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A Modular Architecture for Precision DC Measurements

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This issue's focus is Digital Multimeters (DMMs). 
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Most engineers are very familiar with using digital multimeters (DMMs) for simple DC voltage measurements.  Where DMMs derive their name, however, is from the breadth of measurement options that they offer – typically voltage, resistance, current, and diode test, among others.   As a result of this versatility, DMMs have become one the most widely used and most recognizable measurement instruments today.

By catering to such a wide variety of signals, however, DMMs can also fall short of specific measurement needs.  When engineers must precisely characterize low-level signals, instruments such as nanovoltmeters, micro-ohmmeters, and picoammeters have become common additions to a system.  Typically, a precision measurement station will have a core DMM that performs the majority of the measurements, but will also use multiple specialized instruments for more specific functionality.  Unfortunately, the trades-offs that come with this solution are increased size and cost, as each specific precision measurement requires its own dedicated instrument in addition to the standard DMM. 

A different approach offers both versatility and specialization through the use of modular instrumentation.  By using a single measurement device in conjunction with other modular hardware that provides specialized conditioning, precision functionality can be incorporated into an existing system with minimal overhead.  Such a system can be built using the PXI form factor - an open, multi-vendor platform for instrumentation that offers modularity as one of its key advantages.  For instance, a typical PXI measurement system would incorporate a DMM such as the NI PXI-4070 6˝-Digit DMM.  As mentioned previously, a DMM alone provides measurement versatility – in the case of the PXI-4070, voltages below 1 microvolt, currents below 1 microamp and resistances below 1milliohm can be measured along with a variety of other signals.

Application requirements, however, can dictate that much lower signals must be measured, such as current signals down to 1 picoamp.  A traditional approach to satisfy this requirement would be to add a separate, stand-alone picoammeter.  However, with a modular platform such as PXI, the functionality of the PXI-4070 DMM can be augmented through the use of complimentary modules that provide precision conditioning.   In this case, the module of choice would be the NI PXI-4022 current amplifier, which can be set to convert a current signal to an analogous voltage with a gain of up to 10^8.  With this configuration, a picoamp current signal can be converted to a millivolt voltage signal, which can then be acquired by the PXI-4070 DMM.  In this manner, picoamp measurement functionality can be integrated into an existing PXI DMM with the simple addition of a low-cost, low-profile PXI module, thus saving on the cost and size of a traditional stand-alone pico-ammeter.

Similarly, micro-ohm measurement functionality can be integrated into the same PXI DMM with the addition of another PXI modular instrument.  For typical resistance measurements, a constant current source from the PXI-4070 DMM is applied to the load and the resulting voltage is measured in order to calculate resistance using Ohm’s law.  In the case of micro-ohm resistance measurements such as path and contact resistances, the amount of current that must be applied to develop a measurable voltage exceeds the range of the internal current source on the PXI-4070 DMM.  For these applications, the NI PXI-4110 programmable power supply module can be added to the system and used as a constant current source at up to 1 amp to enhance the low-resistance measurement functionality of the PXI-4070 DMM.  By applying a higher current using the PXI-4110 programmable power supply, voltages in the microvolt range are developed across the load, which can then sent to the PXI-4070 DMM for precision measurement, allowing micro-ohm resistances to be characterized.  Again, through the addition of a low-cost, low-profile PXI module, new measurement functionality can be integrated into an existing PXI DMM.

Historically, there have been two distinct approaches to precision DC measurements that accommodate different needs.  DMMs have achieved cost savings and versatility by measuring a variety of signals on a single device, whereas dedicated precision instruments have achieved superior performance through the use of highly specialized analog circuitry.  With a modular architecture such as PXI, the benefits of versatility and specialization need not be exclusive.  By combining a high-performance PXI DMM with other complimentary PXI modules, engineers can create a system that is both highly precise and highly flexible.  
The system discussed in this article is just one example of the many test configurations that can be built with PXI modular instruments.  For more information visit ni.com/modularinstruments and ni.com/pxi 

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