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Switched Capacitor Circuitry
15 questions By Tony R. Kuphaldt
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Question 10 of 15
Given the fact that a switched-capacitor network has the ability to emulate a variable resistance, what advantage(s) can you think of for using switched-capacitor networks inside of analog integrated circuits? Identify some practical applications of this technology.
Reveal answerSwitched-capacitor networks allow us to have electronically variable resistors inside integrated circuits, with no moving parts, which is a technological advantage over standard resistors. Another advantage of switched-capacitor circuits is that they typically require less substrate area on an integrated circuit than an equivalent resistor. A huge advantage is that switched-capacitor networks may be manufactured to give much more accurate resistances than plain resistors, which is important in filtering applications.
I’ll let you research (or dream up) some practical applications for this technology.
Notes:This question could very well lead to an interesting and lively discussion with your students. Once students recognize the equivalence between switched-capacitor networks and resistors, it is a short cognitive leap from there to practical applications where variable resistances would be useful (especially in an integrated circuit environment, where moving parts have traditionally been out of the question).
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Question 11 of 15
Identify what type of passive filter circuit this is, and then sketch a schematic diagram for an equivalent circuit using a switched-capacitor network instead of a resistor:
Also, write an equation describing the cutoff frequency in terms of switching frequency and capacitor values.
Reveal answerThis is a lowpass filter circuit:
f−3dB = fswitch C1 2 πC2Note: in order for this circuit to be practical, fswitch >> fsignal.
Notes:This question asks students to substitute a switched capacitor network for the fixed resistor in the original filter circuit. Similarly, algebraic substitution is required to derive the equation for cutoff frequency.
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Question 12 of 15
Identify what type of passive filter circuit this is, and then sketch a schematic diagram for an equivalent circuit using a switched-capacitor network instead of a resistor:
Also, write an equation describing the cutoff frequency in terms of switching frequency and capacitor values.
Reveal answerThis is a highpass filter circuit:
f−3dB = fswitch C2 2 πC1Note: in order for this circuit to be practical, fswitch >> fsignal.
Notes:This question asks students to substitute a switched capacitor network for the fixed resistor in the original filter circuit. Similarly, algebraic substitution is required to derive the equation for cutoff frequency.
