Facebook
Google
GitHub
LinkedinDigital Circuits
Analog-to-Digital Conversion
18 questions By Tony R. Kuphaldt
-
Question 13 of 18
Suppose an analog-digital converter IC (“chip”) inputs a voltage ranging from 0 to 5 volts DC and converts the magnitude of that voltage into an 8-bit binary number. How many discrete “steps” are there in the output as the converter circuit resolves the input voltage from one end of its range (0 volts) to the other (5 volts)? How much voltage does each of these steps represent?
Reveal answerThis ADC (Analog-to-Digital Converter) circuit has 256 steps in its output range, each step representing 19.61 mV.
Notes:This question is not so much about ADC circuitry as it is about digital resolution in general. Any digital system with a finite number of parallel bits has a finite range. When representing analog variables in digital form by the limited number of bits available, there will be a certain minimum voltage increment represented by each “step” in the digital output. Here, students get to see how the discrete nature of a binary number translates to real-life measurement “rounding.”
-
Question 14 of 18
One of the idiosyncrasies of analog-to-digital conversion is a phenomenon known as aliasing. It happens when an ADC attempts to digitize a waveform with too high of a frequency.
Explain what aliasing is, how it happens, and what may be done to prevent it from happening to an ADC circuit.
Reveal answerAs the saying goes, a picture is worth a thousand words:
Notes:The point of this question (and of the answer given) is to have students put this important concept into their own words.
Something noteworthy for students and instructors alike is that aliasing may be visually experienced using digital oscilloscopes. Setting the timebase (seconds/division) control too slow may result in a false (aliased) waveform displayed in the oscilloscope. Not only does this make a good classroom demonstration, but it also is a great lesson to learn if one expects to use digital oscilloscopes on a regular basis!
-
Question 15 of 18
Analog-to-digital converter circuits (ADC) are usually equipped with analog low-pass filters to pre-condition the signal prior to digitization. This prevents signals with frequencies greater than the sampling rate from being seen by the ADC, causing a detrimental effect called aliasing. These analog pre-filters are thus known as anti-aliasing filters.
Determine which of the following Sallen-Key active filters is of the correct type to be used as an anti-aliasing filter:
Reveal answerThe low-pass Sallen-Key filter, of course! What’s the matter? You’re not laughing at my answer. What I’m doing here is asking you to some research on Sallen-Key filters to confirm your qualitative analysis. And yes, I do expect you to be able to figure out which of the two filters is low-pass based on your knowledge of capacitors and op-amps, not just look up the answer in an op-amp reference book!
Notes:Discuss with your students various ways of identifying active filter types. What clues are present in these two circuits to reveal their filtering characteristics?
