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Convert signals to proper logic levels

26 Jan 2016  | Abel Raynus

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Another good option is to use any dual- or quad-voltage comparator. You can use an LM393 from National Semiconductor because it's inexpensive and widely available. Figure 2 shows a simple configuration with few components. The 5V power-supply voltage acts as the positive-threshold voltage. The output is 5V for input signals lower than this level. If the input signal exceeds 5V, the output voltage drops to 0V. Resistor R1 connects an open collector of the LM393 to the supply voltage.


Figure 3: Use the voltage divider comprising R2 and R3 for the threshold voltage.


Sometimes, a zero-output signal is undesirable. A missing power-supply voltage, a bad solder joint, or a broken wire in the test fixture could cause this zero-output signal. Use a logic high level when the signal under test is present and logic low when it's absent. At first glance, it seems that just switching the comparator pins of the input and the threshold voltages provides an acceptable approach. However, that assumption is invalid because the positive input voltage may exceed the power-supply level only as long as the other voltage remains within the common-mode range. The upper limit of common-mode input voltage for the LM393 is 1.5V less than the power-supply voltage, or 3.5V. Thus, you should use the voltage divider comprising R2 and R3 for the threshold voltage (figure 3).


About the author
Abel Raynus contributed this article.


This article is a Design Idea selected for re-publication by the editors. It was first published on February 5, 2009 in EDN.com.


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