NI FlexDMM Measurement Capabilities

National Instruments introduced the groundbreaking PXI-4070 FlexDMM in 2002. This new product provided engineers with a solution for solving the measurement challenges inherent in traditional precision instruments – limited measurement throughput and flexibility. The FlexDMM helped overcome these challenges by delivering measurement throughput rivaling higher resolution DMMs costing thousands of dollars more and offering measurements found in two common instruments – 6 1/2-digit digital multimeters and digitizers. NI has continued to innovate on the FlexDMM architecture since the release by:

• Doubling the throughput of its fastest measurement mode
• Adding a 1.8 MS/s isolated-digitizer mode to the PXI-4070
• Releasing a PCI version of the PXI-4070

The latest innovation is the NI PXI-4072, which delivers the identical functionality as the NI 4070, but adds the capacitance and inductance measurement capability found in LCR meters. This measurement capability gives engineers the critical functionality of three common instruments in a single-slot 3U PXI module – 6 1/2-digit multimeter, LCR meter, and 1.8 MS/s isolated digitizer. The PXI-4072 brings digital multimeters one step closer to providing universal measurement capability by offering engineers the 20 most common test measurements, including voltage, current, capacitance, inductance, temperature, and resistance. This document describes the PXI-4072 FlexDMM architecture and how it helps engineers reduce test system size and cost, increase throughput and shorten test development time.

NI FlexDMM Architecture and Measurement Capability

The FlexADC is the backbone of the NI FlexDMM family (PXI-4072, PXI-4070, and PCI-4070). The FlexADC proves the noise, linearity, speed, and flexibility required to achieve high-speed high-precision measurements. The FlexADC converter is based on a combination of off-the-shelf high-speed ADC technology and a custom-designed sigma-delta converter, shown in Figure 1. This combination optimizes linearity and noise for 7-digit precision and stability, yet offering digitizer sampling rates up to 1.8 MS/s.

Figure 1. FlexADC Converter

The block diagram 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 proves the noise shaping necessary for good performance across all resolutions. No ramp down is needed, because the ultrahigh precision 1.8 MS/s modulator proves extremely high-resolution conversion without it. At high speeds, the 1.8 MS/s modulator combines with the fast-sampling ADC to prove continuous-sample digitizing. The DSP proves real-time sequencing, calibration, linearization, AC true-rms computing, decimation, as well as the weighted noise filtering used for the DC functions.

The FlexADC has several advantages:

• The unique architecture of the FlexDMM offers a continuously variable reading rate from <5 S/s at 7 digits to 10 kS/s at 4 1/2 digits
• The FlexADC can be operated as a digitizer with a sampling rate up to 1.8 MS/s
• Because of the custom sigma-delta modulator, noise shaping and digital filtering has been optimized for use in DMM and digitizer applications.
• Unlike in other ADC conversion techniques, it is not necessary to turn the input signal on and off. Therefore, continuous contiguous signal acquisition can be accomplished.
• Direct AC voltage conversion and frequency response calibration can be accomplished without the use of a conventional analog AC Trms converter and analog “trimmers” for flatness correction.
• Input signal noise can be dramatically reduced on all functions with appropriate noise-shaping algorithms
• Advanced host-based functions can be implemented with LabVIEW, once signals are digitized, leading to an almost endless list of signal characterization options (such as FFT, calculating impedances, AC crest factor, peak, and AC average).

Low-Noise Front End
A major source of measurement error in most traditional DMMs is electromechanical relay switching. Contact-induced thermal voltage offsets can cause instability and drift. The FlexDMM eliminates all but one relay in the DC voltage, AC voltage, and resistance paths. A special relay-contact configuration cancels the thermal errors in this single relay. This relay is switched only during self-calibration. All measurement-related switching for function and range changing is done with low thermal, highly-reliable sol-state switching. Thus, electromechanical relay wear-out failures are all but eliminated.

Onboard Precision References
The FlexDMM employs some of the most stable onboard references available. As a voltage reference, the FlexDMM uses a well-known selected thermally stabilized reference that proves unmatched performance. This voltage reference is thermally shielded for a maximum reference temperature coefficient of less than 0.3 ppm/C. Time stability of this device is on the order of 8 ppm/year. No other DMM in this price range offers this reference source and its accompanying stability. That is why the FlexDMM offers a full 2-year accuracy specification.

