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Digital-to-Analog Conversion
13 questions By Tony R. Kuphaldt
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Question 1 of 13
If a pulse-width modulated (PWM) signal is sent to a passive integrator circuit from a circuit capable of both sourcing and sinking current (as is the case with the dual-MOSFET output stage), the output will be a DC voltage (with some ripple):
Determine the relationship between the PWM signal’s duty cycle and the DC voltage output by the integrator. What does this suggest about PWM as a means of communicating information, such as analog data from a measuring device?
Reveal answerThere is a direct-proportional relationship between duty cycle and DC output voltage in this circuit, making it possible for a PWM signal to represent analog data.
Follow-up question #1: why is it important that the circuit generating the PWM signal for the integrator be able to both source and sink current?
Follow-up question #2: what would have to be done to reduce the ripple voltage at the integrator’s output?
Notes:Although it should not be difficult for students to discern the relationship between duty cycle and DC output voltage, the application of this relationship to data communication might be difficult for some students to grasp, especially on their own. Further elaboration on your part may be necessary.
An excellent example of this principle applied is the generation of an analog voltage by a 1-bit digital circuit. This technique is useful in microcontroller systems where output ports may be scarce, provided that ripple voltage (or slow response) is not a problem.
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Question 2 of 13
Explain what the purpose of a digital-to-analog converter, or DAC circuit is, in your own words.
Reveal answerThe most basic definition for this device should be obvious: a circuit that takes a digital input and creates an analog output. What I’m looking for, though, is something a little less obvious. In your own words, explain what it means for a circuit to have a “digital input” and an “analog output.” You may give an example of such a circuit if you find it easier to answer the question in context.
Notes:Informational sources abound which your students can research. Be sure to ask them to answer specifically, explaining what it means for a circuit to have a “digital input” and an “analog output.”
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Question 3 of 13
Explain how this digital-to-analog converter (DAC) circuit is supposed to function:
Reveal answerI’ll let you figure out the operation of this circuit on your own!
Notes:This question is a good review of op-amp theory, especially for students who might not have studied operational amplifiers in a while.
