Have you ever imagined the life without electricity? These days, most people have the smartphone and it can be difficult to do something without the smartphone. If you don’t have the smartphone even for a day, you will feel so empty and anxiety. As you may have an experience, the discharged smartphone is useless. With this simple imagination, we can understand the importance of electricity, although it is hard to know the value of electricity in every life.
I have a job to make several products for industrial applications using electricity. More specifically, our company designs and produces something what can measure the electrical characteristics of device under test(DUT) using electrical signal. We call it the electrical instruments. The word “using electrical signal” in experiment can be divided into two parts. One is the generation of electrical signals, the other is the measurement of electrical signals.. Of course, when we use a electrical instrument for test DUT, DUT have a response characteristics for these electrical signals. Fortunately, we can say that most things have their specific responses for some electrical signals although it is more or less.
As mentioned, the word ‘electrical signals’ also can be divided into two parts, voltage and current. So we can make a table as below for simple understanding of various electrical instruments.
Some of electrical instruments has a specialized design for the generation function of the electrical signals, and some of electrical instruments has a specialized design for the measurement function of the electrical signals. You can also find some multifunctional electrical instruments having simple generation and measurement functions. Let’s imagine that we have a resistor for DUT. We have to use proper electrical instrument for check the resistance of DUT. A resistor is related to voltage and current by Ohm’s law. Therefore, the electrical instruments as stated above can be considered to check the resistance. For example, it is possible to use the voltage source and ammeter. The other way, it is also possible to use the current source and voltmeter.
It is simple answer to use the multifunctional electrical instruments providing two test modes. However, which configuration is the right choice if we have to select. In this situation, the answer is not simple. We have to guess the resistance range of DUT and to understand the limitation of electrical instruments. For example, let’s assume the DUT (resistor) has 1 μΩ. In case of voltage source and ammeter configuration, if you apply 1 V using voltage source to the DUT, the ammeter has to measure 1,000,000 A. It is an enormous amount! Unfortunately, it is difficult to find the ammeter which can measure this kind of range. Similar to this context, if you have typical ammeter having a 10 A range, you must apply 10 μV. It is possible solution but the voltage source with 10 μV range is rare. Finally, you have to pay more time and cost for measure 1 μΩ accurately with general voltage source and ammeter.
On the contrary to this, we can imagine to use the current source and voltmeter. If the 1 A is applied, the voltmeter should be measured to 1 μV. Fortunately, you can easily get these kind of voltmeter. Also, the current source with 1 A output is easy to get. This configuration is more practical then the previous situation in cost and time. Refer to below graph, typical DMM (Digital Multi-Meter) has a performance measuring the range from nanovolt at 1 μΩ.
Reference : ‘Low Level Measurements Handbook’ of Keithley
As we experimented virtually, we can measure the electrical characteristics of DUT or not depending on the DUT level and limitation of electrical instruments. Some people simply think to use high priced electrical instruments or general purposed electrical instruments for various DUT measurements. However, this kind of choice does not guarantee accurate results. You have to pay more attention to use proper electrical instruments for your experiment. I will give more information for 4 kind of electrical instruments as shown in our next issue.
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