Resistance functions are referenced to a single 10 kΩ highly stabilized metal-foil resistor originally designed for demanding aerospace applications. This component has a guaranteed temperature coefficient of less than 0.8 ppm/C and a time stability of less than 25 ppm/year.

Self-Calibration
Traditional DMMs are calibrated at a particular temperature, and this calibration is characterized and specified over a limited temperature range, usually ±5 ºC (or even ±1 ºC in some cases). Thus, whenever the DMM is used outse of this temperature range, its accuracy specifications must be derated by a temperature coefficient, usually on the order of 10 percent of the accuracy specification/ºC. Thus, 10 ºC outse of this specified range, you may have twice the specified measurement error, which can be a serious concern when absolute accuracy is important.

Unfortunately, keeping the environmental temperature of a precision instrument within ±5 ºC can be challenging in a production environment, or in a test system composed of multiple instruments, sources, etc. Instruments in a system are subject to temperature rise caused by inherent compromises in air circulation and other factors. If the excursions in temperature exceed these limits and tight specifications are required, then recalibration is also required at the new temperature.

The PXI-407x self-calibration eliminates this common problem by delivering accuracy over the full operating temperature range as summarized in Table 1. It should be noted that using the FlexDMM with Self-Cal proves accuracy at 50 C that is an 8X improvement over traditional methods.

Condition	Traditional 6 1/2 (1-Year)	NI PXI-4070 (2-Year)
Measurement within 18 to 28 C	225 µV	130 µV
Measurement at 50 C without Self-Cal	1045 µV	470 µV
Measurement at 50 C with Self-Cal	1045 µV (no self-cal available)	130 µV

Table 1. Example Summary – Uncertainty Analysis, Measuring 5 V on 10 V Range

For more information about how the FlexADC compares to traditional DMM ADCs, please refer to the Understanding the FlexDMM Architecture white paper. This architecture whitepaper compares the FlexADC capabilities to the dual-slope, charge-balance-with-ramp-down, and sigma-delta converters.

1.8 MS/s Isolated Digitizer Mode

This NI FlexDMM Family also has the capability to acquire both AC and DC-coupled waveforms up to 300 V (425 Vp AC) input at a maximum sampling rate of 1.8 MS/s. Users can vary the digitizer resolution from 10 bits to 23 bits by simply changing the sampling rate, as reflected in Figure 2. With the isolated digitizer capability, the FlexDMM can minimize overall test system cost by eliminating the need to purchase a separate digitizer and reducing the test fixture size and maintenance costs.

Figure 2. FlexDMM 1.8 MS/s Digitizer Mode

By combining NI LabVIEW graphical development software with the isolated digitizer mode of the FlexDMM, engineers can analyze transients and other nonrepetitive high-voltage AC waveforms in both the time and frequency domain. No other 6 1/2 -digit DMM features this capability.

For example, a common application in the automotive industry is the measurement of the flyback voltage on an ignition coil. The ignition coil creates the high voltages used to drive the spark plugs in the engine. The ignition coil is made up of a primary coil and a secondary coil. The secondary coil generally has many more turns of wire than the primary coil, because the turns ratio times the voltage applied to the primary coil determines the output voltage. When the current is suddenly commutated off, the collapse of the magnetic field induces a large voltage (20,000+ V) onto the secondary coil. This voltage is then routed to the spark plugs.

Because the voltages are so high on the secondary coil, tests are actually made on the primary coil. The flyback waveform is usually on the order of 10 µs with a peak voltage of 40 to 400 V, depending on the ignition coil. The common measurements made on this waveform are peak firing voltage, dwell time, and burn time. Using the FlexDMM digitizer capability and the NI LabVIEW analysis functions, you can build a flyback voltage measurement system.

