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Digital-to-Analog Conversion
13 questions By Tony R. Kuphaldt
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Question 7 of 13
Determine the voltage output by the following R-2R ladder network given the switch states shown in the table:
SW0 SW1 SW2 SW3 Vout
Ground Ground Ground Vref
Ground Ground Vref Ground
Ground Vref Ground Ground
Vref Ground Ground Ground
Ground Ground Ground Ground
Reveal answer
SW0 SW1 SW2 SW3 Vout
Ground Ground Ground Vref 8 volts
Ground Ground Vref Ground 4 volts
Ground Vref Ground Ground 2 volts
Vref Ground Ground Ground 1 volt
Ground Ground Ground Ground 0 volts
Follow-up question: the fact that an R-2R resistor network is inherently linear, we may readily apply the Superposition Theorem to figure out what happens when more than one switch is moved to the Vref position. Explain how you would apply Superposition to determine all output voltages for all possible combinations of switch positions.Notes:As you can see, the reference voltage value of 16 volts was not chosen at random! I wanted students to see the pattern between single switch closures and binary place-weights for a four-bit number. The actual electrical analyses for each condition are best expedited by applying Thévenin’s theorem repeated to the circuit, condensing sections to single resistances and voltage sources until a simple voltage divider circuit is obtained at the output terminal.
The follow-up question is quite important. Be sure to ask your students about it, for it holds the key to figuring out all output voltage values for all binary input possibilities.
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Question 8 of 13
Explain why DAC circuits based on R-2R ladder networks are more popular than binary-weighed resistor networks. Either one will work well if properly designed and built, so why would one design be more widely manufactured?
Reveal answerThe answer to this question has to do with business and production-line priorities. Just because two designs work equally well in theory does not mean they are equally easy to mass-produce!
Notes:It is important for your students to grasp basic principles and practices of business, because that is the arena their technical skills will most likely find challenge and value. This question is a way to get your students thinking about real-life, practical manufacturing concerns that go beyond basic principles of electrical theory.
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Question 9 of 13
Explain what a digital potentiometer is, and give one example of a digital potentiometer in integrated-circuit (IC) form.
Reveal answerAnalog Devices manufactures a 64-position digital potentiometer under the part number AD5227, for example. This is by no means the only digital potentiometer in production!
Follow-up question: would you classify a digital potentiometer as an ADC (analog-to-digital converter) or as a DAC (digital-to-analog converter)?
Notes:Rarely is the digital potentiometer mentioned in introductory textbooks as a digital-to-analog converter device, but it is!
