolution, but the FlexDMM will have 7 digits of resolution (left-most oval), which is twice the resolution. At 90 S/s, the FlexDMM is able to resolve an entire digit more than the traditional digital multimeter, 6 ½ digits to 5 ½ digits (center oval). Finally, at 600 S/s the difference is even greater with the FlexDMM enjoying 6 digits of revolution vs. 4 ½ digits on the traditional digital multimeter (right-most oval), reflecting an improvement of 32 times over the traditional digital multimeter. At any speed, the FlexDMM has a much higher resolution than a traditional digital multimeter.


The NI 4070 FlexDMM also shatters conventional speed performance as shown in Figure 7. At any particular resolution, the FlexDMM can operate at a faster reading rate. With a selected resolution of 6 ½ digits, a traditional digital multimeter may operate at 5 S/s. The FlexDMM can operate at up to 100 S/s for the same 6 ½ digit resolution. At 5 ½ digits, the FlexDMM is 33 times faster, reading at 3 kS/s vs. the 90 S/s of the traditional digital multimeter. And again, at 4 ½ digits, the FlexDMM has a much greater sampling rate. The FlexDMM is the fastest and most accurate 6 ½ digit digital multimeter on any platform, PXI, VXI, and GPIB.


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Summary

The unique architecture of the NI 4070 FlexDMM offers a maximum resolution of 7 digits vs. 6 ½ for a conventional digital multimeter. The custom designed ADC leverages modern technology and design to provide a continuously variable reading rate from 5 S/s at 7 digits to 10 kS/s at 4 ½ digits. Even at 6 ½ digits, the FlexDMM achieves scanning rates of 100 S/s. The FlexDMM architecture allows you to fine-tune the resolution to meet your exact application requirements. Simply stated, the FlexDMM is faster at every resolution and more accurate at any particular reading rate.

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Related Links:
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