Benefits of an Isolated Digitizer
With isolation, engineers can safely measure a small voltage in the presence of a large common-mode signal. The three advantages of isolation are:

• Improved rejection – Isolation increases the ability of the measurement system to reject common-mode voltages. Common-mode voltage is the signal that is present or “common” to both the positive and negative input of a measurement device, but is not part of the signal to be measured. For example, common-mode voltages are often several hundred volts on a fuel cell.
• Improved safety – Isolation creates an insulation barrier so you can make floating measurements while protected against large transient voltage spikes. A properly isolated measurement circuit can generally withstand spikes greater than 2 kV.
• Improved accuracy – Isolation improves measurement accuracy by physically preventing ground loops. Ground loops, a common source of error and noise, are the result of a measurement system having multiple grounds at different potentials.

Innovative Inductance and Capacitance Measurement Algorithm

In many production test applications, it is necessary to characterize or test inductance and capacitance values. Over the years the traditional method for measuring inductance and capacitance has involved a sine wave source and a precision synchronous detector. This rather specialized design approach has its advantages in the design of LCR and impedance brges. These devices are used for DUT characterization across a band of frequencies, voltage, and current excitation. The circuitry can become quite complex, with many precision components. Convenience, low cost, and ease of use are sacrificed. For many applications, this level of flexibility, complexity, and cost is not required or tolerable.

To put this in perspective, conser that for resistance measurements, specialized Wheatstone brges have been used for years. Figure 3 shows the basic block diagram of a Wheatstone brge. These devices can prove flexible excitation levels and deliver accuracy better than 1 ppm. But they have rather narrow application because they are complex to use and require a level of familiarity with the art. DMMs, on the other hand, use a much simpler technique to prove an excellent tradeoff between precision and ease of use. They have been universally adopted for the great majority of resistance measurement applications.

Similarly, Kelvin-Varley dividers can be used to do precision voltage measurement. But again, the complexity of the equipment combined with the manual nature of the technique makes it prohibitive in general laboratory applications or high-speed production test. In addition, most common measurements don’t require the 1 to 2 ppm accuracies achievable with this method. However, these devices are indispensable for calibration and standards comparison – where ultimate accuracy is everything and speed is not.

At the other extreme, there are low-cost DMMs that provide inductance and capacitance capability. Historically, these devices were notorious for having poor performance and repeatability, not surprising because the traditional capabilities of a DMM and inductance/capacitance measurements are somewhat at odds with each other. DMMs are traditionally slow DC devices, while inductance and capacitance measurements require precision, AC excitation, and phase-sensitive detectors.

The PXI-4072 FlexDMM offers an innovative solution to delivering both precision inductance and capacitance capability and the simplicity of a DMM resistance measurement. The PXI-4072 offers a conservatively specified 0.15% +0.1 basic accuracy for both inductance and capacitance, along with the same robust input protection expected from a high-resolution DMM. Sensitivity is 50 fF and 1 nH, with ranges extending to over 10,000 µF and 5 H. These levels are suitable for production test of a broad range of devices. With basic accuracy below 0.15% +0.1, sufficient test accuracy ratio is assured for production test of components with production tolerances less than 1%.

PXI-4072 Measurement Theory
The PXI-4072 is made possible through an innovative use of the FlexADC architecture and the 1.8 MS/s high-speed digitizer capability. The fundamental technique involves application of a precision multitone (modified square wave) current source to the DUT. The resulting voltage across the device is digitized. An FFT is performed on this waveform, from which the magnitude and phase of the impedance can be derived.

When the impedance of an element is expressed in its polar form, it provides the magnitude and phase angle, where this angle is the phase difference between the voltage and current. When the impedance is expressed in its rectangular form,



the real and imaginary parts are called resistance (R) and reactance (X) respectively. In this representation we can model the impedance as an equivalent of a resistive component and a reactive component (such as a capacitor or an inductor).



It is necessary to know both the magnitude and phase of the voltage across and current through an inductor or capacitor to be able to determine its impedance.

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Figure 4. Example of RC and RL circuits

The excitation chosen for the PXI-4072 is a modified square wave current source, which is important because it provides perfect symmetry of design to the DC current source required for measurement of resistance. This feature greatly simplifies the hardware design versus traditional LCR bridge measurement techniques, and takes advantage of the robust input protection already available on the FlexDMM. This design translates to precision and stability of measurement for the user.

