How to select your instrument: oscilloscope, digitizer or DMM...

Author : Christian Plötz | Technical Sales Engineer | VX Instruments

01 May 2020

VX-Instruments-PXI-Supplement_How to select your instrument_oscilloscope, digitizer or DMM_580x280

The task of testing electronic assemblies is becoming increasingly complex. Once the appropriate measurement procedure has been selected and the test setup determined, the next step is to select the appropriate instrument: oscilloscope, digitizer or DMM.

This tutorial was originally featured in EPDT's 2020 PXI for T&M supplement, included in the May 2020 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.

In this tutorial, Christian Plötz, Technical Sales Engineer at test solutions provider, VX Instruments offers some hints & tips on the selection of measurement device...

The task of testing electronic assemblies – even using modern, flexible and powerful modular instrumentation platforms like PXI – has become more and more complex. After selecting suitable measuring methods and test set-up, the selection of measurement device is one of the most important tasks for the test engineer. In this context, the requirements for devices used in test systems are also increasing – and this applies to use in the laboratory, as well as in production test systems. Test speed and reliability in continuously operating production test systems also play a decisive role here.

Figure 1. Input impedance in double logarithmic representation with clearly recognisable 3 dB cutoff frequency (L) & input impedance in simple logarithmic representation for the representation of different frequency responses

The core components of a test system can be divided into two basic types: stimuli devices generate signals for the device-under-test (DUT) and electrical measuring instruments detect the signals supplied by the DUT. Digital multimeters (DMMs) and oscilloscopes are still by far the most widely used measuring instruments. So-called digitizers – test system components from modular instrumentation platforms, such as PXI – are frequently found  in production test, and they are becoming increasingly important, due to increasing requirements for testing.

The problem of different specifications

Users are often faced with the problem that even in seemingly simple devices, such as DMMs, the specifications of individual manufacturers are so differently structured that comparison is possible only with relatively substantial effort. When selecting instruments which can fulfil the requirements of the test specification, measurement uncertainties must be identified from the instrument specification, as well as consideration given to how an instrument itself influences the signal to be measured.

Figure 2. Presentation of the possible danger of using insufficiently isolated measuring instruments

Relevant effects include, for example, the input impedance, whose ohmic component with DC components is usually 10 MOhm, and often only 1 MOhm with AC components. Likewise, input capacities must be considered, which can vary from 10 pF to 300 pF, depending on the device type. In the case of AC measurements, these form a low-pass filter, together with the measurement object, and can thus significantly falsify the measurement result.

Benefits & boundaries of isolation

With the terms ‘not grounded’, ‘isolated’ or ‘floating’, the user must be careful in several respects. When using such a device, it’s essential to check the voltage up to which the measuring inputs are isolated to the protective conductor. If you want to measure a single cell in a stack on a high-voltage battery, the instrument must also be isolated to the protective conductor up to the maximum voltage. In order to rule out malfunction, and thus hazard, it is recommended that the selected instrument is also isolated to the protective conductor across the entire available measuring range. For example, if a device is isolated to the protective conductor only up to 40V, cells above a voltage of 40V can no longer be individually measured. A device with a maximum input voltage of 250V should be isolated to the protective conductor with this value in order to be able to use it without restriction.

Figure 3. Schematic representation of isolated measuring technology with indicated parasitic coupling

With DMMs, many users automatically assume that the devices are not grounded, in other words isolated from the protective conductor. This is usually the case when you look purely at the ohmic resistance between the measuring ground connection and the protective conductor. The ohmic isolation is several gigaohms, and can therefore be disregarded. Some DMMs show a considerable capacity between the measuring ground connection and the protective conductor, which can achieve several nanofarads (parasitic coupling). Although nanofarad capacitances appear small, it must be checked whether the test object is too heavily influenced, thereby causing another relevant measurement error. This happens due to the load of the measuring signal, due to the input impedance of the instrument.

Different parameters in device selection

Parameters such as the duration of measurement are increasingly important, since they significantly influence throughput in test systems, and therefore on the production line. It should be noted that any measurement inaccuracy occurring during test is likely to increase significantly with increasing measurement speed.

Figure 4. Schematic representation of the properties of measuring device groups

In order to test efficiently and rapidly, in addition to pure measurement time, the time for instrument setup, the measurement range change required during the test procedure, and the data transmission time from the instrument to the PC must also be taken into account. To make it even more difficult to plan the test procedure, these times are generally not specified in datasheets and must therefore be determined experimentally for each device. This shows that even choosing a seemingly simple device like a DMM can already raise plenty of questions. For oscilloscopes and digitizers, the parameters are even more diverse.

Conversely, DC mean values and AC RMS values can be measured with an oscilloscope. However, the results are of lower accuracy, as these instruments are mostly equipped with 8 to 12-bit A/D converters. In addition, the measuring ground connection of an oscilloscope is not isolated, but connected directly to the protective conductor. As a result, the use for many types of measurements, for example in the case of in-circuit tests, is significantly restricted or causes further measurement errors due to ground loops. A list of important additional parameters and their typical values for the device groups shown is given in Table 1.

Figure 5. Schematic relationship of the measuring device groups

Meanwhile, high-resolution digitizers, which are isolated at the same time, with a resolution of up to 20 bits are available (in PXI form factor, for example). Depending on the measurement task, they represent an alternative to DMMs and oscilloscopes. The fact that the boundaries between these devices will continue to blur is demonstrated by a new device that will soon be launched.

The Multi Measurement Device (MMD) from VX Instruments combines the following functions:

•    Digitizer 40 MS/s, 16 Bit, ±250V

•    24 Bit-DMM for DC-voltage & current, AC voltage & current, resistance & LCR

Table 1. Properties and typical values of the represented measuring device groups

•    Timer/counter with various measuring functions with a resolution of 25 ns & voltages up to 250V

•    High configurable FPGA trigger matrix

•    Fully isolated up to ±250V

Summary

It is still important for the user to know his test case and its requirements, as well as the effects of the measurement device itself on the DUT, in order to best select the right instrument. Furthermore, the structure and integration of the test system into the production line must be taken into account. The developer should keep an eye on the parasitic capacitances and inductances, as well as coupled-in interferences, which are caused by long cabling, for example.


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