If the appropriate measurement method is selected and the test setup is fixed, then the instrument must be selected: oscilloscope, digitizer or DMM. We give tips on the selection of devices.
The tasks of testing electronic assemblies are becoming increasingly sophisticated. After selecting the appropriate measurement methods and test setup, selecting the appropriate instrument is one of the key tasks of the test engineer. In this context, the requirements for the devices used in test systems are also increasing. This applies to use in the laboratory as well as in production test systems.
Reliability of continuous operation and test speed also play a decisive role here. The core components of a test system can be divided into two basic types. Stimuli devices generate signals for the device under test and electrical measuring devices detect the signals supplied by the device under test. Digital multimeters (DMMs) and oscilloscopes are by far the most widely used devices for measuring instruments.
In the production test, so-called digitizers are frequently found, which are becoming increasingly important due to increasing test requirements. The other articles in this series are dedicated to these three types of gauges. They provide the user with assistance in the selection of the optimal device for him and show possible measures to avoid measurement errors.
The problem of different specifications
The user is often faced with the problem that even in seemingly simple devices such as a DMM, the specifications of the individual manufacturers are constructed so differently that a comparison is possible only with relatively great effort. When selecting measuring instruments which can fulfill the requirements of the test specification, both the measurement uncertainties from the measuring device specification and the fact how a measuring instrument influences the signal to be measured must be taken from it.
Relevant effects, for example, the input impedance, the ohmic component with DC components is usually 10 MOhm and with AC shares very often only 1 MOhm. Likewise, the 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. In the terms earth-free, isolated or floating, however, be careful in several respects.
Measuring inputs isolated from earth
When using such a device, it is essential to check to what voltage the measuring inputs are insulated from earth. If you want to measure a single cell in a stack on a high-voltage battery, the meter must also be isolated up to this voltage concerning ground. To rule out the incorrect operation and therefore a hazard, it is recommended that the selected measuring instrument is also isolated from the earth in the entire available measuring range. For example, if a device is isolated from the ground only up to 40 V, cells above a voltage of 40 V can no longer be individually measured. A device with a maximum input voltage of 250 V should be isolated from the ground with this value to be able to use it without restriction.
In DMMs many users will automatically assume that the devices are floating, are constructed so insulated from the ground conductor. This is usually also the case when one considers the ohmic resistance between the measuring mass and the protective conductor purely. The ohmic isolation is several gigaohms and can, therefore, be disregarded. However, some DMMs additionally have a considerable capacitance between measuring mass and protective conductor, which can assume several nanofarads (parasitic coupling). Although nanofarad capacitances appear small, it must be checked whether the test object is too heavily loaded thereby causing another relevant measurement error. This happens due to the load of the measuring signal due to the input impedance of the measuring device.
The different parameters for the meter selection
Also, parameters such as the duration of measurement are of increasing importance as they significantly influence the throughput in test systems. It should be noted here that the measurement inaccuracy is occurring during the height greatly increases with increasing measuring speed. In order to test efficiently and quickly, in addition to the pure measurement time, the time for instrument setup, the measurement range change necessary in the course of the test, and the data transfer time from the instrument to the PC must also be taken into account.
To complicate the planning of the test procedure is added that these times are not usually specified in data sheets and thus must be determined experimentally for each device. This shows that even selecting a seemingly simple device like a DMM can already raise a large number of questions. For oscilloscopes and digitizers, the parameters are a bit more diverse and besttopreviewsonline.com has discussed how you can buy the best oscilloscope for you.
Also, the boundaries between DMM, oscilloscope and digitizer are beginning to blur more and more: In the past, DMM measured DC / AC voltages and currents, and resistors. The integration times were typically one second. Meanwhile, DMMs can also sample signals, but with a much lower sample rate than with an oscilloscope.
In turn, DC oscilloscopes and AC RMS values can be measured with an oscilloscope. However, the results are of lower accuracy as they are mostly equipped with 8 to 12 bit A / D converters. Also, the measuring mass of an oscilloscope is not ground-free but connected directly to the protective conductor. As a result, the use of many types of measurements, such as in-circuit tests, significantly limits or causes further measurement errors due to ground loops. A list of essential sizes and their typical values for the illustrated device groups is shown in the table in the gallery.
Know test case and requirements exactly
Meanwhile, high-resolution and simultaneously isolated digitizers with a resolution of up to 20 bits are available. 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 shown by a new device that will soon be available on the market. The Multi Measurement Device, short MMD, from VX Instruments combines the following functions:
- Digitizer 40 MS / s, 16 bit, 250 V,
- 24-bit DMM for DC voltage and current, AC voltage and current, resistance and LCR,
- Timer/counter with various measuring functions with a resolution of 25 ns and voltages up to 250 V,
- Highly configurable trigger unit and
- fully insulated to 250V
It is still necessary for the user to know his test case and the requirements, as well as the effects of the measuring device on the size to be measured, to select the right measuring device. Furthermore, the structure and integration of the test system into production must be taken into account. The parasitic capacitances and inductances as well as coupled disturbances, which arise for example by long wiring, should keep the developer, particularly in the eye.
In two further contributions, a comparison between isolated and earth-related measuring technology, on the other hand, possible causes of measured value influencing by the measuring device are presented and assistance for the evaluation and minimization is given.