To measure phase, it is possible to use two signals of different frequencies for which the relative phase angle is known. This is greatly simplified if both frequencies (or tones) are contained in the same signal. This is another major advantage of using the modified square wave excitation. The square wave contains two tones that can be extracted using the FFT algorithm: the fundamental and the third harmonic.

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Figure 5. PXI-4072 Excitation Waveform and Analysis

We can “define” the magnitude of the excitation current fundamental and third harmonic by calibration. Then, we can calculate either impedance magnitude:



From the definitions we know that the impedance should change only its imaginary component when the frequency changes. This means that the different tones on the same signal should “see” a different magnitude across the same reactive element. For example, in the case of a capacitor modeled with a series resistor, the vector representation of the impedance at the two different frequencies should appear as is shown in Figure 6.

The real component should be the same at both frequencies. Given this assumption, we can write the two following equations:



It is also known that X1 = 3X3 for an ideal component. Given these three equations, where Z1 and Z3 are measured values, we can solve for R, X1 and X3. Because the frequency is also a known value, the capacitance or inductance value can be easily calculated. The PXI-4072 implements these mathematical analysis using DSP tools with LabVIEW to extract inductance and capacitance.

Minimizing Overall System Cost

The overall cost of an ATE system includes initial cost, development cost, cost to test, and maintenance cost. The deciding contributor to the cost of a test system (especially when replicating multiple systems) is the selection of test equipment. Designing a system that keeps the overall cost within budget is often difficult because there are many parameters that affect it including:

• Test software development
• Throughput
• Accuracy
• System integration
• System footprint
• Ease of use
• Calibration and maintenance

Lowering Initial Cost

The initial cost of an ATE system generally includes instruments, chassis, computer, instrument controller, cabling, fixturing, software, and switching. Careful selection of these components will minimize the initial cost of your system as well as the development cost, cost to test, and cost of maintenance.

The FlexDMM family benefits greatly from its use of the common PXI platform architecture. Thus, incremental system cost is substantially lower than traditional instruments. This design approach results in measurement functionality at a cost much lower than devices in proprietary platforms. In addition, the PXI platform uses components already present in your computer – the processor, memory, and monitor – to perform the measurement analysis and display instead of duplicating this functionality in each customized box instrument. As these computer components are succeeded by the next generation in speed and capacity, the overall measurement system also improves.

As mentioned above, the PXI-4072 FlexDMM lowers the initial cost further by combining the most frequently used functionality of three common traditional instruments found in an ATE system – digital multimeter, digitizer, and LCR meter. As a digital multimeter, the FlexDMM delivers fast and accurate 7-digit DC voltage and current, true rms AC voltage and current, and resistance measurements. In the high-voltage, isolated digitizer mode, the FlexDMM can acquire both AC and DC-coupled voltage and current waveforms up to 300 V and 1 A input, at a maximum sampling rate of 1.8 MS/s. Lastly, the FlexDMM delivers 0.15% +0.1 basic accuracy for both inductance and capacitance measurements. This combination of functionality can lower system cost by some 50 percent, as shown in Table 2.

	Traditional Instruments	NI 4072 FlexDMM
Digital multimeter	$1,200	$2,500
DAQ/Digitizer	$1,500
LCR meter	$5,200
Switch	$2,000	$1,000
Fixture and Rack	$1,000	$1,000
PXI Chassis	$0	$1,000
Control	$1,100 (GPIB + Cables)	$1,500 (MXI 3)
Computer	$1,500	1,500
Calibration (5 year period)	$4,500 (see text)	$600 (see text)
TOTAL 5-year Cost	$18,000	$9,100

Table 2. Cost Comparison between Traditional Instruments and the PXI-4072 FlexDMM

Reducing Development Cost

The biggest contributor to development cost is the time it takes to develop the test software, given your development software environment and instrument drivers. An instrument driver is a set of software routines that control a programmable instrument. Each routine corresponds to a programmatic operation such as configuring, reading from, writing to, and triggering the instrument.

Traditional test systems include multiple individual instruments, each with its own specific instrument driver. Engineers must learn a different driver for each instrument, which can be time-consuming. It is often cumbersome to integrate these individual drivers in a system because the instruments and drivers were designed by different companies and were not designed and optimized to work together.

The FlexDMM driver software, NI-DMM, provides complete access to the functionality of the FlexDMM. This highly integrated instrument driver, shown in Figure 7, can help engineers reduce software development time.

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Figure 7. The NI PXI-4072 Single API for All Three Measurement Modes

Reducing Cost of Test

One of the most common concerns with developing a production test system is throughput.
Historically, the slowest component of these automated measurement systems is a DMM. For years, test engineers have employed numerous strategies to extract more speed from these devices in R&D laboratories and on the manufacturing floor. Optimization techniques have included cutting down the number of tests, reducing the accuracy requirements, purchasing multiple DMMs, or even purchasing a more expensive 8 1/2-digit DMM and running it at much lower resolution. These compromises can improve the overall test throughput of a DMM, but at the expense of accuracy, system cost, size, or a combination of them. The FlexDMM family eliminates the need to use such techniques by providing the following advantages over traditional DMMs:

• Fast A/D converter with a sampling rate of 1.8 MS/s, so acquisition speed is dramatically improved.
• PCI bus, which provides transfer rates at least 100 times faster than GPIB interface
• High-speed host PC processing, which improves system throughput every time the CPU is upgraded
• Streamlined NI-DMM driver software, which offers tight integration with LabVIEW
• AC-rms measurement (traditionally an analog domain problem) in the digital domain, simultaneously improving sensitivity, accuracy, and speed of precision AC measurements.
• Dramatic improvement of speed and accuracy of function and range changes by using solid-state low-leakage relays
• Seamlessly integration with National Instruments PXI and SCXI switches using the high-speed PXI trigger bus

As a result, the FlexDMMs are very fast, flexible measurement devices, as shown in Figure 8. Traditional DMMs make very dramatic tradeoffs between speed and resolution (represented by the stepped curve). At 6 1/2 digits, the FlexDMM achieves DC voltage scanning rates of 100 S/s. For those applications with higher throughput needs the FlexDMM has a maximum DC voltage scanning rate of 10 kS/s at 4 1/2 digits.

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Figure 8. NI FlexDMM Speed and Resolution Tradeoff

Reducing Cost of Ownership (Maintenance Cost)

The primary source of maintenance cost is calibration and repair of instruments. Calibration cycles interrupt the service life of the product and potentially result in downtime. The cost of yearly calibration can easily offset the cost of the product over a 5-year service life. The FlexDMMs minimize maintenance cost by offering a 2-year external calibration cycle, compared to the 1-year cycle of traditional instruments. Table 3 compares the cost for calibration over a 5-year product service life.

Instrument Functionality	PXI-4072	Traditional Instruments
Digital Multimeter	$6001	$1,5001
Digitizer		$1,500
LCR meter		$1,500
Total Cost	$600	$4,500

¹Assuming $300/cal cycle
Table 3. 5-Year Calibration Cost for the PXI-4072 and Traditional Stand-Alone Instruments


Some users may choose to set up a calibration system in house for their instruments to reduce downtime and the risk involved in shipping the product to an outside calibration facility. However, the internal labor costs are generally similar to the calibration fees charged by the outside facility. In either case, the PXI-4072 can save customers up to $3,900 in calibration cost over a 5-year cycle.

Conclusion

The high-performance, multifunction NI PXI-4072 FlexDMM gives engineers the functionality of three common instruments in a single-slot 3U PXI module – 6 1/2-digit multimeter, 1.8 MS/s isolated digitizer, and LCR meter. The PXI-4072 FlexDMM brings digital multimeters one step closer to providing universal measurement capability, by offering engineers the 20 most common ATE measurements, including voltage, current, capacitance, inductance, temperature and resistance. By integrating these measurements into one PXI module, engineers reduce test system size and total cost, increase throughput, and shorten test development time.

References

Understanding the FlexDMM Architecture

Paul Horowitz and Winfield Hill, The Art of Electronics: University of Cambridge Press, 1989.

Digital Multimeter Measurement Tutorial National Instruments,

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